socket and bind
- C course
- Advanced
- Lesson 26/83
- 5 min
Learn to set up a server with socket and bind.
socket and bind
socket(domain, type, protocol) creates a new socket and returns a descriptor. You pass AF_UNIX for domain, SOCK_STREAM for type, 0 (default) for protocol.
bind(fd, (struct sockaddr *)&addr, sizeof(addr)) binds the socket to an address. addr is of type struct sockaddr_un; you set sun_family to AF_UNIX and sun_path to a file path. Returns 0 on success, -1 on error.
Preparing the address struct
Before bind you must fill the address struct correctly. First zero it with memset(&addr, 0, sizeof(addr)); this ensures sun_path is clean.
Then set addr.sun_family = AF_UNIX; and copy the path with strncpy(addr.sun_path, "/tmp/sock", sizeof(addr.sun_path) - 1);. The trailing -1 in strncpy prevents overflow. Removing the old socket file with unlink is a good habit; otherwise bind fails.
Practice Create and bind a socket
Create a socket with int fd = socket(AF_UNIX, SOCK_STREAM, 0);. Declare struct sockaddr_un addr;; set memset(&addr, 0, sizeof(addr)), addr.sun_family = AF_UNIX;, strncpy(addr.sun_path, "/tmp/cb.sock", sizeof(addr.sun_path) - 1);. Call unlink("/tmp/cb.sock"); and bind(fd, (struct sockaddr *)&addr, sizeof(addr));. Print printf("bound\n"); and close(fd);.
Starter code
#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include <string.h>
// Create socket
// Fill address struct and call bind
// Print bound and close
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include <string.h>
int main(void) {
int fd = socket(AF_UNIX, SOCK_STREAM, 0);
struct sockaddr_un addr;
memset(&addr, 0, sizeof(addr));
addr.sun_family = AF_UNIX;
strncpy(addr.sun_path, "/tmp/cb.sock", sizeof(addr.sun_path) - 1);
unlink("/tmp/cb.sock");
bind(fd, (struct sockaddr *)&addr, sizeof(addr));
printf("bound\n");
close(fd);
return 0;
}Expected output
bound
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match socket and bind parts
| Term | Meaning |
|---|---|
socket |
Creates socket |
bind |
Binds to address |
sockaddr_un |
UNIX address struct |
memset |
Zeroing |
2 What does the bind function do?
- Creates a socket
- Binds socket to address Correct answer
- Starts listening
- Accepts a connection
Explanation: bind links the socket to an address (e.g. a file path).
3 bind call
bind(fd, (struct sockaddr *)&____, sizeof(addr));bind(fd, (struct sockaddr *)&addr, sizeof(addr));Explanation: bind links the addr struct to the socket.