Question

I do the following:

void * myFunction(void) {
    void *someBytes = malloc(1000);
    // fill someBytes

    //check the first three bytes (header)
    if(memcmp(someBytes, "OK+", 3) == 0) {
        // move the pointer (jump over the first three bytes)
        someBytes+=3
    }

    return someBytes;
}

How can the receiver free the malloced pointer?
Of course I could do a -3 on the pointer.

But is there a best practice for that case?
Is there a easy solution for still allowing in the receiver function to call free(someBytes);
Because someBytes could also held multiple megabytes I’d like to avoid memcpy (malloc(1000) is for the example only).

Answer 1

There isn’t any way (unless you happen to know the exact offset). Best practice is to store a copy of the original pointer so you can later use it to free the memory.

void* myFunction(void) {
    void* someBytes = malloc(1000);
    void* pos = someBytes;
    // fill someBytes

    //check the first three bytes (header)
    if(memcmp(pos, "OK+", 3) == 0) {
        // move the pointer (jump over the first three bytes)
        pos+=3
    }

    return someBytes;
}