Question

I am creating a thread for each request from the client to get the files on the server. Thread function normally gets a void pointer; but I need to give it a char pointer as a parameter and want it to be populated with the file names by the thread function.

Code creates a thread:

pt_ret = pthread_create(&thread_id, NULL, getfiles, (void*) thread_buff);
pthread_join(thread_id, pt_ret);

Def. of thread function:

void *getfiles(void *ptr) {
    /* ... */
    char buff[256]; // populating that local buffer with the file names
    // ptr should be as a return of buff
}

I have tried different things but each time after that thread completes, thread_buff becomes just ‘Q’.

Answer 1

Just cast it to a char*, since you know it actually is a char*:

void *getfiles(void *ptr) {
    /* ... */
    const char *ptr_char = (const char*)ptr;
    char buff[256]; 
    memcpy(buff, ptr_char, 256); //just as an example, check the sizes.
    //you could also strcpy, that's up to you
}

Or you could also just work on the buffer pointed by ptr, without copying it, so it’s accessible after the thread has ended:

void *getfiles(void *ptr) {
    /* ... */
    char *buff = (char*)ptr;
    /* do stuff with buff. */
}