Getting the Session ID: getsid()

#define _XOPEN_SOURCE 500
#include <unistd.h>

pid_t getsid(pid_t pid);
/* pid == 0  → session ID of calling process
 * Returns session ID on success, -1 on error (EPERM on some systems) */

Creating a New Session: setsid()

#include <unistd.h>

pid_t setsid(void);
/* Returns: new session ID on success, -1 on error (EPERM if caller is a group leader) */

Why a Process Group Leader Cannot Call setsid()

Code Example 1 — Creating a New Session (t_setsid)

/* t_setsid.c
 * Demonstrates setsid() to create a new session.
 * Verifies the new session has no controlling terminal by trying to open /dev/tty.
 * Compile: gcc -o t_setsid t_setsid.c
 */
#define _XOPEN_SOURCE 500
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>

int main(void)
{
    printf("Before fork:\n");
    printf("  PID=%ld  PGID=%ld  SID=%ld\n",
           (long)getpid(), (long)getpgrp(), (long)getsid(0));

    /* Step 1: fork so child is NOT a process group leader */
    pid_t pid = fork();
    if (pid == -1) { perror("fork"); exit(EXIT_FAILURE); }

    if (pid != 0) {
        /* Parent exits — child is now an orphan (adopted by init) */
        printf("Parent exiting...\n");
        exit(EXIT_SUCCESS);
    }

    /* --- CHILD CONTINUES --- */
    printf("\nChild (PID=%ld) calling setsid()...\n", (long)getpid());

    /* Step 2: child calls setsid() — cannot be group leader here */
    pid_t sid = setsid();
    if (sid == -1) {
        perror("setsid");
        exit(EXIT_FAILURE);
    }

    printf("After setsid():\n");
    printf("  PID=%ld  PGID=%ld  SID=%ld\n",
           (long)getpid(), (long)getpgrp(), (long)getsid(0));
    /* Note: PID == PGID == SID — all equal after setsid() */

    /* Step 3: Verify no controlling terminal by opening /dev/tty */
    int fd = open("/dev/tty", O_RDWR);
    if (fd == -1) {
        /* ENXIO means no controlling terminal — expected! */
        printf("open(\"/dev/tty\") failed: %s (errno=%d)\n",
               strerror(errno), errno);
        printf("  → Confirmed: new session has NO controlling terminal\n");
    } else {
        printf("open(\"/dev/tty\") succeeded (unexpected)\n");
        close(fd);
    }

    return 0;
}
/*
 * Expected output:
 * Before fork:
 *   PID=12352  PGID=12352  SID=12243    ← shell's PID is SID
 * Parent exiting...
 * Child (PID=12353) calling setsid()...
 * After setsid():
 *   PID=12353  PGID=12353  SID=12353    ← PID == PGID == SID
 * open("/dev/tty") failed: No such device or address (errno=6)
 *   → Confirmed: new session has NO controlling terminal
 */

Code Example 2 — Verifying setsid() Fails for Group Leader

/* setsid_group_leader_fail.c
 * Demonstrates that setsid() fails with EPERM if the caller is a process group leader.
 * Compile: gcc -o sgl setsid_group_leader_fail.c
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

int main(void)
{
    printf("=== Testing setsid() restrictions ===\n\n");

    /* Case 1: main process is typically already a group leader */
    printf("Case 1: Calling setsid() directly (process is group leader)\n");
    printf("  PID=%ld  PGID=%ld\n", (long)getpid(), (long)getpgrp());

    if (getpid() == getpgrp()) {
        printf("  We ARE the group leader — setsid() should fail.\n");
    }

    pid_t result = setsid();
    if (result == -1) {
        printf("  setsid() returned -1, errno=%d (%s)\n",
               errno, strerror(errno));  /* EPERM */
    } else {
        printf("  setsid() succeeded (we were not a group leader)\n");
    }

    printf("\nCase 2: fork() first, then setsid() in child\n");
    pid_t child = fork();
    if (child == -1) { perror("fork"); return 1; }

    if (child == 0) {
        /*
         * The child has a new PID which is guaranteed to differ from
         * any existing PGID, so the child is NOT a group leader.
         */
        printf("  Child PID=%ld  PGID=%ld\n",
               (long)getpid(), (long)getpgrp());
        printf("  Child is%s a group leader.\n",
               (getpid() == getpgrp()) ? "" : " NOT");

        pid_t sid = setsid();
        if (sid == -1) {
            perror("  child setsid");
        } else {
            printf("  child setsid() succeeded! SID=%ld\n", (long)sid);
        }
        return 0;
    }
    wait(NULL);
    return 0;
}
/*
 * Expected output:
 * Case 1: Calling setsid() directly (process is group leader)
 *   PID=5000  PGID=5000
 *   We ARE the group leader — setsid() should fail.
 *   setsid() returned -1, errno=1 (Operation not permitted)
 *
 * Case 2: fork() first, then setsid() in child
 *   Child PID=5001  PGID=5000
 *   Child is NOT a group leader.
 *   child setsid() succeeded! SID=5001
 */

Interview Questions — Section 34.3