Undefined reference to pthread_create [FIXED]

In this article, we have explored the reason behind the error "Undefined reference to pthread_create" and presented two fixes in the compilation command to fix this compilation error.

Table of contents:

1. Reason for the error
2. Fix 1: pthread
3. Fix 2: lpthread

In short, the fix is to use pthread flag in compilation:

gcc -pthread -o code code.c

Reason for the error

While compiling a C code using the function pthread_create(), you may face this compilation error:

code.c: In function ‘main’:
/tmp/cc8BMzwx.o: In function `main':
term.c:(.text+0x82): undefined reference to `pthread_create'
collect2: ld returned 1 exit status

This issue can come when you are not linking the pthread library during compilation.

Following is a sample C code where pthread_create is used and where the compilation error will be faced:

#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS  64

void *run(void * id)
{
   // ... Do computation
   pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
   pthread_t threads[NUM_THREADS];
   for(iter=0; iter < NUM_THREADS; ++iter){
      pthread_create(&threads[t], NULL, run, (void *)iter);
   }
   pthread_exit(NULL);
}

Error if you compile using the command:

gcc -o code code.c

We will explore 2 fixes.

Fix 1: pthread

pthread option is used to add multi-threading support in C code with pthread library. It links both preprocessor and linker.

gcc -pthread -o code code.c

Fix 2: lpthread

Another fix is to add lpthread option to the compilation command. It adds pthread library in the linker. Depending on your system, this may fix the error while -pthread option is a complete solution.

gcc -o code code.c -lpthread

With these fixes in this article at OpenGenus, you must have the complete idea of how to fix this compilation error "Undefined reference to pthread_create".