gcc can't link to pthread?

Question

I have recently installed XUbuntu 11.10 64bit, but I am having problem compiling the most simple pthread example.

Here is the code pthread_simple.c:

#include <stdio.h>
#include <pthread.h> 
main()  {
  pthread_t f2_thread, f1_thread; 
  void *f2(), *f1();
  int i1,i2;
  i1 = 1;
  i2 = 2;
  pthread_create(&f1_thread,NULL,f1,&i1);
  pthread_create(&f2_thread,NULL,f2,&i2);
  pthread_join(f1_thread,NULL);
  pthread_join(f2_thread,NULL);
}
void *f1(int *x){
  int i;
  i = *x;
  sleep(1);
  printf("f1: %d",i);
  pthread_exit(0); 
}
void *f2(int *x){
  int i;
  i = *x;
  sleep(1);
  printf("f2: %d",i);
  pthread_exit(0); 
}

And here is the compile command

gcc -lpthread pthread_simple.c

The results:

lptang@tlp-linux:~/test/test-pthread$ gcc -lpthread pthread_simple.c 
/tmp/ccmV0LdM.o: In function `main':
pthread_simple.c:(.text+0x2c): undefined reference to `pthread_create'
pthread_simple.c:(.text+0x46): undefined reference to `pthread_create'
pthread_simple.c:(.text+0x57): undefined reference to `pthread_join'
pthread_simple.c:(.text+0x68): undefined reference to `pthread_join'
collect2: ld returned 1 exit status

Does anyone know what's causing the problem?

Solution 1

In the latest versions of gcc compiler require that libraries follow the object or source files.

So to compile this it should be:

gcc pthread_sample.c -lpthread

Normally though pthread code is compiled this way:

gcc -pthread pthread_sample.c

Solution 2

gcc -o exectable_namme pthread_sample.c -lpthread

Comments

[Frg]

Is it stackexchange's fault that you have empty includes in the first two lines? There should be an #include <pthread.h>

[chtlp]

Yes, I used the pre environment. It should now display correctly.

[sr_]

Tried gcc -pthread...?

[Mat]

BTW, please compile with -Wall, you're missing headers. (And sr_ is correct.)

[dhag]

Hi! It would be helpful if you could edit your answer to explain why it works, and to highlight why you think it adds something the already accepted answer doesn't cover.

[roaima]

It would be helpful to edit your answer to explain why this is different from the already accepted answer.

[Quazi Irfan]

@Karlson Can you please explain why just including the pthread.h file is not enough for gcc to resolve the references?

[Karlson]

@iamcreasy Because declaration is not the same as definition. The program needs to know where the code executing a particular function is.

[Paris Qian Sen]

Sorry, could you please tell us what's the difference between -pthread and -lpthread. Some tutorial add -l some don't, why?