Sockets
*******

This chapter describes the GNU facilities for interprocess communication using sockets.

A socket is a generalized interprocess communication channel. Like a pipe, a socket is represented as a file descriptor.  But, unlike pipes, sockets support communication between unrelated processes, and even between processes running on different machines that communicate over a network.  Sockets are the primary means of communicating with other machines; telnet, rlogin, ftp, talk, and the other familiar network programs use sockets.

Not all operating systems support sockets.  In the GNU library, the header file sys/socket.h exists regardless of the operating system, and the socket functions always exist, but if the system does not really support sockets, these functions always fail.

Incomplete: We do not currently document the facilities for broadcast messages or for configuring Internet interfaces.

Socket Concepts
===============

When you create a socket, you must specify the style of communication you want to use and the type of protocol that should implement it. The communication style of a socket defines the user-level semantics of sending and receiving data on the socket.  Choosing a communication style specifies the answers to questions such as these:

  * What are the units of data transmission?  Some communication styles regard the data as a sequence of bytes, with no larger structure; others group the bytes into records (which are known in this context as packets).

  * Can data be lost during normal operation?  Some communication styles guarantee that all the data sent arrives in the order it was sent (barring system or network crashes); other styles occasionally lose data as a normal part of operation, and may sometimes deliver packets more than once or in the wrong order.

     Designing a program to use unreliable communication styles usually involves taking precautions to detect lost or misordered packets and to retransmit data as needed.

  * Is communication entirely with one partner?  Some communication styles are like a telephone call---you make a connection with one remote socket, and then exchange data freely.  Other styles are like mailing letters---you specify a destination address for each message you send.

You must also choose a namespace for naming the socket.  A socket name (``address'') is meaningful only in the context of a particular namespace.  In fact, even the data type to use for a socket name may depend on the namespace.  Namespaces are also called ``domains'', but we avoid that word as it can be confused with other usage of the same term.  Each namespace has a symbolic name that starts with PF_. A corresponding symbolic name starting with AF_ designates the address format for that namespace.

Finally you must choose the protocol to carry out the communication.  The protocol determines what low-level mechanism is used to transmit and receive data.  Each protocol is valid for a particular namespace and communication style; a namespace is sometimes called a protocol family because of this, which is why the namespace names start with PF_.

The rules of a protocol apply to the data passing between two programs, perhaps on different computers; most of these rules are handled by the operating system, and you need not know about them.  What you do need to know about protocols is this:

  * In order to have communication between two sockets, they must specify the same protocol.

  * Each protocol is meaningful with particular style/namespace combinations and cannot be used with inappropriate combinations.  For example, the TCP protocol fits only the byte stream style of communication and the Internet namespace.

  * For each combination of style and namespace, there is a default protocol which you can request by specifying 0 as the protocol number.  And that's what you should normally do---use the default.

Communication Styles
====================

The GNU library includes support for several different kinds of sockets, each with different characteristics.  This section describes the supported socket types.  The symbolic constants listed here are defined in sys/socket.h.

 -- Macro: int SOCK_STREAM
     The SOCK_STREAM style is like a pipe (see Pipes and FIFOs); it operates over a connection with a particular remote socket, and transmits data reliably as a stream of bytes.

     Use of this style is covered in detail in Connections.

 -- Macro: int SOCK_DGRAM
     The SOCK_DGRAM style is used for sending individually-addressed packets, unreliably. It is the diametrical opposite of SOCK_STREAM.

     Each time you write data to a socket of this kind, that data becomes one packet.  Since SOCK_DGRAM sockets do not have connections, you must specify the recipient address with each packet.

     The only guarantee that the system makes about your requests to transmit data is that it will try its best to deliver each packet you send.  It may succeed with the sixth packet after failing with the fourth and fifth packets; the seventh packet may arrive before the sixth, and may arrive a second time after the sixth.

     The typical use for SOCK_DGRAM is in situations where it is acceptable to simply resend a packet if no response is seen in a reasonable amount of time.

     See Datagrams, for detailed information about how to use datagram sockets.

 -- Macro: int SOCK_RAW
     This style provides access to low-level network protocols and interfaces.  Ordinary user programs usually have no need to use this style.

Socket Addresses
================

The name of a socket is normally called an address.  The functions and symbols for dealing with socket addresses were named inconsistently, sometimes using the term ``name'' and sometimes using ``address''.  You can regard these terms as synonymous where sockets are concerned.

A socket newly created with the socket function has no address.  Other processes can find it for communication only if you give it an address.  We call this binding the address to the socket, and the way to do it is with the bind function.

You need be concerned with the address of a socket if other processes are to find it and start communicating with it.  You can specify an address for other sockets, but this is usually pointless; the first time you send data from a socket, or use it to initiate a connection, the system assigns an address automatically if you have not specified one.

Occasionally a client needs to specify an address because the server discriminates based on addresses; for example, the rsh and rlogin protocols look at the client's socket address and don't bypass password checking unless it is less than IPPORT_RESERVED (see Ports).

The details of socket addresses vary depending on what namespace you are using.  See File Namespace, or Internet Namespace, for specific information.

Regardless of the namespace, you use the same functions bind and getsockname to set and examine a socket's address.  These functions use a phony data type, struct sockaddr *, to accept the address.  In practice, the address lives in a structure of some other data type appropriate to the address format you are using, but you cast its address to struct sockaddr * when you pass it to bind.

Address Formats
---------------

The functions bind and getsockname use the generic data type struct sockaddr * to represent a pointer to a socket address.  You can't use this data type effectively to interpret an address or construct one; for that, you must use the proper data type for the socket's namespace.

Thus, the usual practice is to construct an address in the proper namespace-specific type, then cast a pointer to struct sockaddr * when you call bind or getsockname.

The one piece of information that you can get from the struct sockaddr data type is the address format designator which tells you which data type to use to understand the address fully.

The symbols in this section are defined in the header file sys/socket.h.

 -- Date Type: struct sockaddr
     The struct sockaddr type itself has the following members:

     short int sa_family
          This is the code for the address format of this address.  It identifies the format of the data which follows.

     char sa_data[14]
          This is the actual socket address data, which is format-dependent.  Its length also depends on the format, and may well be more than 14.  The length 14 of sa_data is essentially arbitrary.

Each address format has a symbolic name which starts with AF_. Each of them corresponds to a PF_ symbol which designates the corresponding namespace.  Here is a list of address format names:

AF_FILE
     This designates the address format that goes with the file namespace. (PF_FILE is the name of that namespace.)  See File Namespace Details, for information about this address format.

AF_UNIX
     This is a synonym for AF_FILE, for compatibility. (PF_UNIX is likewise a synonym for PF_FILE.)

AF_INET
     This designates the address format that goes with the Internet namespace.  (PF_INET is the name of that namespace.) See Internet Address Format.

AF_UNSPEC
     This designates no particular address format.  It is used only in rare cases, such as to clear out the default destination address of a ``connected'' datagram socket.  See Sending Datagrams.

     The corresponding namespace designator symbol PF_UNSPEC exists for completeness, but there is no reason to use it in a program.

sys/socket.h defines symbols starting with AF_ for many different kinds of networks, all or most of which are not actually implemented.  We will document those that really work, as we receive information about how to use them.

Setting the Address of a Socket
-------------------------------

Use the bind function to assign an address to a socket.  The prototype for bind is in the header file sys/socket.h. For examples of use, see File Namespace, or see Inet Example.

 -- Function: int bind (int SOCKET, struct sockaddr *ADDR, size_t LENGTH)
     The bind function assigns an address to the socket SOCKET.  The ADDR and LENGTH arguments specify the address; the detailed format of the address depends on the namespace. The first part of the address is always the format designator, which specifies a namespace, and says that the address is in the format for that namespace.

     The return value is 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

     EBADF
          The SOCKET argument is not a valid file descriptor.

     ENOTSOCK
          The descriptor SOCKET is not a socket.

     EADDRNOTAVAIL
          The specified address is not available on this machine.

     EADDRINUSE
          Some other socket is already using the specified address.

     EINVAL
          The socket SOCKET already has an address.

     EACCES
          You do not have permission to access the requested address.  (In the Internet domain, only the super-user is allowed to specify a port number in the range 0 through IPPORT_RESERVED minus one; see Ports.)

     Additional conditions may be possible depending on the particular namespace of the socket.

Reading the Address of a Socket
-------------------------------

Use the function getsockname to examine the address of an Internet socket.  The prototype for this function is in the header file sys/socket.h.

 -- Function: int getsockname (int SOCKET, struct sockaddr *ADDR, size_t *LENGTH-PTR)
     The getsockname function returns information about the address of the socket SOCKET in the locations specified by the ADDR and LENGTH-PTR arguments.  Note that the LENGTH-PTR is a pointer; you should initialize it to be the allocation size of ADDR, and on return it contains the actual size of the address data.

     The format of the address data depends on the socket namespace.  The length of the information is usually fixed for a given namespace, so normally you can know exactly how much space is needed and can provide that much.  The usual practice is to allocate a place for the value using the proper data type for the socket's namespace, then cast its address to struct sockaddr * to pass it to getsockname.

     The return value is 0 on success and -1 on error.  The following errno error conditions are defined for this function:

     EBADF
          The SOCKET argument is not a valid file descriptor.

     ENOTSOCK
          The descriptor SOCKET is not a socket.

     ENOBUFS
          There are not enough internal buffers available for the operation.

You can't read the address of a socket in the file namespace.  This is consistent with the rest of the system; in general, there's no way to find a file's name from a descriptor for that file.

The File Namespace
==================

This section describes the details of the file namespace, whose symbolic name (required when you create a socket) is PF_FILE.

File Namespace Concepts
-----------------------

In the file namespace, socket addresses are file names.  You can specify any file name you want as the address of the socket, but you must have write permission on the directory containing it.  In order to connect to a socket, you must have read permission for it.  It's common to put these files in the /tmp directory.

One peculiarity of the file namespace is that the name is only used when opening the connection; once that is over with, the address is not meaningful and may not exist.

Another peculiarity is that you cannot connect to such a socket from another machine--not even if the other machine shares the file system which contains the name of the socket.  You can see the socket in a directory listing, but connecting to it never succeeds.  Some programs take advantage of this, such as by asking the client to send its own process ID, and using the process IDs to distinguish between clients. However, we recommend you not use this method in protocols you design, as we might someday permit connections from other machines that mount the same file systems.  Instead, send each new client an identifying number if you want it to have one.

After you close a socket in the file namespace, you should delete the file name from the file system.  Use unlink or remove to do this; see Deleting Files.

The file namespace supports just one protocol for any communication style; it is protocol number 0.

Details of File Namespace
-------------------------

To create a socket in the file namespace, use the constant PF_FILE as the NAMESPACE argument to socket or socketpair.  This constant is defined in sys/socket.h.

 -- Macro: int PF_FILE
     This designates the file namespace, in which socket addresses are file names, and its associated family of protocols.

 -- Macro: int PF_UNIX
     This is a synonym for PF_FILE, for compatibility's sake.

The structure for specifying socket names in the file namespace is defined in the header file sys/un.h:

 -- Data Type: struct sockaddr_un
     This structure is used to specify file namespace socket addresses.  It has the following members:

     short int sun_family
          This identifies the address family or format of the socket address. You should store the value AF_FILE to designate the file namespace.  See Socket Addresses.

     char sun_path[108]
          This is the file name to use.

          Incomplete:  Why is 108 a magic number?  RMS suggests making this a zero-length array and tweaking the example following to use alloca to allocate an appropriate amount of storage based on the length of the filename.

You should compute the LENGTH parameter for a socket address in the file namespace as the sum of the size of the sun_family component and the string length (not the allocation size!) of the file name string.

Example of File-Namespace Sockets
---------------------------------

Here is an example showing how to create and name a socket in the file namespace.

          [file mkfsock.c.texi not included]

The Internet Namespace
======================

This section describes the details the protocols and socket naming conventions used in the Internet namespace.

To create a socket in the Internet namespace, use the symbolic name PF_INET of this namespace as the NAMESPACE argument to socket or socketpair.  This macro is defined in sys/socket.h.

 -- Macro: int PF_INET
     This designates the Internet namespace and associated family of protocols.

A socket address for the Internet namespace includes the following components:

  * The address of the machine you want to connect to.  Internet addresses can be specified in several ways; these are discussed in Internet Address Format, Host Addresses, and Host Names.

  * A port number for that machine.  See Ports.

You must ensure that the address and port number are represented in a canonical format called network byte order.  See Byte Order, for information about this.

Internet Socket Address Format
------------------------------

In the Internet namespace, a socket address consists of a host address and a port on that host.  In addition, the protocol you choose serves effectively as a part of the address because local port numbers are meaningful only within a particular protocol.

The data type for representing socket addresses in the Internet namespace is defined in the header file netinet/in.h.

 -- Data Type: struct sockaddr_in
     This is the data type used to represent socket addresses in the Internet namespace.  It has the following members:

     short int sin_family
          This identifies the address family or format of the socket address. You should store the value of AF_INET in this member. See Socket Addresses.

     struct in_addr sin_addr
          This is the Internet address of the host machine.  See Host Addresses, and Host Names, for how to get a value to store here.

     unsigned short int sin_port
          This is the port number.  See Ports.

When you call bind or getsockname, you should specify sizeof (struct sockaddr_in) as the LENGTH parameter if you are using an Internet namespace socket address.

Host Addresses
--------------

Each computer on the Internet has one or more Internet addresses, numbers which identify that computer among all those on the Internet. Users typically write numeric host addresses as sequences of four numbers, separated by periods, as in 128.52.46.32.

Each computer also has one or more host names, which are strings of words separated by periods, as in churchy.gnu.ai.mit.edu.

Programs that let the user specify a host typically accept both numeric addresses and host names.  But the program needs a numeric address to open a connection; to use a host name, you must convert it to the numeric address it stands for.

Internet Host Addresses
.......................

Each computer on the Internet has one or more Internet addresses, numbers which identify that computer among all those on the Internet.

An Internet host address is a number containing four bytes of data. These are divided into two parts, a network number and a local network address number within that network.  The network number consists of the first one, two or three bytes; the rest of the bytes are the local address.

Network numbers are registered with the Network Information Center (NIC), and are divided into three classes---A, B, and C.  The local network address numbers of individual machines are registered with the administrator of the particular network.

Class A networks have single-byte numbers in the range 0 to 127.  There are only a small number of Class A networks, but they can each support a very large number of hosts.  Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191.  Class C networks are the smallest; they have three-byte network numbers, with the first byte in the range 192-255.  Thus, the first 1, 2, or 3 bytes of an Internet address specifies a network.  The remaining bytes of the Internet address specify the address within that network.

The Class A network 0 is reserved for broadcast to all networks.  In addition, the host number 0 within each network is reserved for broadcast to all hosts in that network.

The Class A network 127 is reserved for loopback; you can always use the Internet address 127.0.0.1 to refer to the host machine.

Since a single machine can be a member of multiple networks, it can have multiple Internet host addresses.  However, there is never supposed to be more than one machine with the same host address.

There are four forms of the standard numbers-and-dots notation for Internet addresses:

A.B.C.D
     This specifies all four bytes of the address individually.

A.B.C
     The last part of the address, C, is interpreted as a 2-byte quantity. This is useful for specifying host addresses in a Class B network with network address number A.B.

A.B
     The last part of the address, C, is interpreted as a 3-byte quantity. This is useful for specifying host addresses in a Class A network with network address number A.

A
     If only one part is given, this corresponds directly to the host address number.

Within each part of the address, the usual C conventions for specifying the radix apply.  In other words, a leading 0x or 0X implies hexadecimal radix; a leading 0 implies octal; and otherwise decimal radix is assumed.

Host Address Data Type
......................

Internet host addresses are represented in some contexts as integers (type unsigned long int).  In other contexts, the integer is packaged inside a structure of type struct in_addr.  It would be better if the usage were made consistent, but it is not hard to extract the integer from the structure or put the integer into a structure.

The following basic definitions for Internet addresses appear in the header file netinet/in.h:

 -- Data Type: struct in_addr
     This data type is used in certain contexts to contain an Internet host address.  It has just one field, named s_addr, which records the host address number as an unsigned long int.

 -- Macro: unsigned long int INADDR_LOOPBACK
     You can use this constant to stand for ``the address of this machine,'' instead of finding its actual address.  It is the Internet address 127.0.0.1, which is usually called localhost.  This special constant saves you the trouble of looking up the address of your own machine.  Also, the system usually implements INADDR_LOOPBACK specially, avoiding any network traffic for the case of one machine talking to itself.

 -- Macro: unsigned long int INADDR_ANY
     You can use this constant to stand for ``any incoming address,'' when binding to an address.  See Setting Address.  This is the usual address to give in the sin_addr member of struct sockaddr_in when you want to accept Internet connections.

 -- Macro: unsigned long int INADDR_BROADCAST
     This constant is the address you use to send a broadcast message.

 -- Macro: unsigned long int INADDR_NONE
     This constant is returned by some functions to indicate an error.

Host Address Functions
......................

These additional functions for manipulating Internet addresses are declared in arpa/inet.h.  They represent Internet addresses in network byte order; they represent network numbers and local-address-within-network numbers in host byte order. See Byte Order, for an explanation of network and host byte order.

 -- Function: int inet_aton (const char *NAME, struct in_addr *ADDR)
     This function converts the Internet host address NAME from the standard numbers-and-dots notation into binary data and stores it in the struct in_addr that ADDR points to. inet_aton returns nonzero if the address is valid, zero if not.

 -- Function: unsigned long int inet_addr (const char *NAME)
     This function converts the Internet host address NAME from the standard numbers-and-dots notation into binary data.  If the input is not valid, inet_addr returns INADDR_NONE.  This is an obsolete interface to inet_aton, described immediately above; it is obsolete because INADDR_NONE is a valid address (255.255.255.255), and inet_aton provides a cleaner way to indicate error return.

 -- Function: unsigned long int inet_network (const char *NAME)
     This function extracts the network number from the address NAME, given in the standard numbers-and-dots notation. If the input is not valid, inet_network returns -1.

 -- Function: char * inet_ntoa (struct in_addr ADDR)
     This function converts the Internet host address ADDR to a string in the standard numbers-and-dots notation.  The return value is a pointer into a statically-allocated buffer.  Subsequent calls will overwrite the same buffer, so you should copy the string if you need to save it.

 -- Function: struct in_addr inet_makeaddr (int NET, int LOCAL)
     This function makes an Internet host address by combining the network number NET with the local-address-within-network number LOCAL.

 -- Function: int inet_lnaof (struct in_addr ADDR)
     This function returns the local-address-within-network part of the Internet host address ADDR.

 -- Function: int inet_netof (struct in_addr ADDR)
     This function returns the network number part of the Internet host address ADDR.

Host Names
..........

Besides the standard numbers-and-dots notation for Internet addresses, you can also refer to a host by a symbolic name.  The advantage of a symbolic name is that it is usually easier to remember.  For example, the machine with Internet address 128.52.46.32 is also known as churchy.gnu.ai.mit.edu; and other machines in the gnu.ai.mit.edu domain can refer to it simply as churchy.

Internally, the system uses a database to keep track of the mapping between host names and host numbers.  This database is usually either the file /etc/hosts or an equivalent provided by a name server. The functions and other symbols for accessing this database are declared in netdb.h.  They are BSD features, defined unconditionally if you include netdb.h.

 -- Data Type: struct hostent
     This data type is used to represent an entry in the hosts database.  It has the following members:

     char *h_name
          This is the ``official'' name of the host.

     char **h_aliases
          These are alternative names for the host, represented as a null-terminated vector of strings.

     int h_addrtype
          This is the host address type; in practice, its value is always AF_INET.  In principle other kinds of addresses could be represented in the data base as well as Internet addresses; if this were done, you might find a value in this field other than AF_INET. See Socket Addresses.

     int h_length
          This is the length, in bytes, of each address.

     char **h_addr_list
          This is the vector of addresses for the host.  (Recall that the host might be connected to multiple networks and have different addresses on each one.)  The vector is terminated by a null pointer.

     char *h_addr
          This is a synonym for h_addr_list[0]; in other words, it is the first host address.

As far as the host database is concerned, each address is just a block of memory h_length bytes long.  But in other contexts there is an implicit assumption that you can convert this to a struct in_addr or an unsigned long int.  Host addresses in a struct hostent structure are always given in network byte order; see Byte Order.

You can use gethostbyname or gethostbyaddr to search the hosts database for information about a particular host.  The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

 -- Function: struct hostent * gethostbyname (const char *NAME)
     The gethostbyname function returns information about the host named NAME.  If the lookup fails, it returns a null pointer.

 -- Function: struct hostent * gethostbyaddr (const char *ADDR, int LENGTH, int FORMAT)
     The gethostbyaddr function returns information about the host with Internet address ADDR.  The LENGTH argument is the size (in bytes) of the address at ADDR.  FORMAT specifies the address format; for an Internet address, specify a value of AF_INET.

     If the lookup fails, gethostbyaddr returns a null pointer.

If the name lookup by gethostbyname or gethostbyaddr fails, you can find out the reason by looking at the value of the variable h_errno.  (It would be cleaner design for these functions to set errno, but use of h_errno is compatible with other systems.)  Before using h_errno, you must declare it like this:

     extern int h_errno;

Here are the error codes that you may find in h_errno:

HOST_NOT_FOUND
     No such host is known in the data base.

TRY_AGAIN
     This condition happens when the name server could not be contacted.  If you try again later, you may succeed then.

NO_RECOVERY
     A non-recoverable error occurred.

NO_ADDRESS
     The host database contains an entry for the name, but it doesn't have an associated Internet address.

You can also scan the entire hosts database one entry at a time using sethostent, gethostent, and endhostent.  Be careful in using these functions, because they are not reentrant.

 -- Function: void sethostent (int STAYOPEN)
     This function opens the hosts database to begin scanning it.  You can then call gethostent to read the entries.

     If the STAYOPEN argument is nonzero, this sets a flag so that subsequent calls to gethostbyname or gethostbyaddr will not close the database (as they usually would).  This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

 -- Function: struct hostent * gethostent ()
     This function returns the next entry in the hosts database.  It returns a null pointer if there are no more entries.

 -- Function: void endhostent ()
     This function closes the hosts database.

Internet Ports
--------------

A socket address in the Internet namespace consists of a machine's Internet address plus a port number which distinguishes the sockets on a given machine (for a given protocol).  Port numbers range from 0 to 65,535.

Port numbers less than IPPORT_RESERVED are reserved for standard servers, such as finger and telnet.  There is a database that keeps track of these, and you can use the getservbyname function to map a service name onto a port number; see Services Database.

If you write a server that is not one of the standard ones defined in the database, you must choose a port number for it.  Use a number greater than IPPORT_USERRESERVED; such numbers are reserved for servers and won't ever be generated automatically by the system. Avoiding conflicts with servers being run by other users is up to you.

When you use a socket without specifying its address, the system generates a port number for it.  This number is between IPPORT_RESERVED and IPPORT_USERRESERVED.

On the Internet, it is actually legitimate to have two different sockets with the same port number, as long as they never both try to communicate with the same socket address (host address plus port number).  You shouldn't duplicate a port number except in special circumstances where a higher-level protocol requires it.  Normally, the system won't let you do it; bind normally insists on distinct port numbers.  To reuse a port number, you must set the socket option SO_REUSEADDR.  See Socket-Level Options.

These macros are defined in the header file netinet/in.h.

 -- Macro: int IPPORT_RESERVED
     Port numbers less than IPPORT_RESERVED are reserved for superuser use.

 -- Macro: int IPPORT_USERRESERVED
     Port numbers greater than or equal to IPPORT_USERRESERVED are reserved for explicit use; they will never be allocated automatically.

The Services Database
---------------------

The database that keeps track of ``well-known'' services is usually either the file /etc/services or an equivalent from a name server. You can use these utilities, declared in netdb.h, to access the services database.

 -- Data Type: struct servent
     This data type holds information about entries from the services database. It has the following members:

     char *s_name
          This is the ``official'' name of the service.

     char **s_aliases
          These are alternate names for the service, represented as an array of strings.  A null pointer terminates the array.

     int s_port
          This is the port number for the service.  Port numbers are given in network byte order; see Byte Order.

     char *s_proto
          This is the name of the protocol to use with this service. See Protocols Database.

To get information about a particular service, use the getservbyname or getservbyport functions.  The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

 -- Function: struct servent * getservbyname (const char *NAME, const char *PROTO)
     The getservbyname function returns information about the service named NAME using protocol PROTO.  If it can't find such a service, it returns a null pointer.

     This function is useful for servers as well as for clients; servers use it to determine which port they should listen on (see Listening).

 -- Function: struct servent * getservbyport (int PORT, const char *PROTO)
     The getservbyport function returns information about the service at port PORT using protocol PROTO.  If it can't find such a service, it returns a null pointer.

You can also scan the services database using setservent, getservent, and endservent.  Be careful in using these functions, because they are not reentrant.

 -- Function: void setservent (int STAYOPEN)
     This function opens the services database to begin scanning it.

     If the STAYOPEN argument is nonzero, this sets a flag so that subsequent calls to getservbyname or getservbyport will not close the database (as they usually would).  This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

 -- Function: struct servent * getservent (void)
     This function returns the next entry in the services database.  If there are no more entries, it returns a null pointer.

 -- Function: void endservent (void)
     This function closes the services database.

Byte Order Conversion
---------------------

Different kinds of computers use different conventions for the ordering of bytes within a word.  Some computers put the most significant byte within a word first (this is called ``big-endian'' order), and others put it last (``little-endian'' order).

So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network.  This is known as the network byte order.

When establishing an Internet socket connection, you must make sure that the data in the sin_port and sin_addr members of the sockaddr_in structure are represented in the network byte order. If you are encoding integer data in the messages sent through the socket, you should convert this to network byte order too.  If you don't do this, your program may fail when running on or talking to other kinds of machines.

If you use getservbyname and gethostbyname or inet_addr to get the port number and host address, the values are already in the network byte order, and you can copy them directly into the sockaddr_in structure.

Otherwise, you have to convert the values explicitly.  Use htons and ntohs to convert values for the sin_port member.  Use htonl and ntohl to convert values for the sin_addr member.  (Remember, struct in_addr is equivalent to unsigned long int.)  These functions are declared in netinet/in.h.

 -- Function: unsigned short int htons (unsigned short int HOSTSHORT)
     This function converts the short integer HOSTSHORT from host byte order to network byte order.

 -- Function: unsigned short int ntohs (unsigned short int NETSHORT)
     This function converts the short integer NETSHORT from network byte order to host byte order.

 -- Function: unsigned long int htonl (unsigned long int HOSTLONG)
     This function converts the long integer HOSTLONG from host byte order to network byte order.

 -- Function: unsigned long int ntohl (unsigned long int NETLONG)
     This function converts the long integer NETLONG from network byte order to host byte order.

Protocols Database
------------------

The communications protocol used with a socket controls low-level details of how data is exchanged.  For example, the protocol implements things like checksums to detect errors in transmissions, and routing instructions for messages.  Normal user programs have little reason to mess with these details directly.

The default communications protocol for the Internet namespace depends on the communication style.  For stream communication, the default is TCP (``transmission control protocol'').  For datagram communication, the default is UDP (``user datagram protocol'').  For reliable datagram communication, the default is RDP (``reliable datagram protocol''). You should nearly always use the default.

Internet protocols are generally specified by a name instead of a number.  The network protocols that a host knows about are stored in a database.  This is usually either derived from the file /etc/protocols, or it may be an equivalent provided by a name server.  You look up the protocol number associated with a named protocol in the database using the getprotobyname function.

Here are detailed descriptions of the utilities for accessing the protocols database.  These are declared in netdb.h.

 -- Data Type: struct protoent
     This data type is used to represent entries in the network protocols database.  It has the following members:

     char *p_name
          This is the official name of the protocol.

     char **p_aliases
          These are alternate names for the protocol, specified as an array of strings.  The last element of the array is a null pointer.

     int p_proto
          This is the protocol number (in host byte order); use this member as the PROTOCOL argument to socket.

You can use getprotobyname and getprotobynumber to search the protocols database for a specific protocol.  The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

 -- Function: struct protoent * getprotobyname (const char *NAME)
     The getprotobyname function returns information about the network protocol named NAME.  If there is no such protocol, it returns a null pointer.

 -- Function: struct protoent * getprotobynumber (int PROTOCOL)
     The getprotobynumber function returns information about the network protocol with number PROTOCOL.  If there is no such protocol, it returns a null pointer.

You can also scan the whole protocols database one protocol at a time by using setprotoent, getprotoent, and endprotoent. Be careful in using these functions, because they are not reentrant.

 -- Function: void setprotoent (int STAYOPEN)
     This function opens the protocols database to begin scanning it.

     If the STAYOPEN argument is nonzero, this sets a flag so that subsequent calls to getprotobyname or getprotobynumber will not close the database (as they usually would).  This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

 -- Function: struct protoent * getprotoent (void)
     This function returns the next entry in the protocols database.  It returns a null pointer if there are no more entries.

 -- Function: void endprotoent (void)
     This function closes the protocols database.

Internet Socket Example
-----------------------

Here is an example showing how to create and name a socket in the Internet namespace.  The newly created socket exists on the machine that the program is running on.  Rather than finding and using the machine's Internet address, this example specifies INADDR_ANY as the host address; the system replaces that with the machine's actual address.

          [file mkisock.c.texi not included]

Here is another example, showing how you can fill in a sockaddr_in structure, given a host name string and a port number:

          [file isockad.c.texi not included]

Other Namespaces
================

Certain other namespaces and associated protocol families are supported but not documented yet because they are not often used.  PF_NS refers to the Xerox Network Software protocols.  PF_ISO stands for Open Systems Interconnect.  PF_CCITT refers to protocols from CCITT.  socket.h defines these symbols and others naming protocols not actually implemented.

PF_IMPLINK is used for communicating between hosts and Internet Message Processors.  For information on this, and on PF_ROUTE, an occasionally-used local area routing protocol, see the GNU Hurd Manual (to appear in the future).

Opening and Closing Sockets
===========================

This section describes the actual library functions for opening and closing sockets.  The same functions work for all namespaces and connection styles.

Creating a Socket
-----------------

The primitive for creating a socket is the socket function, declared in sys/socket.h.

 -- Function: int socket (int NAMESPACE, int STYLE, int PROTOCOL)
     This function creates a socket and specifies communication style STYLE, which should be one of the socket styles listed in Communication Styles.  The NAMESPACE argument specifies the namespace; it must be PF_FILE (see File Namespace) or PF_INET (see Internet Namespace).  PROTOCOL designates the specific protocol (see Socket Concepts); zero is usually right for PROTOCOL.

     The return value from socket is the file descriptor for the new socket, or -1 in case of error.  The following errno error conditions are defined for this function:

     EPROTONOSUPPORT
          The PROTOCOL or STYLE is not supported by the NAMESPACE specified.

     EMFILE
          The process already has too many file descriptors open.

     ENFILE
          The system already has too many file descriptors open.

     EACCESS
          The process does not have privilege to create a socket of the specified STYLE or PROTOCOL.

     ENOBUFS
          The system ran out of internal buffer space.

     The file descriptor returned by the socket function supports both read and write operations.  But, like pipes, sockets do not support file positioning operations.

For examples of how to call the socket function, see File Namespace, or Inet Example.

Closing a Socket
----------------

When you are finished using a socket, you can simply close its file descriptor with close; see Opening and Closing Files. If there is still data waiting to be transmitted over the connection, normally close tries to complete this transmission.  You can control this behavior using the SO_LINGER socket option to specify a timeout period; see Socket Options.

You can also shut down only reception or only transmission on a connection by calling shutdown, which is declared in sys/socket.h.

 -- Function: int shutdown (int SOCKET, int HOW)
     The shutdown function shuts down the connection of socket SOCKET.  The argument HOW specifies what action to perform:

     0
          Stop receiving data for this socket.  If further data arrives, reject it.

     1
          Stop trying to transmit data from this socket.  Discard any data waiting to be sent.  Stop looking for acknowledgement of data already sent; don't retransmit it if it is lost.

     2
          Stop both reception and transmission.

     The return value is 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

     EBADF
          SOCKET is not a valid file descriptor.

     ENOTSOCK
          SOCKET is not a socket.

     ENOTCONN
          SOCKET is not connected.

Socket Pairs
------------

A socket pair consists of a pair of connected (but unnamed) sockets.  It is very similar to a pipe and is used in much the same way.  Socket pairs are created with the socketpair function, declared in sys/socket.h.  A socket pair is much like a pipe; the main difference is that the socket pair is bidirectional, whereas the pipe has one input-only end and one output-only end (see Pipes and FIFOs).

 -- Function: int socketpair (int NAMESPACE, int STYLE, int PROTOCOL, int FILEDES[2])
     This function creates a socket pair, returning the file descriptors in FILEDES[0] and FILEDES[1].  The socket pair is a full-duplex communications channel, so that both reading and writing may be performed at either end.

     The NAMESPACE, STYLE, and PROTOCOL arguments are interpreted as for the socket function.  STYLE should be one of the communication styles listed in Communication Styles. The NAMESPACE argument specifies the namespace, which must be AF_FILE (see File Namespace); PROTOCOL specifies the communications protocol, but zero is the only meaningful value.

     If STYLE specifies a connectionless communication style, then the two sockets you get are not connected, strictly speaking, but each of them knows the other as the default destination address, so they can send packets to each other.

     The socketpair function returns 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

     EMFILE
          The process has too many file descriptors open.

     EAFNOSUPPORT
          The specified namespace is not supported.

     EPROTONOSUPPORT
          The specified protocol is not supported.

     EOPNOTSUPP
          The specified protocol does not support the creation of socket pairs.

Using Sockets with Connections
==============================

The most common communication styles involve making a connection to a particular other socket, and then exchanging data with that socket over and over.  Making a connection is asymmetric; one side (the client) acts to request a connection, while the other side (the server) makes a socket and waits for the connection request.

Making a Connection
-------------------

In making a connection, the client makes a connection while the server waits for and accepts the connection.  Here we discuss what the client program must do, using the connect function, which is declared in sys/socket.h.

 -- Function: int connect (int SOCKET, struct sockaddr *ADDR, size_t LENGTH)
     The connect function initiates a connection from the socket with file descriptor SOCKET to the socket whose address is specified by the ADDR and LENGTH arguments.  (This socket is typically on another machine, and it must be already set up as a server.)  See Socket Addresses, for information about how these arguments are interpreted.

     Normally, connect waits until the server responds to the request before it returns.  You can set nonblocking mode on the socket SOCKET to make connect return immediately without waiting for the response.  See File Status Flags, for information about nonblocking mode.

     The normal return value from connect is 0.  If an error occurs, connect returns -1.  The following errno error conditions are defined for this function:

     EBADF
          The socket SOCKET is not a valid file descriptor.

     ENOTSOCK
          The socket SOCKET is not a socket.

     EADDRNOTAVAIL
          The specified address is not available on the remote machine.

     EAFNOSUPPORT
          The namespace of the ADDR is not supported by this socket.

     EISCONN
          The socket SOCKET is already connected.

     ETIMEDOUT
          The attempt to establish the connection timed out.

     ECONNREFUSED
          The server has actively refused to establish the connection.

     ENETUNREACH
          The network of the given ADDR isn't reachable from this host.

     EADDRINUSE
          The socket address of the given ADDR is already in use.

     EINPROGRESS
          The socket SOCKET is non-blocking and the connection could not be established immediately.

     EALREADY
          The socket SOCKET is non-blocking and already has a pending connection in progress.

Listening for Connections
-------------------------

Now let us consider what the server process must do to accept connections on a socket.  First it must use the listen function to enable connection requests on the socket, and then accept each incoming connection with a call to accept (see Accepting Connections).  Once connection requests are enabled on a server socket, the select function reports when the socket has a connection ready to be accepted (see Waiting for I/O).

The listen function is not allowed for sockets using connectionless communication styles.

You can write a network server that does not even start running until a connection to it is requested.  See Inetd Servers.

In the Internet namespace, there are no special protection mechanisms for controlling access to connect to a port; any process on any machine can make a connection to your server.  If you want to restrict access to your server, make it examine the addresses associated with connection requests or implement some other handshaking or identification protocol.

In the File namespace, the ordinary file protection bits control who has access to connect to the socket.

 -- Function: int listen (int SOCKET, unsigned int N)
     The listen function enables the socket SOCKET to accept connections, thus making it a server socket.

     The argument N specifies the length of the queue for pending connections.  When the queue fills, new clients attempting to connect fail with ECONNREFUSED until the server calls accept to accept a connection from the queue.

     The listen function returns 0 on success and -1 on failure.  The following errno error conditions are defined for this function:

     EBADF
          The argument SOCKET is not a valid file descriptor.

     ENOTSOCK
          The argument SOCKET is not a socket.

     EOPNOTSUPP
          The socket SOCKET does not support this operation.

Accepting Connections
---------------------

When a server receives a connection request, it can complete the connection by accepting the request.  Use the function accept to do this.

A socket that has been established as a server can accept connection requests from multiple clients.  The server's original socket does not become part of the connection; instead, accept makes a new socket which participates in the connection. accept returns the descriptor for this socket.  The server's original socket remains available for listening for further connection requests.

The number of pending connection requests on a server socket is finite. If connection requests arrive from clients faster than the server can act upon them, the queue can fill up and additional requests are refused with a ECONNREFUSED error.  You can specify the maximum length of this queue as an argument to the listen function, although the system may also impose its own internal limit on the length of this queue.

 -- Function: int accept (int SOCKET, struct sockaddr *ADDR, size_t *LENGTH-PTR)
     This function is used to accept a connection request on the server socket SOCKET.

     The accept function waits if there are no connections pending, unless the socket SOCKET has nonblocking mode set.  (You can use select to wait for a pending connection, with a nonblocking socket.)  See File Status Flags, for information about nonblocking mode.

     The ADDR and LENGTH-PTR arguments are used to return information about the name of the client socket that initiated the connection.  See Socket Addresses, for information about the format of the information.

     Accepting a connection does not make SOCKET part of the connection.  Instead, it creates a new socket which becomes connected.  The normal return value of accept is the file descriptor for the new socket.

     After accept, the original socket SOCKET remains open and unconnected, and continues listening until you close it.  You can accept further connections with SOCKET by calling accept again.

     If an error occurs, accept returns -1.  The following errno error conditions are defined for this function:

     EBADF
          The SOCKET argument is not a valid file descriptor.

     ENOTSOCK
          The descriptor SOCKET argument is not a socket.

     EOPNOTSUPP
          The descriptor SOCKET does not support this operation.

     EWOULDBLOCK
          SOCKET has nonblocking mode set, and there are no pending connections immediately available.

The accept function is not allowed for sockets using connectionless communication styles.

Who is Connected to Me?
-----------------------

 -- Function: int getpeername (int SOCKET, struct sockaddr *ADDR, size_t *LENGTH-PTR)
     The getpeername function returns the address of the socket that SOCKET is connected to; it stores the address in the memory space specified by ADDR and LENGTH-PTR.  It stores the length of the address in *LENGTH-PTR.

     See Socket Addresses, for information about the format of the address.  In some operating systems, getpeername works only for sockets in the Internet domain.

     The return value is 0 on success and -1 on error.  The following errno error conditions are defined for this function:

     EBADF
          The argument SOCKET is not a valid file descriptor.

     ENOTSOCK
          The descriptor SOCKET is not a socket.

     ENOTCONN
          The socket SOCKET is not connected.

     ENOBUFS
          There are not enough internal buffers available.

Transferring Data
-----------------

Once a socket has been connected to a peer, you can use the ordinary read and write operations (see I/O Primitives) to transfer data.  A socket is a two-way communications channel, so read and write operations can be performed at either end.

There are also some I/O modes that are specific to socket operations. In order to specify these modes, you must use the recv and send functions instead of the more generic read and write functions.  The recv and send functions take an additional argument which you can use to specify various flags to control the special I/O modes.  For example, you can specify the MSG_OOB flag to read or write out-of-band data, the MSG_PEEK flag to peek at input, or the MSG_DONTROUTE flag to control inclusion of routing information on output.

Sending Data
............

The send function is declared in the header file sys/socket.h.  If your FLAGS argument is zero, you can just as well use write instead of send; see I/O Primitives.  If the socket was connected but the connection has broken, you get a SIGPIPE signal for any use of send or write (see Miscellaneous Signals).

 -- Function: int send (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS)
     The send function is like write, but with the additional flags FLAGS.  The possible values of FLAGS are described in Socket Data Options.

     This function returns the number of bytes transmitted, or -1 on failure.  If the socket is nonblocking, then send (like write) can return after sending just part of the data. See File Status Flags, for information about nonblocking mode.

     Note, however, that a successful return value merely indicates that the message has been sent without error, not necessarily that it has been received without error.

     The following errno error conditions are defined for this function:

     EBADF
          The SOCKET argument is not a valid file descriptor.

     EINTR
          The operation was interrupted by a signal before any data was sent. See Interrupted Primitives.

     ENOTSOCK
          The descriptor SOCKET is not a socket.

     EMSGSIZE
          The socket type requires that the message be sent atomically, but the message is too large for this to be possible.

     EWOULDBLOCK
          Nonblocking mode has been set on the socket, and the write operation would block.  (Normally send blocks until the operation can be completed.)

     ENOBUFS
          There is not enough internal buffer space available.

     ENOTCONN
          You never connected this socket.

     EPIPE
          This socket was connected but the connection is now broken.  In this case, send generates a SIGPIPE signal first; if that signal is ignored or blocked, or if its handler returns, then send fails with EPIPE.

Receiving Data
..............

The recv function is declared in the header file sys/socket.h.  If your FLAGS argument is zero, you can just as well use read instead of recv; see I/O Primitives.

 -- Function: int recv (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS)
     The recv function is like read, but with the additional flags FLAGS.  The possible values of FLAGS are described In Socket Data Options.

     If nonblocking mode is set for SOCKET, and no data is available to be read, recv fails immediately rather than waiting.  See File Status Flags, for information about nonblocking mode.

     This function returns the number of bytes received, or -1 on failure. The following errno error conditions are defined for this function:

     EBADF
          The SOCKET argument is not a valid file descriptor.

     ENOTSOCK
          The descriptor SOCKET is not a socket.

     EWOULDBLOCK
          Nonblocking mode has been set on the socket, and the read operation would block.  (Normally, recv blocks until there is input available to be read.)

     EINTR
          The operation was interrupted by a signal before any data was read. See Interrupted Primitives.

     ENOTCONN
          You never connected this socket.

Socket Data Options
...................

The FLAGS argument to send and recv is a bit mask.  You can bitwise-OR the values of the following macros together to obtain a value for this argument.  All are defined in the header file sys/socket.h.

 -- Macro: int MSG_OOB
     Send or receive out-of-band data.  See Out-of-Band Data.

 -- Macro: int MSG_PEEK
     Look at the data but don't remove it from the input queue.  This is only meaningful with input functions such as recv, not with send.

 -- Macro: int MSG_DONTROUTE
     Don't include routing information in the message.  This is only meaningful with output operations, and is usually only of interest for diagnostic or routing programs.  We don't try to explain it here.

Byte Stream Socket Example
--------------------------

Here is an example client program that makes a connection for a byte stream socket in the Internet namespace.  It doesn't do anything particularly interesting once it has connected to the server; it just sends a text string to the server and exits.

          [file inetcli.c.texi not included]

Byte Stream Connection Server Example
-------------------------------------

The server end is much more complicated.  Since we want to allow multiple clients to be connected to the server at the same time, it would be incorrect to wait for input from a single client by simply calling read or recv.  Instead, the right thing to do is to use select (see Waiting for I/O) to wait for input on all of the open sockets.  This also allows the server to deal with additional connection requests.

This particular server doesn't do anything interesting once it has gotten a message from a client.  It does close the socket for that client when it detects an end-of-file condition (resulting from the client shutting down its end of the connection).

This program uses make_socket and init_sockaddr to set up the socket address; see Inet Example.

          [file inetsrv.c.texi not included]

Out-of-Band Data
----------------

Streams with connections permit out-of-band data that is delivered with higher priority than ordinary data.  Typically the reason for sending out-of-band data is to send notice of an exceptional condition.  The way to send out-of-band data is using send, specifying the flag MSG_OOB (see Sending Data).

Out-of-band data is received with higher priority because the receiving process need not read it in sequence; to read the next available out-of-band data, use recv with the MSG_OOB flag (see Receiving Data).  Ordinary read operations do not read out-of-band data; they read only the ordinary data.

When a socket finds that out-of-band data is on its way, it sends a SIGURG signal to the owner process or process group of the socket.  You can specify the owner using the F_SETOWN command to the fcntl function; see Interrupt Input.  You must also establish a handler for this signal, as described in Signal Handling, in order to take appropriate action such as reading the out-of-band data.

Alternatively, you can test for pending out-of-band data, or wait until there is out-of-band data, using the select function; it can wait for an exceptional condition on the socket.  See Waiting for I/O, for more information about select.

Notification of out-of-band data (whether with SIGURG or with select) indicates that out-of-band data is on the way; the data may not actually arrive until later.  If you try to read the out-of-band data before it arrives, recv fails with an EWOULDBLOCK error.

Sending out-of-band data automatically places a ``mark'' in the stream of ordinary data, showing where in the sequence the out-of-band data ``would have been''.  This is useful when the meaning of out-of-band data is ``cancel everything sent so far''.  Here is how you can test, in the receiving process, whether any ordinary data was sent before the mark:

     success = ioctl (socket, SIOCATMARK, &result);

Here's a function to discard any ordinary data preceding the out-of-band mark:

     int
     discard_until_mark (int socket)
     {
       while (1)
         {
           /* This is not an arbitrary limit; any size will do.  */
           char buffer[1024];
           int result, success;

           /* If we have reached the mark, return.  */
           success = ioctl (socket, SIOCATMARK, &result);
           if (success < 0)
             perror ("ioctl");
           if (result)
             return;

           /* Otherwise, read a bunch of ordinary data and discard it.
              This is guaranteed not to read past the mark
              if it starts before the mark.  */
           success = read (socket, buffer, sizeof buffer);
           if (success < 0)
             perror ("read");
         }
     }

If you don't want to discard the ordinary data preceding the mark, you may need to read some of it anyway, to make room in internal system buffers for the out-of-band data.  If you try to read out-of-band data and get an EWOULDBLOCK error, try reading some ordinary data (saving it so that you can use it when you want it) and see if that makes room.  Here is an example:

     struct buffer
     {
       char *buffer;
       int size;
       struct buffer *next;
     };

     /* Read the out-of-band data from SOCKET and return it
        as a `struct buffer', which records the address of the data
        and its size.

        It may be necessary to read some ordinary data
        in order to make room for the out-of-band data.
        If so, the ordinary data is saved as a chain of buffers
        found in the `next' field of the value.  */

     struct buffer *
     read_oob (int socket)
     {
       struct buffer *tail = 0;
       struct buffer *list = 0;

       while (1)
         {
           /* This is an arbitrary limit.
              Does anyone know how to do this without a limit?  */
           char *buffer = (char *) xmalloc (1024);
           struct buffer *link;
           int success;
           int result;

           /* Try again to read the out-of-band data.  */
           success = recv (socket, buffer, sizeof buffer, MSG_OOB);
           if (success >= 0)
             {
               /* We got it, so return it.  */
               struct buffer *link
                 = (struct buffer *) xmalloc (sizeof (struct buffer));
               link->buffer = buffer;
               link->size = success;
               link->next = list;
               return link;
             }

           /* If we fail, see if we are at the mark.  */
           success = ioctl (socket, SIOCATMARK, &result);
           if (success < 0)
             perror ("ioctl");
           if (result)
             {
               /* At the mark; skipping past more ordinary data cannot help.
                  So just wait a while.  */
               sleep (1);
               continue;
             }

           /* Otherwise, read a bunch of ordinary data and save it.
              This is guaranteed not to read past the mark
              if it starts before the mark.  */
           success = read (socket, buffer, sizeof buffer);
           if (success < 0)
             perror ("read");

           /* Save this data in the buffer list.  */
           {
             struct buffer *link
               = (struct buffer *) xmalloc (sizeof (struct buffer));
             link->buffer = buffer;
             link->size = success;

             /* Add the new link to the end of the list.  */
             if (tail)
               tail->next = link;
             else
               list = link;
             tail = link;
           }
         }
     }

Datagram Socket Operations
==========================

This section describes how to use communication styles that don't use connections (styles SOCK_DGRAM and SOCK_RDM).  Using these styles, you group data into packets and each packet is an independent communication.  You specify the destination for each packet individually.

Datagram packets are like letters: you send each one independently, with its own destination address, and they may arrive in the wrong order or not at all.

The listen and accept functions are not allowed for sockets using connectionless communication styles.

Sending Datagrams
-----------------

The normal way of sending data on a datagram socket is by using the sendto function, declared in sys/socket.h.

You can call connect on a datagram socket, but this only specifies a default destination for further data transmission on the socket.  When a socket has a default destination, then you can use send (see Sending Data) or even write (see I/O Primitives) to send a packet there.  You can cancel the default destination by calling connect using an address format of AF_UNSPEC in the ADDR argument.  See Connecting, for more information about the connect function.

 -- Function: int sendto (int SOCKET, void *BUFFER. size_t SIZE, int FLAGS, struct sockaddr *ADDR, size_t LENGTH)
     The sendto function transmits the data in the BUFFER through the socket SOCKET to the destination address specified by the ADDR and LENGTH arguments.  The SIZE argument specifies the number of bytes to be transmitted.

     The FLAGS are interpreted the same way as for send; see Socket Data Options.

     The return value and error conditions are also the same as for send, but you cannot rely on the system to detect errors and report them; the most common error is that the packet is lost or there is no one at the specified address to receive it, and the operating system on your machine usually does not know this.

     It is also possible for one call to sendto to report an error due to a problem related to a previous call.

Receiving Datagrams
-------------------

The recvfrom function reads a packet from a datagram socket and also tells you where it was sent from.  This function is declared in sys/socket.h.

 -- Function: int recvfrom (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS, struct sockaddr *ADDR, size_t *LENGTH-PTR)
     The recvfrom function reads one packet from the socket SOCKET into the buffer BUFFER.  The SIZE argument specifies the maximum number of bytes to be read.

     If the packet is longer than SIZE bytes, then you get the first SIZE bytes of the packet, and the rest of the packet is lost. There's no way to read the rest of the packet.  Thus, when you use a packet protocol, you must always know how long a packet to expect.

     The ADDR and LENGTH-PTR arguments are used to return the address where the packet came from.  See Socket Addresses.  For a socket in the file domain, the address information won't be meaningful, since you can't read the address of such a socket (see File Namespace).  You can specify a null pointer as the ADDR argument if you are not interested in this information.

     The FLAGS are interpreted the same way as for recv (see Socket Data Options).  The return value and error conditions are also the same as for recv.

You can use plain recv (see Receiving Data) instead of recvfrom if you know don't need to find out who sent the packet (either because you know where it should come from or because you treat all possible senders alike).  Even read can be used if you don't want to specify FLAGS (see I/O Primitives).

Datagram Socket Example
-----------------------

Here is a set of example programs that send messages over a datagram stream in the file namespace.  Both the client and server programs use the make_named_socket function that was presented in File Namespace, to create and name their sockets.

First, here is the server program.  It sits in a loop waiting for messages to arrive, bouncing each message back to the sender. Obviously, this isn't a particularly useful program, but it does show the general ideas involved.

          [file filesrv.c.texi not included]

Example of Reading Datagrams
----------------------------

Here is the client program corresponding to the server above.

It sends a datagram to the server and then waits for a reply.  Notice that the socket for the client (as well as for the server) in this example has to be given a name.  This is so that the server can direct a message back to the client.  Since the socket has no associated connection state, the only way the server can do this is by referencing the name of the client.

          [file filecli.c.texi not included]

Keep in mind that datagram socket communications are unreliable.  In this example, the client program waits indefinitely if the message never reaches the server or if the server's response never comes back.  It's up to the user running the program to kill it and restart it, if desired.  A more automatic solution could be to use select (see Waiting for I/O) to establish a timeout period for the reply, and in case of timeout either resend the message or shut down the socket and exit.

The inetd Daemon
================

We've explained above how to write a server program that does its own listening.  Such a server must already be running in order for anyone to connect to it.

Another way to provide service for an Internet port is to let the daemon program inetd do the listening.  inetd is a program that runs all the time and waits (using select) for messages on a specified set of ports.  When it receives a message, it accepts the connection (if the socket style calls for connections) and then forks a child process to run the corresponding server program.  You specify the ports and their programs in the file /etc/inetd.conf.

inetd Servers
-------------

Writing a server program to be run by inetd is very simple.  Each time someone requests a connection to the appropriate port, a new server process starts.  The connection already exists at this time; the socket is available as the standard input descriptor and as the standard output descriptor (descriptors 0 and 1) in the server process.  So the server program can begin reading and writing data right away.  Often the program needs only the ordinary I/O facilities; in fact, a general-purpose filter program that knows nothing about sockets can work as a byte stream server run by inetd.

You can also use inetd for servers that use connectionless communication styles.  For these servers, inetd does not try to accept a connection, since no connection is possible.  It just starts the server program, which can read the incoming datagram packet from descriptor 0.  The server program can handle one request and then exit, or you can choose to write it to keep reading more requests until no more arrive, and then exit.  You must specify which of these two techniques the server uses, when you configure inetd.

Configuring inetd
-----------------

The file /etc/inetd.conf tells inetd which ports to listen to and what server programs to run for them.  Normally each entry in the file is one line, but you can split it onto multiple lines provided all but the first line of the entry start with whitespace.  Lines that start with # are comments.

Here are two standard entries in /etc/inetd.conf:

     ftp	stream	tcp	nowait	root	/libexec/ftpd	ftpd
     talk	dgram	udp	wait	root	/libexec/talkd	talkd

An entry has this format:

     SERVICE STYLE PROTOCOL WAIT USERNAME PROGRAM ARGUMENTS

The SERVICE field says which service this program provides.  It should be the name of a service defined in /etc/services. inetd uses SERVICE to decide which port to listen on for this entry.

The fields STYLE and PROTOCOL specify the communication style and the protocol to use for the listening socket.  The style should be the name of a communication style, converted to lower case and with SOCK_ deleted---for example, stream or dgram.  PROTOCOL should be one of the protocols listed in /etc/protocols.  The typical protocol names are tcp for byte stream connections and udp for unreliable datagrams.

The WAIT field should be either wait or nowait. Use wait if STYLE is a connectionless style and the server, once started, handles multiple requests, as many as come in. Use nowait if inetd should start a new process for each message or request that comes in.  If STYLE uses connections, then WAIT must be nowait.

USER is the user name that the server should run as.  inetd runs as root, so it can set the user ID of its children arbitrarily.  It's best to avoid using root for USER if you can; but some servers, such as Telnet and FTP, read a username and password themselves.  These servers need to be root initially so they can log in as commanded by the data coming over the network.

PROGRAM together with ARGUMENTS specifies the command to run to start the server.  PROGRAM should be an absolute file name specifying the executable file to run.  ARGUMENTS consists of any number of whitespace-separated words, which become the command-line arguments of PROGRAM.  The first word in ARGUMENTS is argument zero, which should by convention be the program name itself (sans directories).

If you edit /etc/inetd.conf, you can tell inetd to reread the file and obey its new contents by sending the inetd process the SIGHUP signal.  You'll have to use ps to determine the process ID of the inetd process, as it is not fixed.

Socket Options
==============

This section describes how to read or set various options that modify the behavior of sockets and their underlying communications protocols.

When you are manipulating a socket option, you must specify which level the option pertains to.  This describes whether the option applies to the socket interface, or to a lower-level communications protocol interface.

Socket Option Functions
-----------------------

Here are the functions for examining and modifying socket options. They are declared in sys/socket.h.

 -- Function: int getsockopt (int SOCKET, int LEVEL, int OPTNAME, void *OPTVAL, size_t *OPTLEN-PTR)
     The getsockopt function gets information about the value of option OPTNAME at level LEVEL for socket SOCKET.

     The option value is stored in a buffer that OPTVAL points to. Before the call, you should supply in *OPTLEN-PTR the size of this buffer; on return, it contains the number of bytes of information actually stored in the buffer.

     Most options interpret the OPTVAL buffer as a single int value.

     The actual return value of getsockopt is 0 on success and -1 on failure.  The following errno error conditions are defined:

     EBADF
          The SOCKET argument is not a valid file descriptor.

     ENOTSOCK
          The descriptor SOCKET is not a socket.

     ENOPROTOOPT
          The OPTNAME doesn't make sense for the given LEVEL.

 -- Function: int setsockopt (int SOCKET, int LEVEL, int OPTNAME, void *OPTVAL, size_t OPTLEN)
     This function is used to set the socket option OPTNAME at level LEVEL for socket SOCKET.  The value of the option is passed in the buffer OPTVAL, which has size OPTLEN.

     The return value and error codes for setsockopt are the same as for getsockopt.

Socket-Level Options
--------------------

 -- Constant: int SOL_SOCKET
     Use this constant as the LEVEL argument to getsockopt or setsockopt to manipulate the socket-level options described in this section.

Here is a table of socket-level option names; all are defined in the header file sys/socket.h.

SO_DEBUG

     This option toggles recording of debugging information in the underlying protocol modules.  The value has type int; a nonzero value means ``yes''.

SO_REUSEADDR
     This option controls whether bind (see Setting Address) should permit reuse of local addresses for this socket.  If you enable this option, you can actually have two sockets with the same Internet port number; but the system won't allow you to use the two identically-named sockets in a way that would confuse the Internet.  The reason for this option is that some higher-level Internet protocols, including FTP, require you to keep reusing the same socket number.

     The value has type int; a nonzero value means ``yes''.

SO_KEEPALIVE
     This option controls whether the underlying protocol should periodically transmit messages on a connected socket.  If the peer fails to respond to these messages, the connection is considered broken.  The value has type int; a nonzero value means ``yes''.

SO_DONTROUTE
     This option controls whether outgoing messages bypass the normal message routing facilities.  If set, messages are sent directly to the network interface instead.  The value has type int; a nonzero value means ``yes''.

SO_LINGER
     This option specifies what should happen when the socket of a type that promises reliable delivery still has untransmitted messages when it is closed; see Closing a Socket.  The value has type struct linger.

      -- Data Type: struct linger
          This structure type has the following members:

          int l_onoff
               This field is interpreted as a boolean.  If nonzero, close blocks until the data is transmitted or the timeout period has expired.

          int l_linger
               This specifies the timeout period, in seconds.

SO_BROADCAST
     This option controls whether datagrams may be broadcast from the socket. The value has type int; a nonzero value means ``yes''.

SO_OOBINLINE
     If this option is set, out-of-band data received on the socket is placed in the normal input queue.  This permits it to be read using read or recv without specifying the MSG_OOB flag.  See Out-of-Band Data.  The value has type int; a nonzero value means ``yes''.

SO_SNDBUF
     This option gets or sets the size of the output buffer.  The value is a size_t, which is the size in bytes.

SO_RCVBUF
     This option gets or sets the size of the input buffer.  The value is a size_t, which is the size in bytes.

SO_STYLE
SO_TYPE
     This option can be used with getsockopt only.  It is used to get the socket's communication style.  SO_TYPE is the historical name, and SO_STYLE is the preferred name in GNU. The value has type int and its value designates a communication style; see Communication Styles.

SO_ERROR

     This option can be used with getsockopt only.  It is used to reset the error status of the socket.  The value is an int, which represents the previous error status.

Networks Database
=================

Many systems come with a database that records a list of networks known to the system developer.  This is usually kept either in the file /etc/networks or in an equivalent from a name server.  This data base is useful for routing programs such as route, but it is not useful for programs that simply communicate over the network.  We provide functions to access this data base, which are declared in netdb.h.

 -- Data Type: struct netent
     This data type is used to represent information about entries in the networks database.  It has the following members:

     char *n_name
          This is the ``official'' name of the network.

     char **n_aliases
          These are alternative names for the network, represented as a vector of strings.  A null pointer terminates the array.

     int n_addrtype
          This is the type of the network number; this is always equal to AF_INET for Internet networks.

     unsigned long int n_net
          This is the network number.  Network numbers are returned in host byte order; see Byte Order.

Use the getnetbyname or getnetbyaddr functions to search the networks database for information about a specific network.  The information is returned in a statically-allocated structure; you must copy the information if you need to save it.

 -- Function: struct netent * getnetbyname (const char *NAME)
     The getnetbyname function returns information about the network named NAME.  It returns a null pointer if there is no such network.

 -- Function: struct netent * getnetbyaddr (long NET, int TYPE)
     The getnetbyaddr function returns information about the network of type TYPE with number NET.  You should specify a value of AF_INET for the TYPE argument for Internet networks.

     getnetbyaddr returns a null pointer if there is no such network.

You can also scan the networks database using setnetent, getnetent, and endnetent.  Be careful in using these functions, because they are not reentrant.

 -- Function: void setnetent (int STAYOPEN)
     This function opens and rewinds the networks database.

     If the STAYOPEN argument is nonzero, this sets a flag so that subsequent calls to getnetbyname or getnetbyaddr will not close the database (as they usually would).  This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

 -- Function: struct netent * getnetent (void)
     This function returns the next entry in the networks database.  It returns a null pointer if there are no more entries.

 -- Function: void endnetent (void)
     This function closes the networks database.