Networking in C · beginner · ~6 min

socket() — creating an endpoint

Call `socket()` to obtain a fresh, unbound socket file descriptor.

Lesson

What `socket()` does

int socket(int family, int type, int proto) asks the kernel to allocate a new socket. It returns a file descriptor (an integer the kernel uses to identify an open resource) for that socket.

The socket starts out empty: it is not yet tied to an address or a peer.

The three arguments

family — the address family.
- AF_INET for IPv4.
- AF_INET6 for IPv6.
- Use AF_INET throughout this course.
type — the kind of communication.
- SOCK_STREAM for TCP (reliable, connection-based).
- SOCK_DGRAM for UDP (message-based, no connection).
proto — the protocol.
- Pass 0 to let the kernel choose the default for the family/type pair.
- That default is TCP for SOCK_STREAM and UDP for SOCK_DGRAM.

Return value

socket() returns the new file descriptor on success.

On failure it returns -1 and sets errno. Always check the result before using the descriptor.

Code examples

#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <unistd.h>

int main(void) {
    int fd = socket(AF_INET, SOCK_STREAM, 0);
    if (fd < 0) { perror("socket"); return 1; }
    printf("got socket fd %d\n", fd);
    close(fd);
    return 0;
}

Common mistakes

Leaking the descriptor on error paths. A socket is a file descriptor. If you do not close() it, it leaks exactly like any other open file descriptor.
Confusing socket() with connect(). socket() only allocates the socket. You still need bind() and listen() (for a server) or connect() (for a client) before any communication happens.

Summary

socket(AF_INET, SOCK_STREAM, 0) creates a TCP socket and returns its file descriptor.
family picks IPv4/IPv6, type picks TCP/UDP, and proto = 0 selects the default protocol.
On error it returns -1 and sets errno — always check.
A new socket is unbound; you still need bind/listen or connect to use it.