bind.2

.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
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.\" Modified Mon Oct 21 23:05:29 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1998 by Andi Kleen
.\" $Id: bind.2,v 1.3 1999/04/23 19:56:07 freitag Exp $
.\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.TH BIND 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
bind \- bind a name to a socket
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.PP
.BI "int bind(int " sockfd ", const struct sockaddr *" addr ,
.BI "         socklen_t " addrlen );
.fi
.SH DESCRIPTION
When a socket is created with
.BR socket (2),
it exists in a name space (address family) but has no address assigned to it.
.BR bind ()
assigns the address specified by
.I addr
to the socket referred to by the file descriptor
.IR sockfd .
.I addrlen
specifies the size, in bytes, of the address structure pointed to by
.IR addr .
Traditionally, this operation is called \(lqassigning a name to a socket\(rq.
.PP
It is normally necessary to assign a local address using
.BR bind ()
before a
.B SOCK_STREAM
socket may receive connections (see
.BR accept (2)).
.PP
The rules used in name binding vary between address families.
Consult the manual entries in Section 7 for detailed information.
For
.BR AF_INET ,
see
.BR ip (7);
for
.BR AF_INET6 ,
see
.BR ipv6 (7);
for
.BR AF_UNIX ,
see
.BR unix (7);
for
.BR AF_APPLETALK ,
see
.BR ddp (7);
for
.BR AF_PACKET ,
see
.BR packet (7);
for
.BR AF_X25 ,
see
.BR x25 (7);
and for
.BR AF_NETLINK ,
see
.BR netlink (7).
.PP
The actual structure passed for the
.I addr
argument will depend on the address family.
The
.I sockaddr
structure is defined as something like:
.PP
.in +4n
.EX
struct sockaddr {
    sa_family_t sa_family;
    char        sa_data[14];
}
.EE
.in
.PP
The only purpose of this structure is to cast the structure
pointer passed in
.I addr
in order to avoid compiler warnings.
See EXAMPLES below.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EACCES
.\" e.g., privileged port in AF_INET domain
The address is protected, and the user is not the superuser.
.TP
.B EADDRINUSE
The given address is already in use.
.TP
.B EADDRINUSE
(Internet domain sockets)
The port number was specified as zero in the socket address structure,
but, upon attempting to bind to an ephemeral port,
it was determined that all port numbers in the ephemeral port range
are currently in use.
See the discussion of
.I /proc/sys/net/ipv4/ip_local_port_range
.BR ip (7).
.TP
.B EBADF
.I sockfd
is not a valid file descriptor.
.TP
.B EINVAL
The socket is already bound to an address.
.\" This may change in the future: see
.\" .I linux/unix/sock.c for details.
.TP
.B EINVAL
.I addrlen
is wrong, or
.I addr
is not a valid address for this socket's domain.
.TP
.B ENOTSOCK
The file descriptor
.I sockfd
does not refer to a socket.
.PP
The following errors are specific to UNIX domain
.RB ( AF_UNIX )
sockets:
.TP
.B EACCES
Search permission is denied on a component of the path prefix.
(See also
.BR path_resolution (7).)
.TP
.B EADDRNOTAVAIL
A nonexistent interface was requested or the requested
address was not local.
.TP
.B EFAULT
.I addr
points outside the user's accessible address space.
.TP
.B ELOOP
Too many symbolic links were encountered in resolving
.IR addr .
.TP
.B ENAMETOOLONG
.I addr
is too long.
.TP
.B ENOENT
A component in the directory prefix of the socket pathname does not exist.
.TP
.B ENOMEM
Insufficient kernel memory was available.
.TP
.B ENOTDIR
A component of the path prefix is not a directory.
.TP
.B EROFS
The socket inode would reside on a read-only filesystem.
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD
.RB ( bind ()
first appeared in 4.2BSD).
.\" SVr4 documents an additional
.\" .B ENOSR
.\" general error condition, and
.\" additional
.\" .B EIO
.\" and
.\" .B EISDIR
.\" UNIX-domain error conditions.
.SH NOTES
For background on the
.I socklen_t
type, see
.BR accept (2).
.SH BUGS
The transparent proxy options are not described.
.\" FIXME Document transparent proxy options
.SH EXAMPLES
An example of the use of
.BR bind ()
with Internet domain sockets can be found in
.BR getaddrinfo (3).
.PP
The following example shows how to bind a stream socket in the UNIX
.RB ( AF_UNIX )
domain, and accept connections:
.\" listen.7 refers to this example.
.\" accept.7 refers to this example.
.\" unix.7 refers to this example.
.PP
.EX
#include <sys/socket.h>
#include <sys/un.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define MY_SOCK_PATH "/somepath"
#define LISTEN_BACKLOG 50

#define handle_error(msg) \e
    do { perror(msg); exit(EXIT_FAILURE); } while (0)

int
main(int argc, char *argv[])
{
    int sfd, cfd;
    struct sockaddr_un my_addr, peer_addr;
    socklen_t peer_addr_size;

    sfd = socket(AF_UNIX, SOCK_STREAM, 0);
    if (sfd == \-1)
        handle_error("socket");

    memset(&my_addr, 0, sizeof(my_addr));
    my_addr.sun_family = AF_UNIX;
    strncpy(my_addr.sun_path, MY_SOCK_PATH,
            sizeof(my_addr.sun_path) \- 1);

    if (bind(sfd, (struct sockaddr *) &my_addr,
            sizeof(my_addr)) == \-1)
        handle_error("bind");

    if (listen(sfd, LISTEN_BACKLOG) == \-1)
        handle_error("listen");

    /* Now we can accept incoming connections one
       at a time using accept(2). */

    peer_addr_size = sizeof(peer_addr);
    cfd = accept(sfd, (struct sockaddr *) &peer_addr,
                 &peer_addr_size);
    if (cfd == \-1)
        handle_error("accept");

    /* Code to deal with incoming connection(s)... */

    /* When no longer required, the socket pathname, MY_SOCK_PATH
       should be deleted using unlink(2) or remove(3). */
}
.EE
.SH SEE ALSO
.BR accept (2),
.BR connect (2),
.BR getsockname (2),
.BR listen (2),
.BR socket (2),
.BR getaddrinfo (3),
.BR getifaddrs (3),
.BR ip (7),
.BR ipv6 (7),
.BR path_resolution (7),
.BR socket (7),
.BR unix (7)