The kernel assigns a unique positive integer PID to each new process. PIDs are issued sequentially. The getpid() system call returns the PID of the calling process. It always succeeds — it never returns an error.

Every process (except PID 1) was created by another process — its parent. getppid() returns the PID of the calling process’s parent. If the parent dies before the child, the child is adopted by init (PID 1), so getppid() returns 1.

Linux exposes every process’s attributes through the virtual filesystem at /proc/PID/. You can read a process’s parent PID from /proc/PID/status.

# Check your shell's PID and parent PID
$ echo $$               # PID of current bash shell → e.g., 4100
$ cat /proc/4100/status | grep -E "^(Pid|PPid)"
Pid:    4100
PPid:   3990

#include <stdio.h>
#include <unistd.h>

int main(void)
{
    printf("My PID   : %d\n", getpid());
    printf("Parent PID: %d\n", getppid());

    /* Tip: Run this in a terminal.
       Your parent is the shell (bash/zsh).
       Verify: $ ps -p <PPID> */
    return 0;
}

/* Sample output:
   My PID   : 7820
   Parent PID: 7680   ← this is your bash shell */

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>

int main(void)
{
    pid_t child_pid = fork();

    if (child_pid == 0) {
        /* --- CHILD PROCESS --- */
        printf("[Child]  My PID    = %d\n", getpid());
        printf("[Child]  My PPID   = %d  (parent's PID)\n", getppid());
    } else {
        /* --- PARENT PROCESS --- */
        printf("[Parent] My PID    = %d\n", getpid());
        printf("[Parent] Child PID = %d\n", child_pid);
        wait(NULL);   /* Wait for child to finish */
    }
    return 0;
}

/* Sample output:
   [Parent] My PID    = 9100
   [Parent] Child PID = 9101
   [Child]  My PID    = 9101
   [Child]  My PPID   = 9100  ← matches parent's PID */

A common use of PIDs is creating temporary filenames that won’t collide with other processes.

#include <stdio.h>
#include <unistd.h>

int main(void)
{
    char tmpfile[64];

    /* Build a unique filename using our PID */
    snprintf(tmpfile, sizeof(tmpfile), "/tmp/myapp_%d.tmp", getpid());

    printf("Creating unique temp file: %s\n", tmpfile);

    FILE *fp = fopen(tmpfile, "w");
    if (fp) {
        fprintf(fp, "Process %d was here\n", getpid());
        fclose(fp);
        printf("Done. File created.\n");
        /* In real code, unlink(tmpfile) to delete when done */
    }
    return 0;
}

/* Output:
   Creating unique temp file: /tmp/myapp_9250.tmp
   Done. File created.

   Each run produces a different filename because
   each process gets a unique PID. */