getting error in c program "undefined reference to gettid"

Question

This is my thread sub routine... Here, I am creating 4 threads and passing structure as a argument to thread sub routine.

I am trying to print thread id with getid() function,

I am getting error saying "undefined reference to gettid()".

I have added necessary header files...

#include <pthread.h>
#include <stdio.h>
#include <sys/types.h>
#define ARRAYSIZE 17
#define NUMTHREADS 4

struct ThreadData {
        int start, stop;
        int* array; 
};

void* squarer(void* td) 
{
     struct ThreadData* data=(struct ThreadData*) td;

     int start=data->start;
     int stop=data->stop;
     int* array=data->array;
     int i;
     pid_t tid1;

     tid1 = gettid(); //error at this statement//`
     printf("tid : %d\n",tid1);

     for (i=start; i<stop; i++) {
         sleep(1);
         array[i]=i*i;
         printf("arr[%d] = [%d]\n",i,array[i]);
     } 
   return NULL;
}

int main(void) {
    int array[ARRAYSIZE];
    pthread_t thread[NUMTHREADS];
    struct ThreadData data[NUMTHREADS];
    int i;

    int tasksPerThread=(ARRAYSIZE+NUMTHREADS-1)/NUMTHREADS;

    for (i=0; i<NUMTHREADS; i++) {
            data[i].start=i*tasksPerThread;
            data[i].stop=(i+1)*tasksPerThread;
            data[i].array=array;
    }

    data[NUMTHREADS-1].stop=ARRAYSIZE;

    for (i=0; i<NUMTHREADS; i++) {
            pthread_create(&thread[i], NULL, squarer, &data[i]);
    }

    for (i=0; i<NUMTHREADS; i++) {
            pthread_join(thread[i], NULL);
    }

    for (i=0; i<ARRAYSIZE; i++) {
            printf("%d ", array[i]);
    }
    printf("\n");

    return 0;
}

Crutchfield

If you are using libc, gettid() is not implemented. You need to create it yourself – Photomechanical

@Photomechanical Thanks for the answer....! how to implement gettid function – Crutchfield

Never tried it myself. But according to the link below, you can simply (long int)syscall(224). Maybe that helps you ubuntuforums.org/showthread.php?t=345317 – Photomechanical

@Photomechanical Do not hardcode syscall ids in your code please. It can be different on different Linux distros. Mine (Red Hat 6) for instance has gettid on ID 186. Use the SYS_* Macros instead. – Vickery

@Photomechanical thanks for the information, I have checked the syscall.h file and replaced the syscall gettid function id with sys_gettid instade of using 224 / 186. now the statement is tid1 = syscall(SYS_gettid);. – Crutchfield

syscall numbers are definitely different on different archs. be aware - use SYS_gettid instead. – Furbelow

Answer 1

Try

#include <unistd.h>
#include <sys/syscall.h>

#ifdef SYS_gettid
pid_t tid = syscall(SYS_gettid);
#else
#error "SYS_gettid unavailable on this system"
#endif

Vickery

On Linux Ubuntu 18.04 with gcc --version gcc (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0 I had to also add #define _GNU_SOURCE above all of the includes in order for syscall() to be defined. I learned that from the example at the bottom of this documentation here: man7.org/linux/man-pages/man2/syscall.2.html. – Reeves

Answer 2

Macro to be pasted (improved over previous answer):

#include <unistd.h>
#include <sys/syscall.h>

#ifndef SYS_gettid
#error "SYS_gettid unavailable on this system"
#endif

#define gettid() ((pid_t)syscall(SYS_gettid))

Example:

#include <unistd.h>
#include <sys/syscall.h>

#ifndef SYS_gettid
#error "SYS_gettid unavailable on this system"
#endif

#define gettid() ((pid_t)syscall(SYS_gettid))

#include <stdio.h>

void main()
{
        printf("tid is %d\n", gettid());
}

Laryngology

Awesome! Best answer. Works on Ubuntu 20.04 with glibc Version 2.31-0ubuntu9.2, as shown by apt show libc6. – Reeves

Answer 3

If you're using glibc or musl, define _GNU_SOURCE to have this symbol defined by unistd.h.

Angelinaangeline

I solved my similar problem by define _GNU_SOURCE. Last question : gcc -D_GNU_SOURCE source.c solve problem. Adding #define _GNU_SOURCE in code, before #include <unistd.h> have no effect. – Flump

Answer 4

I have followed the suggestions provided on errro and corrected the source *below is the error free code *

#include <pthread.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/syscall.h>

#define ARRAYSIZE 17
#define NUMTHREADS 4

struct ThreadData {
    int start, stop;
    int* array; 
};

void* squarer(void* td) 
{
 struct ThreadData* data=(struct ThreadData*) td;

 int start=data->start;
 int stop=data->stop;
 int* array=data->array;
 int i;
 pid_t tid1;

 tid1 = syscall(SYS_gettid); // here is the correct statement //
 printf("tid : %d\n",tid1);

 for (i=start; i<stop; i++) {
         sleep(1);
         array[i]=i*i;
         printf("arr[%d] = [%d]\n",i,array[i]);
 } 
 return NULL;
}

int main(void) {
    int array[ARRAYSIZE];
    pthread_t thread[NUMTHREADS];
    struct ThreadData data[NUMTHREADS];
    int i;

    int tasksPerThread=(ARRAYSIZE+NUMTHREADS-1)/NUMTHREADS;

    for (i=0; i<NUMTHREADS; i++) {
        data[i].start=i*tasksPerThread;
        data[i].stop=(i+1)*tasksPerThread;
        data[i].array=array;
    }

    data[NUMTHREADS-1].stop=ARRAYSIZE;

    for (i=0; i<NUMTHREADS; i++) {
            pthread_create(&thread[i], NULL, squarer, &data[i]);
    }

    for (i=0; i<NUMTHREADS; i++) {
            pthread_join(thread[i], NULL);
    }

    for (i=0; i<ARRAYSIZE; i++) {
            printf("%d ", array[i]);
    }
    printf("\n");

    return 0;
}

Crutchfield