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Problem with pointers, structures, and Pthread

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# 1  
Old 05-23-2007
Problem with pointers, structures, and Pthread
Hello all,

I'm working on a small wrapper library for a bigger project, and i've been killing my self over (what I think is) a pointer problem.

Here is the code (I extracted the part of the code where the problem is for better reading, I tested the code below, and I get the same problem):
wrapper.c
Code:
#include <stdio.h>
#include <pthread.h>
#include "rwmutex.h"

rwmutex_t* create_rwmutex()
{
	rwmutex_t m;
	
	pthread_mutex_init(&m.rm, NULL);
	pthread_mutex_init(&m.wm, NULL);
	pthread_cond_init(&m.rc, NULL);

	return &m;
}

void write_lock(rwmutex_t *m)
{
	pthread_mutex_lock(&m->wm);
}

void write_unlock(rwmutex_t *m)
{
	pthread_mutex_unlock(&m->wm);
}

wrapper.h
Code:
typedef struct
{
	pthread_mutex_t wm;		//write mutex
	pthread_mutex_t rm;		//read mutex
	pthread_cond_t rc;		//read condition var

} rwmutex_t;

rwmutex_t* create_rwmutex ();
void write_lock (rwmutex_t *m);
void write_unlock (rwmutex_t *m);

tester.c
Code:
#include <stdio.h>
#include <pthread.h>
#include "rwmutex.h"

void* f1(void* p);

rwmutex_t *m;
pthread_t t1, t2;

int main(int argc, char* argv[])
{
	m = create_rwmutex();

	pthread_create(&t1, NULL, f1, 4);
	sleep(1);
	pthread_create(&t2, NULL, f1, 5);
	
	pthread_join(t1, NULL);
	pthread_join(t2, NULL);

	return 0;
}

void* f1(void* p)
{
	int i = (int)p;
	write_lock(m);
	printf("[%d] WRITE IN\n", i);
	sleep(2);
	printf("[%d] WRITE OUT\n", i);
	write_unlock(m);
}

After compiling with 'gcc wrapper.c tester.c -o t -lpthread' .. I get some warnings.
When I execute './t' I get the output:
Code:
[4] WRITE IN
[5] WRITE IN
[4] WRITE OUT
[5] WRITE OUT

What I should get is a synchronized output, that is, the second thread waits for the first thread to complete, so the output should be:
Code:
[4] WRITE IN
[4] WRITE OUT
[5] WRITE IN
[5] WRITE OUT

I'm running this on a Windows environment using Cygwin.

Thanks in advance for your time,
Naimi
# 2  
Old 05-23-2007
Code:
#include <stdio.h>
#include <pthread.h>
#include "rwmutex.h"

rwmutex_t* create_rwmutex()
{
	rwmutex_t m;
	
	pthread_mutex_init(&m.rm, NULL);
	pthread_mutex_init(&m.wm, NULL);
	pthread_cond_init(&m.rc, NULL);

	return &m;
}

You shot yourself in the foot right at the start. You can't return a pointer to memory on the stack and expect it to be valid when it goes out of scope.

You either need to make it static or use malloc.
# 3  
Old 05-23-2007
Thank you Porter,
I changed my code as you said to
Code:
rwmutex_t* create_rwmutex()
{
	rwmutex_t *m;
	m = (rwmutex_t*)malloc(sizeof(rwmutex_t));
	
	pthread_mutex_init(&m->rm, NULL);
	pthread_mutex_init(&m->wm, NULL);
	pthread_cond_init(&m->rc, NULL);

	return m;
}

Now it is working fine.
Thanks again.
Cheers
# 4  
Old 12-19-2007
pls brother, may i get a copy of this code - Read/Write Locks with Quota code with its methodes bcoz i need it in my studies Smilie
# 5  
Old 12-19-2007
Quote:
Originally Posted by just student
bcoz i need it in my studies Smilie
FYI: Rule 6, no homework.
# 6  
Old 12-19-2007
Quote:
Originally Posted by porter
FYI: Rule 6, no homework.
just some mercy mr porterSmilie
# 7  
Old 12-19-2007
... but how will you learn?
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