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/proc/self

/proc/self is a symbolic link to the directory of the «querying» process (like /proc/$$ in a Shell).

Because readlink (or ls -ld) spawns a new process, /proc/self cannot be used in a Shell to determine the Shell's PID:

$ echo $$
6184

$ readlink /proc/self
7392

$ readlink /proc/self
7393

$ ls -ld  /proc/self
lrwxrwxrwx 1 root root 0 Jan 16 07:35 /proc/self -> 7395

However, with a C program, it can be shown /proc/self points to its own PID because the same process prints the PID and executes the readlink syscall:

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

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

    char symlink_path[256];
    ssize_t len = readlink("/proc/self", symlink_path, sizeof(symlink_path)-1);
    if (len != -1) {
        symlink_path[len] = '\0';
        printf("proc/self -> %s\n", symlink_path);
    } else {
        perror("Failed to read the symlink");
    }

    return 0;
}

$ gcc pid-and-proc-self.c -o pid-and-proc-self
$ ./pid-and-proc-self
My PID 7847
proc/self -> 7847

/proc/self

See also