Get Thread ID in C

  1. Use the pthread_self Function to Get Thread ID in C
  2. Use the gettid Function to Get Thread ID in C
  3. Use the thrd_current Function to Get Thread ID in C

This article will demonstrate multiple methods about how to get thread ID in C.

Use the pthread_self Function to Get Thread ID in C

Threads are the cornerstone of performance on contemporary CPUs as the latter tend to add more virtual or physical cores nowadays to support better multi-threaded workflows. Generally, a thread is denoted as a single flow of control in a process (i.e., running program). Thus, threads are used to implement multiple logic flows that would execute concurrently and form a program that utilizes multiple CPU cores. Note that POSIX threads have been the oldest standard interface for accessing threading facilities in C programs. pthread_self is one of the functions provided by the pthreads API that can retrieve the ID of the calling thread. It takes zero arguments and returns the integer denoting the thread ID as the pthread_t type variable.

In the following example code, we implemented the basic multi-threaded scenario, where the main program(thread) creates additional four threads that execute the printHello function and terminate using the pthread_exit function call. Notice that five threads are running during this program, and all exit after the given code path is executed. In the printHello function, all of them print their thread ID using the pthread_self function.

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

#ifndef NUM_THREADS
#define NUM_THREADS 4
#endif

void *printHello(void *threadid) {
    long tid;
    tid = (long)threadid;
    printf("Hello There! thread %ld, pthread ID - %lu\n", tid, pthread_self());
    pthread_exit(NULL);
}


int main(int argc, char const *argv[]) {
    pthread_t threads[NUM_THREADS];
    int rc;
    long t;

    for (t = 0; t < NUM_THREADS; t++) {
        rc = pthread_create(&threads[t], NULL, printHello, (void *)t);
        if (rc) {
            printf("ERORR; return code from pthread_create() is %d\n", rc);
            exit(EXIT_FAILURE);
        }
    }
    pthread_exit(NULL);
}

Output:

Hello There! thread 1, pthread ID - 140388002486016
Hello There! thread 0, pthread ID - 140388010878720
Hello There! thread 2, pthread ID - 140387994093312
Hello There! thread 3, pthread ID - 140387985700608

Use the gettid Function to Get Thread ID in C

gettid is a Linux-specific system call that is provided using the function wrapper in the C program, and it returns the caller’s thread ID. The function takes no arguments similar to the pthread_self and returns the pid_t type integer value. Note that the value returned by the gettid call is not the same as the ID retrieved from the pthread_self function, which is called POSIX thread ID. If the program is single-threaded, gettid returns the same value as the getpid - equal to the process ID.

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

#ifndef NUM_THREADS
#define NUM_THREADS 4
#endif

void *printHello(void *threadid) {
    long tid;
    tid = (long)threadid;
    printf("Hello There! thread %ld, kthread ID - %d\n", tid, gettid());
    pthread_exit(NULL);
}


int main(int argc, char const *argv[]) {
    pthread_t threads[NUM_THREADS];
    int rc;
    long t;

    for (t = 0; t < NUM_THREADS; t++) {
        rc = pthread_create(&threads[t], NULL, printHello, (void *)t);
        if (rc) {
            printf("ERORR; return code from pthread_create() is %d\n", rc);
            exit(EXIT_FAILURE);
        }
    }
    pthread_exit(NULL);
}

Output:

Hello There! thread 0, kthread ID - 10389
Hello There! thread 1, kthread ID - 10390
Hello There! thread 2, kthread ID - 10391
Hello There! thread 3, kthread ID - 10392

Use the thrd_current Function to Get Thread ID in C

thrd_current is part of ISO C threads API added to the standard language specification in 2011. Note that this API provides a standard interface for POSIX compatible operating systems and any platform that provides a standard-compliant C compiler. Thus, this method is recommended way of interacting with threads in the C language. We use the pthread functions for creating and exiting the threads in the following example, though, one should choose a single interface (Pthreads or ISO C) when working on a project.

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

#ifndef NUM_THREADS
#define NUM_THREADS 4
#endif

void *printHello(void *threadid) {
    long tid;
    tid = (long)threadid;
    printf("Hello There! thread %ld, pthread ID - %lu\n", tid, thrd_current());
    pthread_exit(NULL);
}


int main(int argc, char const *argv[]) {
    pthread_t threads[NUM_THREADS];
    int rc;
    long t;

    for (t = 0; t < NUM_THREADS; t++) {
        rc = pthread_create(&threads[t], NULL, printHello, (void *)t);
        if (rc) {
            printf("ERORR; return code from pthread_create() is %d\n", rc);
            exit(EXIT_FAILURE);
        }
    }
    pthread_exit(NULL);
}

Output:

Hello There! thread 0, pthread ID - 139727514998528
Hello There! thread 1, pthread ID - 139727506605824
Hello There! thread 2, pthread ID - 139727498213120
Hello There! thread 3, pthread ID - 139727489820416
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