Question

I am a beginner in C. While reading git’s source code, I found this wrapper function around malloc.

void *xmalloc(size_t size)
{
    void *ret = malloc(size);
    if (!ret && !size)
        ret = malloc(1);
    if (!ret) {
        release_pack_memory(size, -1);
        ret = malloc(size);
        if (!ret && !size)
            ret = malloc(1);
        if (!ret)
            die("Out of memory, malloc failed");
    }
#ifdef XMALLOC_POISON
    memset(ret, 0xA5, size);
#endif
    return ret;
}

Questions

1. I couldn’t understand why are they using malloc(1)?
2. What does release_pack_memory does and I can’t find this functions implementation in the whole source code.
3. What does the #ifdef XMALLOC_POISON memset(ret, 0xA5, size); does?

I am planning to reuse this function on my project. Is this a good wrapper around malloc?

Any help would be great.

Answer 1

1. malloc(0) does not work on all a platforms, in which case a one-byte allocation is made instead. Allowing the allocation of 0-length memory blocks simplifies the higher-level logic of the program.

2. Don’t know.

3. By filling the allocated memory with a non-zero value, it is easier to find bugs in the program where the memory is used without proper initialization: the program will crash almost immediately in such cases. As filling the memory takes time, it is wrapped in a preprocessor define, so it is compiled in only when desired.