BIND(2) System Calls Manual BIND(2)

bind - bind a name to a socket

Standard C library (libc, -lc)

#include <sys/socket.h>
int bind(int sockfd, const struct sockaddr *addr,
         socklen_t addrlen);

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.

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

The address is protected, and the user is not the superuser.
The given address is already in use.
(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).
sockfd is not a valid file descriptor.
The socket is already bound to an address.
addrlen is wrong, or addr is not a valid address for this socket's domain.
The file descriptor sockfd does not refer to a socket.

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

Search permission is denied on a component of the path prefix. (See also path_resolution(7).)
A nonexistent interface was requested or the requested address was not local.
addr points outside the user's accessible address space.
Too many symbolic links were encountered in resolving addr.
addr is too long.
A component in the directory prefix of the socket pathname does not exist.
Insufficient kernel memory was available.
A component of the path prefix is not a directory.
The socket inode would reside on a read-only filesystem.

POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (bind() first appeared in 4.2BSD).

For background on the socklen_t type, see accept(2).

The transparent proxy options are not described.

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 <stdio.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
#define handle_error(msg) \

do { perror(msg); exit(EXIT_FAILURE); } while (0) 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)
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)... */
if (close(sfd) == -1)
handle_error("close");
if (unlink(MY_SOCK_PATH) == -1)
handle_error("unlink"); }

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)

2022-10-09 Linux man-pages 6.01