using pthread with memory leak

Question

Just a small program to test multi-threading. It supposed to print out a 'Hello' message with the index of the thread and the thread location.

I read why pthread cause memory leak and tried to use pthread_join. It seems that the memory leak still remains

Following is my code:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <sys/types.h>
#include <unistd.h>

void runMe(int *arg) {
    printf("Hello %d from %x\n", *arg, (unsigned int)pthread_self());
    int *ret = (int*)malloc(sizeof(int));
    *ret = *arg + 4; // note this value '4' will be different in the quiz!

    pthread_exit((void*)ret);
}

int run_threads(int n) {
    pthread_t thr[n];
    int thr_args[n];
    int *ptr = 0;
    int total = 0;

    for (int i=0; i<n; i++) {
        thr_args[i] = i;
        pthread_create(&thr[i], NULL, (void*)runMe, &thr_args[i]);

    }
    for (int j=0; j<n; j++) {
        pthread_join(thr[j], (void*)ptr++);
        total += thr_args[j];
    }
    return total;
}

int main() {

    run_threads(10);

}

Following is the result of running valgrind:

==10292== 
==10292== HEAP SUMMARY:
==10292==     in use at exit: 1,654 bytes in 14 blocks
==10292==   total heap usage: 26 allocs, 12 frees, 5,454 bytes allocated
==10292== 
==10292== LEAK SUMMARY:
==10292==    definitely lost: 40 bytes in 10 blocks
==10292==    indirectly lost: 0 bytes in 0 blocks
==10292==      possibly lost: 0 bytes in 0 blocks
==10292==    still reachable: 1,614 bytes in 4 blocks
==10292==         suppressed: 0 bytes in 0 blocks
==10292== Rerun with --leak-check=full to see details of leaked memory
==10292== 
==10292== For counts of detected and suppressed errors, rerun with: -v
==10292== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Answer 1

Your leak is because the memory you allocate has no corresponding free action.

The code you're using appears to be trying to convey a dynamic allocation back to the caller. Proper use of pthread_join and its second parameter can recoup that memory pointer, which can then be properly freed.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <sys/types.h>
#include <unistd.h>

void* runMe(void *pv)
{
    int *arg = pv;
    printf("Hello %d from %x\n", *arg, (unsigned int)pthread_self());
    int *ret = malloc( sizeof *ret );
    *ret = *arg + 4; // note this value '4' will be different in the quiz!
    return ret;
}

int run_threads(int n) {
    pthread_t thr[n];
    int thr_args[n];
    int total = 0;

    for (int i=0; i<n; i++) {
        thr_args[i] = i;
        pthread_create(thr+i, NULL, runMe, thr_args+i);

    }
    for (int j=0; j<n; j++)
    {
        // reap pointer from resulting thread.
        void *res = NULL;
        pthread_join(thr[j], &res);

        int *ires = res;
        printf("ires = %p; *ires = %d\n", ires, *ires);
        free(ires);

        total += thr_args[j];
    }
    return total;
}

int main()
{
    run_threads(10);
}

Output (varies)

Hello 9 from b0a71000
Hello 0 from b05df000
Hello 7 from b096d000
Hello 2 from b06e3000
Hello 1 from b0661000
Hello 4 from b07e7000
Hello 3 from b0765000
Hello 5 from b0869000
Hello 6 from b08eb000
Hello 8 from b09ef000
ires = 0x600000; *ires = 4
ires = 0x2009e0; *ires = 5
ires = 0x600010; *ires = 6
ires = 0x500010; *ires = 7
ires = 0x2009f0; *ires = 8
ires = 0x2012c0; *ires = 9
ires = 0x600020; *ires = 10
ires = 0x400040; *ires = 11
ires = 0x400050; *ires = 12
ires = 0x500000; *ires = 13

I also took liberty to fix your incorrect, non-compliant function signature you were using. pthread_create requires the form:

void *proc(void *)

for the thread procedure. Anything not of that form isn't compliant, and should be avoided.