data type conversion in C

Question

#include <pthread.h>
#include "../header/tlpi_hdr.h"

static void *
threadFunc(void *arg)
{
    char *s = (char *) arg;

    printf("%s", s); 

    return (void *) strlen(s);
}

int
main(int argc, char *argv[])
{
    pthread_t t1; 
    void *res;
    int s;

    s = pthread_create(&t1, NULL, threadFunc, "Hello world\n");
    if (s != 0)
        errExitEN(s, "pthread_create");

    printf("Message from main()\n");
    s = pthread_join(t1, &res);
    if (s != 0)
        errExitEN(s, "pthread_join");

    printf("Thread returned %ld\n", (long) res);

    exit(EXIT_SUCCESS);
}

Above code is just what I'm studying now, but I can't understand about conversion of data type.

On threadFunc(), the return type is void *. But from this function, strlen(s) is type size_t. But how this can be changed to (void *). I mean, why not (void *) &strlen(s) ?

And, also in the main(), there is printf("Thread returned %ld\n", (long) res);.

From this, variable res is the type (void *), not int or long. So, it should be changed like this:

(long) *res.

But, how this can be changed to (long) res?

Answer 1

Here strlen returns an integer which is cast to void * pointer and later back to integer in printf call.

Regarding conversions between pointers and integers, C99 standard only guarantees that void * pointer can be converted to uintptr_t or intptr_t integer and then back to void * pointer and the result will compare equal to the original pointer, if those optional types are defined (Reference).

The conversion done here is different and not guaranteed to work, but since nearly all modern computers internally represent pointers as integers, it will generally work as long as integer is of same size as void * pointer.

So if you know how your compiler handles this kind of conversion, then you can do something like this, but in general this should not be done.

Proper way to return integer value would be to e.g. allocate space for the integer value, and then return pointer to it.

Answer 2

But how this can be changed to (void *). I mean, why not (void *) &strlen(s)(?)

The return type of strlen() is size_t, some unsigned integer type. Casting an arbitrary integer value to void* is undefined behavior (UB).

return (void *) strlen(s); // Undefined behavior

Code could save the length somewhere, say in static/global or allocated memory and return the address. I have doubts this would meets OP's other coding goals.

static size_t len = 0;
len = strlen(s);
return (void *) &len;

The return value from strlen() 1) cannot have its address taken with &strlen(...) 2) Even if the language allowed such, code still needs to consider the lifetime of the pointer.

how this can be changed to (long) res?

It is important that the calling code interprets the returned pointer as the same type to prevent UB.

void *res;
...
s = pthread_join(t1, &res);
if (s != 0) {
  errExitEN(s, "pthread_join");
}
if (res != NULL)
  size_t *len_ptr = (size_t*) res;
  printf("Thread returned %zu\n", *len_ptr);
  // or if long is truly needed
  long len_as_long = (long) *len_ptr;
  printf("Thread returned %ld\n", len_as_long);
}

Answer 3

You cannot return (void *) &strlen(s) because you will get an error. What you want to do is var = strlen(s); return (void *) &var;. But in fact this is not correct because you are returning a pointer to a variable that will be destroyed at the end of the function, so your pointer will be pointing to an empty or overrided part of memory. It's a little strange what pthread function is doing, because you get the value of the position 12. This is a non sense thing, except to check that pthread was running. And strlen returns size_t, that can be an int or short o something like that, according to What is size_t in C?