Query on return value of Thread Function in C

Question

I am using below function which will be launched as seperate thread using pthread_create() api.

static void * threadFunc(void *arg)
{
    char *s = (char *) arg;
    printf("%s", s);
    return (void *) strlen(s);
}

My question:

If the value of strlen(s) is 8(say), On some architectures, return values are stored in %eax. Does this value 8 be visible to main thread in pthread_join() call that launched the above thread?

every program has global/stack/heap segment. I guess return values should be in global segment otherwise pthread_join() would not be able to access it, if return value is from stack area. — overexchange, Jun 06 '14 at 12:34
To have `pthread_join()` pull a value, use `pthread_exit()` to return from a (P)thread. — alk, Jun 06 '14 at 12:39
In general, C function call, as per my knowledge return value is stored in accumulator(AC) registre and caller will read that accumulator. but how strlen() works i dont know — overexchange, Jun 06 '14 at 12:40
@alk In 'void pthread_exit(void *retval);' retval should also not point to stack area. may be we can concentrate on what is currently happening with '(void *)strelen(s)' as per my 2 questions — overexchange, Jun 06 '14 at 12:41
I did not say that. And your example wouldn't to this either if you'd passed the argument to `return` to `pthread_exit()` instead. It would pass/return `strlen()`'s `size_t` typed result of `8` camouflaged as a `void*`. Dirty, dirty ... ;-) — alk, Jun 06 '14 at 12:42
One question per question please. This is not a message board. — Lightness Races in Orbit, Jun 06 '14 at 13:01

score 0 · Accepted Answer · answered Jun 06 '14 at 13:07

0

1) Nope. strlen returns a size_t value on the stack (or in a register depending on the architecture). strlen won't allocate any memory to hold its result.

2) Casting it's return value to (void *) is a pretty bad idea. All you are doing is taking a size_t value (say 8 in your example) and telling the compiler to pretend it is a pointer to a memory area.

None of this is really pthread specific, just normal C. If you want to use the void * as intended, the idea is that you return a pointer to some data structure you have allocated which then needs to be free by the caller.

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

void *testFunc( char * );


void *testFunc( char *s ){
  size_t *returnMemory = malloc( sizeof( size_t ) );
  size_t len = strlen(s);
  *returnMemory = len;
  return returnMemory;
}

int main( int argc, char *argv[] ){
  void *valPtr = testFunc( "Test String" );
  size_t *sizePtr = (size_t *)valPtr;

  printf( "%p => %zu\n", valPtr, *sizePtr );
  exit(0);
}

answered Jun 06 '14 at 13:07

tad

506
3
7

even if i return just 'strlen(s)' implementation internally casts to void * otherwise pthread_join() cannot accept the returned value. – overexchange Jun 06 '14 at 13:18
Yes. The function is defined to return a void *. The point is, if you are sneaking a non-pointer value into it, you will find yourself broken at some point. The intent is that you return a pointer to a data structure that can then be safely used by the parent thread. Instead of passing a char * in the void *arg, you could pass a structure with space for both. – tad Jun 06 '14 at 13:51
here returnMemory is passing its value to valPtr, which is heap address, but if i launch testFunc() on a thread then second arg of pthread_join(t,void**) will store &returnMemory, which is stack area. – overexchange Jun 06 '14 at 15:07
You might want to have a look at http://stackoverflow.com/questions/8513894/pthread-join-and-pthread-exit – tad Jun 06 '14 at 15:37

Query on return value of Thread Function in C

1 Answers1