Member Avatar for hassanalisalama

c prog that take int x and pass it to thread to return x+10
i tried to free()mem i allocate in prog using malloc
// problem i get this error Aborted (core dumped)

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
void *fun(void *TX);
int main()

{
int x =50;
void *exitstat;
pthread_t thread1;
pthread_create(&thread1, NULL, fun, &x);
pthread_join(thread1, &exitstat);
int *result=(int *)exitstat;
printf("%d",*result);
// problem is here i get this error  Aborted (core dumped)
//free(exitstat);
return 0;
}

void *fun(void *TX)
{
int *num = (int*)malloc(sizeof(int*)) ;
num=(int*)TX;
(*num)=(*num)+10;
//free(num); // can't free it here need to return num
return num;
}

note :
prog work fine (test on cygwin) if i remove free(exitstat);
but that lead to mem leak

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  • Latest Post Latest Post by deceptikon

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Generated

Short diagnosis and context: hit undefined behavior by freeing a pointer that does not come from malloc. 's question ("Why are you freeing TX?") and 's correction point to the same root cause: the thread ends up returning a pointer that refers to non-heap memory (or to memory you did not actually keep from malloc), so calling free on it typically aborts the program.

Two safe patterns to use (pick one):

  1. Caller-allocated result (no heap in the thread)
    
/* caller allocates and passes a writable buffer */
struct args { int in; int out; };

void worker(void p) {
struct args *a = p;
a->out = a->in + 10;
return NULL;
}

/ main: create a struct on the stack, pass its address, join, read a.out /


2) Thread-allocated result (heap owned by caller after join)
```c
/* thread allocates and returns a heap pointer; main frees it */
void *worker(void *p) {
    int *r = malloc(sizeof(int));
    if (!r) pthread_exit(NULL);
    *r = *(int*)p + 10;
    return r;
}

/* main: void *res; pthread_join(t,&res); use *(int*)res; free(res); */

A compact alternative when you only need a small integer is to return it via an integer<->pointer cast using intptr_t (portable way to convert integer to/from void*), but document and use it sparingly.

Quick troubleshooting checklist

  • Do not call free() on a pointer that wasn’t returned by malloc/calloc/realloc.
  • Allocate the correct size (e.g., sizeof(int) or sizeof *p).
  • Check return values from malloc, pthread_create and pthread_join.
  • Use AddressSanitizer or Valgrind to catch invalid frees and leaks.

Following one of the two safe patterns above (and checking errors) will eliminate the abort and avoid memory leaks.

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Member Avatar for Unimportant

Why are you freeing TX?

Member Avatar for deceptikon

i tried to free()mem i allocate in prog using malloc

But you don't. You try to free the result of fun(), which is a pointer to int that corresponds to the address of x in main(). In other words, you're trying to free something that should not be freed. The problem is here:

// Memory is allocated to num, all is well
int *num = (int*)malloc(sizeof(int*));

// The memory allocated above is leaked away and now num points to TX,
// which is a pointer to x in main (*not* a pointer to dynamic memory)
num=(int*)TX;

At this point you already have a memory leak, and any attempt to free num or any reference to num will fail miserably because the pointer was not previously returned by malloc(), calloc(), or realloc().

Remove that second line as it's wholely unnecessary in the first place. What you probably wanted to do is this:

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

void *fun(void *TX)

int main(void)
{
    void *exitstat;
    pthread_t t;
    int x = 50;

    pthread_create(&t, NULL, fun, &x);
    pthread_join(t, &exitstat);

    printf("%d\n", *(int*)exitstat);
    free(exitstat);

    return 0;
}

void *fun(void *TX)
{
    int *num = malloc(sizeof *num);

    *num = *(int*)TX;
    *num += 10;

    return num;
}

Why are you freeing TX?

While correct, that's not informative.

commented: Respect! +14
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