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How do you create a new malloc() function defined in C language ?

I don't even have an elflike hint as to how to do this , how to map virtual space address with physical space , what algorithm to follow ? I do know that malloc() is not so efficient as it can cause fragmentation. So something efficient can be created. Even if not efficient , how to implement a naive C malloc function?

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  • You would need OS-specic APIs. Besides I wonder why malloc would be inefficient.. Commented Aug 10, 2012 at 16:44
  • There's a fairly decent implementation somewhere near the end of K&R. Commented Aug 10, 2012 at 16:44
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    i think it's inefficient for small allocations Commented Aug 10, 2012 at 16:44
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    @Thrustmaster: There are a number of areas where the default implementation of malloc is less efficient than a custom implementation for many platforms. For example, for heavily concurrent applications, an implementation that attempts to avoid false sharing can improve performance noticeably. Implementations that are optimized for the target platform can also be much more efficient than generalized implementations (e.g. embedded devices vs. high-memory servers). Commented Aug 10, 2012 at 16:50
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    @Thrustmaster: Added an answer with a summary of a very relevant Wikipedia entry. It outlines the different kinds of things that malloc implementations try to optimize for. That Wikipedia article is a great jumping off point for a deep dive in the world of malloc. Commented Aug 10, 2012 at 16:58

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Wikipedia actually provides a good summary of various malloc implementations, including ones optimized for specific conditions, along with links to learn more about the implementation.

http://en.wikipedia.org/wiki/C_dynamic_memory_allocation

dlmalloc

A general-purpose allocator. The GNU C library (glibc) uses an allocator based on dlmalloc.

jemalloc

In order to avoid lock contention, jemalloc uses separate "arenas" for each CPU. Experiments measuring number of allocations per second in multithreading application have shown that this makes it scale linearly with the number of threads, while for both phkmalloc and dlmalloc performance was inversely proportional to the number of threads.

Hoard memory allocator

Hoard uses mmap exclusively, but manages memory in chunks of 64 kilobytes called superblocks. Hoard's heap is logically divided into a single global heap and a number of per-processor heaps. In addition, there is a thread-local cache that can hold a limited number of superblocks. By allocating only from superblocks on the local per-thread or per-processor heap, and moving mostly-empty superblocks to the global heap so they can be reused by other processors, Hoard keeps fragmentation low while achieving near linear scalability with the number of threads.

tcmalloc

Every thread has local storage for small allocations. For large allocations mmap or sbrk can be used. TCMalloc, a malloc developed by Google, has garbage-collection for local storage of dead threads. The TCMalloc is considered to be more than twice as fast as glibc's ptmalloc for multithreaded programs.

dmalloc (not covered by Wikipedia)

The debug memory allocation or dmalloc library has been designed as a drop in replacement for the system's malloc, realloc, calloc, free and other memory management routines while providing powerful debugging facilities configurable at runtime. These facilities include such things as memory-leak tracking, fence-post write detection, file/line number reporting, and general logging of statistics.

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It's pretty surprising how selecting a malloc implementation tailored to your specific application's memory allocation needs can impact performance. Also, using something like dmalloc while developing / testing is essential to finding memory management errors before they turn into insidious bugs.
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I think it's a nice example of malloc in An example in Chapter 8.7 of C Programming Language book:

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