bind(2)

NAME
bind - bind a name to a socket

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#include <sys/socket.h>

int bind(int sockfd, const struct sockaddr *addr,
         socklen_t addrlen);

DESCRIPTION
When a socket is created with socket(2), it exists in a name space (address family) but has no address assigned to it. bind() assigns the address specified by addr to the socket referred to by the file descriptor sockfd. addrlen specifies the size, in bytes, of the address structure pointed to by addr. Traditionally, this operation is called “assigning a name to a socket”.

It is normally necessary to assign a local address using bind() before a SOCK_STREAM socket may receive connections (see accept(2)).

The rules used in name binding vary between address families. Consult the manual entries in Section 7 for detailed information. For AF_INET, see ip(7); for AF_INET6, see ipv6(7); for AF_UNIX, see unix(7); for AF_APPLETALK, see ddp(7); for AF_PACKET, see packet(7); for AF_X25, see x25(7); and for AF_NETLINK, see netlink(7).

The actual structure passed for the addr argument will depend on the address family. The sockaddr structure is defined as something like:

    struct sockaddr {
        sa_family_t sa_family;
        char        sa_data[14];
    }

The only purpose of this structure is to cast the structure pointer passed in addr in order to avoid compiler warnings. See EXAMPLES below.

RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error.

ERRORS

EACCES
    The address is protected, and the user is not the superuser.

EADDRINUSE
    The given address is already in use.

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 /proc/sys/net/ipv4/ip_local_port_range ip(7).

EBADF
    sockfd is not a valid file descriptor.

EINVAL
    The socket is already bound to an address.

EINVAL
    addrlen is wrong, or addr is not a valid address for this socket's domain.

ENOTSOCK
    The file descriptor sockfd does not refer to a socket.

EADDRNOTAVAIL
    A nonexistent interface was requested or the requested address was not local.

The following errors are specific to UNIX domain (AF_UNIX) sockets:

EACCES
    Search permission is denied on a component of the path prefix. (See also path_resolution(7).)

EFAULT
    addr points outside the user's accessible address space.

ELOOP
    Too many symbolic links were encountered in resolving addr.

ENAMETOOLONG
    addr is too long.

ENOENT
    A component in the directory prefix of the socket pathname does not exist.

ENOMEM
    Insufficient kernel memory was available.

ENOTDIR
    A component of the path prefix is not a directory.

EROFS
    The socket inode would reside on a read-only filesystem.

Other errors may be generated by the underlying protocol modules.

VERSIONS
Portable programs must ensure that addr.sun_path is a null-terminated string for AF_UNIX sockets.

STANDARDS
POSIX.1-2024.

HISTORY
POSIX.1-2001, SVr4, 4.2BSD.

BUGS
The transparent proxy options are not described.

EXAMPLES
An example of the use of bind() with Internet domain sockets can be found in getaddrinfo(3).

The following example shows how to bind a stream socket in the UNIX (AF_UNIX) domain, and accept connections:

    #include <err.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/socket.h>
    #include <sys/un.h>
    #include <unistd.h>

    #define MY_SOCK_PATH "/somepath"
    #define LISTEN_BACKLOG 50

    int
    main(void)
    {
        int                 sfd, cfd;
        socklen_t           peer_addr_size;
        struct sockaddr_un  my_addr, peer_addr;

        sfd = socket(AF_UNIX, SOCK_STREAM, 0);
        if (sfd == -1)
            err(EXIT_FAILURE, "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)
            err(EXIT_FAILURE, "bind");

        if (listen(sfd, LISTEN_BACKLOG) == -1)
            err(EXIT_FAILURE, "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)
            err(EXIT_FAILURE, "accept");

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

        if (close(sfd) == -1)
            err(EXIT_FAILURE, "close");

        if (unlink(MY_SOCK_PATH) == -1)
            err(EXIT_FAILURE, "unlink");
    }

SEE ALSO
accept(2), connect(2), getsockname(2), listen(2), socket(2), getaddrinfo(3), getifaddrs(3), ip(7), ipv6(7), path_resolution(7), socket(7), unix(7)