Thread: Weird compile errors (gcc, pthread, math)

Hi. I'm having compile errors with the following code which I cannot debug:

Code:

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define NTHREADS 32

void *Hello(void *threadid){
	int i;
	double result = 0.0;
	sleep(3);
	
	for(i = 0; i < 1000; i++){
		double di = (double) i;
		result += sin(di) * tan(di);
	}
	
	printf("From thread %d:, Hello world!\n", (int) threadid);
	
	pthread_exit(NULL);
}

int main(){
	pthread_t threads[NTHREADS];
	int rc;
	int t;
	
	for(t = 0; t < NTHREADS; t++){
		rc = pthread_create(&threads[t], NULL, Hello, (void *) t);
		
		if(rc){
			printf("OMG BEARZZZZ with return code %d\n", rc);
			exit(-1);
		}
	}
	
	pthread_exit(NULL);
}

The errors:

Code:

/tmp/ccaU3MYJ.o: In function `Hello':
threads-math.c:(.text+0x2d): undefined reference to `sin'
threads-math.c:(.text+0x3b): undefined reference to `tan'
/tmp/ccaU3MYJ.o: In function `main':
threads-math.c:(.text+0xc3): undefined reference to `pthread_create'
collect2: ld returned 1 exit status

I even try compiling with the following options but still to no avail:

Code:

gcc -L /usr/include/pthread.h /usr/include/math.h -o threads-math threads-math.c

I am particularly frustrated by math.h . The following code compiles well:

Code:

#include <stdio.h>
#include <math.h>

main(){
	double test = sin(42.0);
	printf("sin(42) = %f\n", test);
}

I am compiling under Ubuntu 9.10 with gcc version: gcc (Ubuntu 4.4.1-4ubuntu9) 4.4.1. Any ideas why the "undefined reference"s?

You need to link in the math library with -lm.

Why don't you need to in your second example? Because gcc has built-in support for some math functions, and can decide to use its built-in math functions when it thinks it's appropriate. If you build your larger program with optimization (-O2 for example), that might get gcc to use the built-in versions of sin() and tan(); but even so, you should still use -lm to link in the math library.

It's kind of silly that some standard functions are not part of libc, but it's done for historical reasons: years ago linkers were not as smart as they are today and by not linking in the math library you avoided bloating your program. Not so with today's libraries (though I suppose there still may exist systems with unintelligent linkers).

It's also a bad idea to cast an int to a void* when passing it to pthread_create(). There's no guarantee that such a conversion will work. In your case, you can pass the address of your int and things will be fine (assuming you modify Hello() appropriately). Also, you should include unistd.h for sleep().

Also, trigonometric functions from math.h such as sin(), cos() and tan() all take their argument expressed in radians. You may know this already but it's hard to tell from your code.

To convert from degrees to radians you multiply by pi/180.

Replies...

You need to link in the math library with -lm.

That solves my problems with math.h. Thank you.

Also, trigonometric functions from math.h such as sin(), cos() and tan() all take their argument expressed in radians. You may know this already but it's hard to tell from your code.

I didn't know this but my main agenda is pthreads so it doesn't matter. Thank you for the info though.

It's also a bad idea to cast an int to a void* when passing it to pthread_create(). There's no guarantee that such a conversion will work.

Err...not really my code. I'm just following a threads tutorial I got from school. So, while I'm at it...

1) Can anyone give me a link to a nice pthreads tutorial? The one I'm currently working on just gives me a bunch of codes without explaining anything (like why it casted a void* to int).

2) I'm still having compile errors with pthread. My compile call is

Code:

gcc -L /usr/include/pthread.h -lm -o threads-math threads-math.c

And I get the following error:

Code:

/tmp/ccIxQjDy.o: In function `main':
threads-math.c:(.text+0xc3): undefined reference to `pthread_create'
collect2: ld returned 1 exit status

Any thoughts?

Ooops. While I typed my lengthy update, Bayint posted a reply, which might've answered my new issues. Working on it now till I get new questions. Thanks Bayint!

This example performs argument passing incorrectly. It passes the address of variable t, which is shared memory space and visible to all threads. As the loop iterates, the value of this memory location changes, possibly before the created threads can access it.

Nice catch. I didn't look at how the variable was actually being used; big mistake!