X Alloc

Custom heap memory allocator using brk and sbrk.

Usage

• Build with make.
• Copy library from target/.
• Copy header file from target/.
• Visit test.c for example use.
• Follow the api below.

API

libxalloc provides an api similar to *alloc from libc.

It includes the following functions:

• void *xmalloc(size_t)
• void *xcalloc(size_t, size_t)
• void *xrealloc(void *, size_t)
• size_t xfree(void *)

xmalloc

• param: allocation size
• return: non-NULL pointer to allocation
• return: NULL if size is 0

Note that xmalloc never returns NULL unless size of allocation is 0.

In case an allocation fails, xmalloc will directly call abort.

Therefore it's unnecessary to check for a NULL pointer when using xmalloc.

This trend is followed by all the allocator functions as all of them use xmalloc internally.

xcalloc

• param: allocation count
• param: size per count
• return: non-NULL pointer to allocation
• return: NULL if size is 0

Internally, xcalloc calls xmalloc and memsets the entire bloc with 0.

xrealloc

• param: pervious pointer
• param: new allocation size
• return: non-NULL pointer to allocation
• return: NULL if size is 0

xfree

• param: pointer
• return: total bytes released to OS

There are a few things to be noted when it comes to xfree.

If pointer is NULL, xfree returns 0 and does nothing else.

If pointer is not derived from libxalloc or was previously freed, xfree will call abort with error message invalid pointer.

Insight into xfree

Suppose the argument to xfree is a pointer ptr, and suppose ptr points to a valid memory bloc. The return value of xfree isn't the size of that memory bloc.

Instead, xfree returns the amount of bytes of memory that libxalloc was able to return to the OS during that xfree call.

For allocation and optimization purposes, xfree may only mark our memory bloc as free and then reserve it for recycling. This is because allocation of new memory in the heap is costly (and slow due to system calls involved). By reserving and recycling, we avoid that.

If such reserved memory is released back to the OS during xfree call then the total of all memory released is returned.

Warning

• Overflowing buffer allocated by libxalloc may cause an abort (see Test Fail).
• Segmentation faults during execution can be attributed to libc.
• libc will break functionality of library.

How libc affects libxalloc

libc uses brk and sbrk calls to allocate memory, and libxalloc also uses the same.

As a result, the two libraries will end up corrupting the memory tracking data structures of each other.

This will result in undefined behaviour.

To know how libxalloc works, visit working.md.

Test results

• Visit tests for details.
• Visit test-no-malloc for insight into how the allocator works.