Socket layer
- C course
- Advanced
- Lesson 81/83
- 6 min
Write a network communication layer with sockets.
Socket layer
In this lesson you'll define the server_setup function. It creates an AF_UNIX socket, binds it to /tmp/cb_p.sock and starts listening. On success it returns the descriptor; on failure -1.
Steps: socket(AF_UNIX, SOCK_STREAM, 0), zero the address struct with memset, set sun_family = AF_UNIX, copy the path with strncpy, remove the old socket with unlink, bind, listen(fd, 1). This is the classic Unix socket server setup. The main from previous lessons is included in the starting code.
Address struct
You declare struct sockaddr_un addr;. Zero it with memset(&addr, 0, sizeof(addr)); set addr.sun_family = AF_UNIX; and strncpy(addr.sun_path, "/tmp/cb_p.sock", sizeof(addr.sun_path) - 1);.
Remove the old socket file with unlink("/tmp/cb_p.sock");; otherwise bind fails. Then bind(fd, (struct sockaddr *)&addr, sizeof(addr)) and listen(fd, 1). return fd;. In main add int sfd = server_setup(); and printf("soket: %d\n", sfd >= 0 ? 1 : 0);. The output gains "soket: 1".
Practice Add server_setup
Write int server_setup(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_p.sock", sizeof(addr.sun_path) - 1);, unlink("/tmp/cb_p.sock");, bind(fd, (struct sockaddr *)&addr, sizeof(addr));, listen(fd, 1);, return fd;. Add int sfd = server_setup(); and printf("soket: %d\n", sfd >= 0 ? 1 : 0); to main.
Starter code
#include <stdio.h>
#include <pthread.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <string.h>
#include <unistd.h>
int worker_init(void);
int server_setup(void);
int ipc_setup(void);
int worker_count = 0;
pthread_mutex_t worker_lock = PTHREAD_MUTEX_INITIALIZER;
void *worker_fn(void *arg) {
(void)arg;
pthread_mutex_lock(&worker_lock);
worker_count++;
pthread_mutex_unlock(&worker_lock);
return NULL;
}
int worker_init(void) {
pthread_t t;
pthread_create(&t, NULL, worker_fn, NULL);
pthread_join(t, NULL);
return 1;
}
// server_setup fonksiyonunu yaz: AF_UNIX soket oluştur, bind, listen yap
// soket tanıtıcısı döndür (>= 0 ise başarılı)
int main(void) {
printf("Proje hazır\n");
worker_init();
printf("sayaç: %d\n", worker_count);
// server_setup çağır ve "soket: 1" yazdır
return 0;
}Sample solution
#include <stdio.h>
#include <pthread.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <string.h>
#include <unistd.h>
int worker_init(void);
int server_setup(void);
int ipc_setup(void);
int worker_count = 0;
pthread_mutex_t worker_lock = PTHREAD_MUTEX_INITIALIZER;
void *worker_fn(void *arg) {
(void)arg;
pthread_mutex_lock(&worker_lock);
worker_count++;
pthread_mutex_unlock(&worker_lock);
return NULL;
}
int worker_init(void) {
pthread_t t;
pthread_create(&t, NULL, worker_fn, NULL);
pthread_join(t, NULL);
return 1;
}
int server_setup(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_p.sock", sizeof(addr.sun_path) - 1);
unlink("/tmp/cb_p.sock");
bind(fd, (struct sockaddr *)&addr, sizeof(addr));
listen(fd, 1);
return fd;
}
int main(void) {
printf("Proje hazır\n");
worker_init();
printf("sayaç: %d\n", worker_count);
int sfd = server_setup();
printf("soket: %d\n", sfd >= 0 ? 1 : 0);
return 0;
}Expected output
Proje hazır sayaç: 1 soket: 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 socket call
int fd = socket(AF_UNIX, SOCK_STREAM, ____);int fd = socket(AF_UNIX, SOCK_STREAM, 0);Explanation: The third parameter is 0; the default protocol.
2 Match socket steps
| Term | Meaning |
|---|---|
socket |
Create socket |
bind |
Bind to address |
listen |
Start listening |
unlink |
Remove old socket |
3 What does bind do?
- Binds socket to address Correct answer
- Creates a socket
- Accepts a connection
- Sends data
Explanation: bind links the socket to an address (e.g. a file path).