setsid() — Create session, set process group ID

Standards

Standards / Extensions C or C++ Dependencies
POSIX.1
XPG4
XPG4.2
Single UNIX Specification, Version 3
 both  

Format

#define _POSIX_SOURCE
#include <unistd.h>

pid_t setsid(void);

General description

Creates a new session with the calling process as its session leader. The caller becomes the process group leader of a new process group. The calling process must not be a process group leader already. The caller does not have a controlling terminal.

The process group ID (PGID) of the new process group is equal to the process ID (PID) of the caller. The caller starts as the only process in the new process group and in the new session.

Returned value

If successful, setsid() returns the value of the caller's new PGID.

If unsuccessful, setsid() returns -1 and sets errno to one of the following values:
Error Code
Description
EPERM
One of the following error conditions exists:
  • The caller is already a process group leader.
  • The caller's PID matches the PGID of some other process.

Example

CELEBS10 
/* CELEBS10

   This example creates a new session.

 */
#define _POSIX_SOURCE
#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>

main() {
  pid_t pid;
  int p[2];
  char c='?';

  if (pipe(p) != 0)
    perror("pipe() error");
  else
    if ((pid = fork()) == 0) {
      printf("child's process group id is %d\n", (int) getpgrp());
      write(p[1], &c, 1);
      setsid();
      printf("child's process group id is now %d\n", (int) getpgrp());
      exit(0);
    }
    else {
      printf("parent's process group id is %d\n", (int) getpgrp());
      read(p[0], &c, 1);
      sleep(5);
    }
}
Output 
child's process group id is 262152
child's process group id is now 262150
parent's process group id is 262152

Related information