What are Foreground and Background Groups?

Getting the Foreground Group: tcgetpgrp()

#include <unistd.h>

pid_t tcgetpgrp(int fd);
/* Returns PGID of foreground process group, or -1 on error */

int tcsetpgrp(int fd, pid_t pgid);
/* Sets foreground process group to pgid.
 * Restrictions:
 *   - Calling process must have a controlling terminal
 *   - fd must refer to that controlling terminal
 *   - pgid must match a process group in the calling process's session
 * Returns 0 on success, -1 on error */

How the Shell Uses tcsetpgrp()

Code Example 1 — Reading the Foreground Process Group

/* show_fg_pgrp.c
 * Shows the foreground process group of the controlling terminal.
 * Compile: gcc -o show_fg_pgrp show_fg_pgrp.c
 * Run interactively (not with & ) so it is itself the foreground group.
 */
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>

int main(void)
{
    /* Open the controlling terminal */
    int fd = open("/dev/tty", O_RDONLY);
    if (fd == -1) {
        fprintf(stderr, "open /dev/tty: %s\n", strerror(errno));
        return 1;
    }

    /* Our own identity */
    printf("Our PID  = %ld\n", (long)getpid());
    printf("Our PGID = %ld\n", (long)getpgrp());
    printf("Our SID  = %ld\n\n", (long)getsid(0));

    /* Query the terminal for its foreground process group */
    pid_t fg = tcgetpgrp(fd);
    if (fg == -1) {
        fprintf(stderr, "tcgetpgrp: %s\n", strerror(errno));
    } else {
        printf("Terminal foreground PGID = %ld\n", (long)fg);
        if (fg == getpgrp())
            printf("  → We ARE in the foreground group\n");
        else
            printf("  → We are NOT the foreground group\n");
    }

    close(fd);
    return 0;
}
/* Run in foreground:
 * $ ./show_fg_pgrp
 * Our PID  = 8001
 * Our PGID = 8001
 * Our SID  = 1200
 * Terminal foreground PGID = 8001
 *   → We ARE in the foreground group
 */

Code Example 2 — Simulating Foreground/Background Switch with tcsetpgrp()

/* fg_bg_switch.c
 * Demonstrates how a shell-like program uses tcsetpgrp() to
 * move a child to the foreground and back.
 * Compile: gcc -o fg_bg_switch fg_bg_switch.c
 * Must be run as the foreground process.
 */
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>

int main(void)
{
    int tty_fd = open("/dev/tty", O_RDWR);
    if (tty_fd == -1) { perror("open /dev/tty"); return 1; }

    pid_t shell_pgid = getpgrp();
    printf("Shell PGID = %ld (currently in foreground)\n",
           (long)shell_pgid);
    printf("Terminal foreground PGID = %ld\n",
           (long)tcgetpgrp(tty_fd));

    /* Fork a child to represent a "job" */
    pid_t child = fork();
    if (child == -1) { perror("fork"); return 1; }

    if (child == 0) {
        /* Child: create new process group */
        setpgid(0, 0);
        printf("Child PID=%ld  PGID=%ld — running as background\n",
               (long)getpid(), (long)getpgrp());
        /* Simulate some work */
        sleep(2);
        printf("Child done\n");
        return 0;
    }

    /* Parent (shell): give child its own group */
    setpgid(child, child); /* child's PGID = child's PID */
    printf("\nBringing child (PGID=%ld) to foreground...\n",
           (long)child);

    /* Move child to foreground */
    tcsetpgrp(tty_fd, child);
    printf("Terminal foreground PGID now = %ld\n",
           (long)tcgetpgrp(tty_fd));

    /* Wait for child */
    int status;
    waitpid(child, &status, WUNTRACED);

    /* Restore shell to foreground */
    tcsetpgrp(tty_fd, shell_pgid);
    printf("\nRestored shell (PGID=%ld) to foreground\n",
           (long)shell_pgid);
    printf("Terminal foreground PGID now = %ld\n",
           (long)tcgetpgrp(tty_fd));

    close(tty_fd);
    return 0;
}
/*
 * Shell PGID = 8500 (currently in foreground)
 * Terminal foreground PGID = 8500
 * Child PID=8501  PGID=8501 — running as background
 *
 * Bringing child (PGID=8501) to foreground...
 * Terminal foreground PGID now = 8501
 * Child done
 *
 * Restored shell (PGID=8500) to foreground
 * Terminal foreground PGID now = 8500
 */

Interview Questions — Section 34.5