bind(2)

bind(2) — bestie, we're giving your socket a NAME 💖

okay so real talk, this is straight out of the System Calls Manual section of your friendly neighborhood man pages, and it's about to be your new fave syscall. no cap. 🫶

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✨ NAME ✨

bind — literally bind a name to a socket. that's it. that's the whole job. iconic behavior.

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🎀 LIBRARY 🎀

Standard C library (libc, -lc) — so you just link -lc like always and you're golden ✨

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🌸 SYNOPSIS 🌸

#include <sys/socket.h>

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

look at her. three little args. so tidy. 💅

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🦋 DESCRIPTION 🦋

okay so here's the tea. when you make a socket with socket(2), it shows up in a name space (aka an address family) but it has no address. it's giving... newborn. blank slate. main character with no backstory yet 😭

bind() fixes that. it assigns the address that addr points to, to the socket that file descriptor sockfd refers to. and addrlen? that's the size, in bytes, of the address structure that addr is pointing at. simple as.

traditionally this whole operation is called “assigning a name to a socket” — which, honestly, is such a cute way to describe it. they ATE with that phrasing 🔥

ngl, it's normally necessary to assign a local address with bind() before a SOCK_STREAM socket can receive connections (go peek at accept(2) if you need the full picture).

the rules for name binding? they vary between address families. so like, consult the manual entries in Section 7 for the detailed lore:

• 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)
• for AF_NETLINK, see netlink(7)

kinda a whole roster, lowkey. 💜

the actual structure you pass for the addr argument depends on the address family. the sockaddr structure is defined as something like this:

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

and the only purpose of this structure — listen up, this is the part people always get wrong — is to cast the structure pointer passed in addr, purely to avoid compiler warnings. that's it!! it's a vibe-check struct. see the EXAMPLES below for how it actually goes down 👇

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⭐ RETURN VALUE ⭐

• on success — zero is returned. slay.
• on error — -1 is returned, and errno is set to tell you what went wrong 🙈

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💅 ERRORS 💅

here's the full drama list. every single one, no skips:

• EACCES — the address is protected, and the user is not the superuser. (root only, bestie 😔)
• 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 in 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, so if you're in that lane, this is your section:

• 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.

🫶 also note: other errors may be generated by the underlying protocol modules. so like, it's not always gonna be one of these. keep an open mind, bestie.

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🌈 VERSIONS 🌈

portable programs must ensure that addr.sun_path is a null-terminated string for AF_UNIX sockets. do not forget this. it WILL bite you. 🦋

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👑 STANDARDS 👑

POSIX.1-2024. periodt.

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💖 HISTORY 💖

POSIX.1-2001, SVr4, 4.2BSD. she's been iconic for DECADES, we're just livin in her world 😌

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💀 BUGS 💀

the transparent proxy options are not described. so, y'know. there's that. 🙈

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✨ EXAMPLES ✨

an example of the use of bind() with Internet domain sockets can be found in getaddrinfo(3) — go look, it's cute.

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");
}

copy that whole thing, it's perfect, it's canon, it's the blueprint. 🫶💜

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🌟 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)

read them all. i'm not joking. bestie homework. 📚✨

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and that's the whole thing, gorgeous — this page came to you from Linux man-pages 6.20, dated 2026-02-08, for bind(2). go forth and bind responsibly 💅🌈💖