Question

I have some trouble with the placement of the -l option when using gcc. Here’s a stripped down version for reproduce the problem.

t.c:

#include <pthread.h>

int main() {
    pthread_create(0, 0, 0, 0);
}

and in terminal:

$ gcc -lpthread t.c  
/tmp/ccmkwV7B.o: In function `main':  
t.c:(.text+0x29): undefined reference to `pthread_create'  
collect2: ld returned 1 exit status

$ gcc t.c -lpthread  
$ (compiles ok)

Why do I have to put -lpthread in the end to make it work? And it seems that this problem only occurs on 32bit linux.

My environment info is attached below:
gcc -lpthread t.c fails on this machine.

$ gcc --version  
gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1  

$ uname -rm  
3.0.0-12-generic i686

gcc -lpthread t.c works on this machine.

$ uname -rm  
2.6.18-274.3.1.el5 x86_64  

$ gcc --version  
gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-51)  

---

I looked up the gcc manual, and it says that “the placement of -l is significant”. What exactly does it mean?

Answer 1

From the manual,

It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded.

This means it is very interesting that linking the library first works on gcc 4.1.2. This might have to do with the default libraries linked to by the compiler. I know on some installations I don’t need to explicitly link to pthreads.

On further reflection, I think the issue is with the flag --as-needed, which may be on by default in your gcc 4.6 system. See this link for some discussion.