Implementing your own malloc/free with mmap and munmap

Question

I have implemented by own malloc and free using mmap. Now since unlike free, munmap also takes length as parameter, therefore I put length as an additional information in the mapped memory.

The code for my malloc and free is shown below. I want to ask, if this code is good or am I still missing anything or doing something in a wrong way.

void * malloc ( size_t size )
{
    int *plen;
    int len = size + sizeof( size ); // Add sizeof( size ) for holding length.

    plen = mmap( 0, len, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0 );

    *plen = len;                     // First 4 bytes contain length.
    return (void*)(&plen[1]);        // Memory that is after length variable.
}

void free ( void * ptr )
{
    int *plen = (int*)ptr;
    int len;

    plen--;                          // Reach top of memory
    len = *plen;                     // Read length

    munmap( (void*)plen, len );
}

Answer 1

Some observations:

• You assume that int and size_t have the same size. If you want to store a size_t value at the head of the allocation, then why don't you just do that? Why introduce int?
• This will very likely be quite inefficient, both in terms of memory usage and speed. There is significant overhead to mmap(), and typically allocations cannot be smaller than a "page". Most real allocators try to avoid calling OS-level functionality on every malloc(), in various ways.
• If mmap() fails, it will return MAP_FAILED, and so should malloc(). Thus, you need to test for that before de-referencing the pointer returned by mmap().
• Calling free(NULL) should be a valid thing to do; with your implementation it will very likely cause a crash since you don't NULL-check the argument before assuming it's valid.