Should a C library offer ability to use custom memory allocators?

Question

I see that some C libraries have ability to specify custom memory allocators (malloc/free replacements).

• In what systems/environments/conditions is that useful? Isn't this feature just a leftover from MSDOS era or similar no-longer-relevant problems?

Background story:

I'm planning to make pngquant a library that can be embedded in various software (from iOS apps to Apache modules). I'm using malloc()/free() and my own memory pools for small allocations. I use 2MB-50MB of memory in total. I use threads, but only need to alloc on the main thread.

Answer 1

In any application where control over memory allocation is critical (for example my field, game development, or other real or near real time systems) the inability to control memory allocations in a library immediately disqualifies it from use.

Answer 2

Many malloc/free algorithms exist. The system malloc is sometimes not optimized for the task that the library is handling, so the caller might want to try a few different ones to optimize performance.

A few that come to mind are:

• dlmalloc
• jemalloc
• TCMalloc

There are also Garbage Collection libraries such as the Boehm Garbage Collector which are usable in C by calling the provided malloc/free replacements (even though free is then a dummy function call, kept for compatibility).

There are also many possible uses, for example one may write a debug malloc/free function that could trace memory allocations and liberations in the library, such as one that I wrote that uses SQLite to record statistics about how the memory is used (admittedly at the cost of performance, but it is a debugging situation).