I am trying to use mutex in my multi-tread project, but they don't seem to work.
I have created a simple demonstration of the problem. This is NOT how I would use a mutex, only a demonstration of the problem:
Code:
#include <stdio.h>
#include <pthread.h>
int main()
{
int val;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
printf("Test of mutex\n");
val = pthread_mutex_lock(&mutex);
printf("Lock 1 returned %d\n",val);
val = pthread_mutex_lock(&mutex);
printf("Lock 2 returned %d\n",val);
val = pthread_mutex_trylock(&mutex);
printf("Lock 3 returned %d\n",val);
return(0);
}
Basically, I have a mutex which I lock. It returns 0, its OK.
I then try to lock it a second time. It should block (I think). The program comes straight back with val = 0?
There is also a compilation problem:
The call pthread_mutex_trylock() will compile OK (it is prototyped in pthread.h)
but it will not link! It says 'undefined reference'
It has to be commented out for the program to run.
I am running ubuntu, and GCC - v gives the following:
the first lock works and the second lock block the program because he is waiting the first lock to be unlocked. (which cannot be done since the program is waiting the lock to be unlocked before continue :])
Yes that makes the difference. It works for me now as well.
The strange behaviour is why did it link WOTHOUT the added library?
Its as though there are some dummy functions for when you don't link in the library!
Chris.
The thread functions and data types are declared in pthread.h, but the pthread functions are not included in the standard C library "libc". Instead, they are in a separate library "libpthread", so you should add lpthread to the command line when you link your program.
That's also an issue with trigonometric functions such as sin and cos, they are declared in math.h but the functions are in the library "libm". To compile programs with these functions you have to add -lm to the command line.
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