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?

Is it stackexchange's fault that you have empty includes in the first two lines? There should be an `#include ` — Frg, Mar 04 '12 at 13:24
Yes, I used the pre environment. It should now display correctly. — chtlp, Mar 04 '12 at 13:27
[Tried `gcc -pthread...`?](http://stackoverflow.com/a/1665110/993018) — sr_, Mar 04 '12 at 13:42
BTW, please compile with `-Wall`, you're missing headers. (And sr_ is correct.) — Mat, Mar 04 '12 at 13:43

score 31 · Accepted Answer · answered Mar 04 '12 at 13:45

31

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

answered Mar 04 '12 at 13:45

Karlson

5,845
32
51

1

@Karlson Can you please explain why just including the `pthread.h` file is not enough for gcc to resolve the references? – Quazi Irfan Dec 31 '16 at 21:58
3

@iamcreasy Because declaration is not the same as definition. The program needs to know where the code executing a particular function is. – Karlson Dec 31 '16 at 23:24
Sorry, could you please tell us what's the difference between `-pthread` and `-lpthread`. Some tutorial add `-l` some don't, why? – Paris Qian Sen Jun 02 '21 at 13:49

score 1 · Answer 2 · edited Nov 20 '15 at 11:34

1

gcc -o exectable_namme pthread_sample.c -lpthread

edited Nov 20 '15 at 11:34

chaos

47,463
11
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answered Nov 20 '15 at 10:50

Rodrigue SAOUNGOUMI SOURPELE

11
1

4

It would be helpful to [edit your answer](http://unix.stackexchange.com/posts/244330/edit) to explain why this is different from the already accepted answer. – roaima Nov 20 '15 at 11:17

score 0 · Answer 3 · edited Apr 08 '15 at 05:24

0

compile code using following command

gcc filename.c -lpthread -lrt

edited Apr 08 '15 at 05:24

PersianGulf

10,728
8
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answered Apr 08 '15 at 04:18

user109578

11

3

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. – dhag Apr 08 '15 at 04:28

gcc can't link to pthread?

3 Answers3