Isaac Turner 2022
Portable qsort_r / qsort_s
Discussion here: https://stackoverflow.com/questions/4300896/how-portable-is-the-re-entrant-qsort-r-function-compared-to-qsort
License: Public Domain - use as you wish, no warranty

If you want to qsort() an array with a comparison operator that takes parameters you need to use global variables to pass those parameters (not possible when writing multithreaded code), or use qsort_r/qsort_s which are not portable (there are separate GNU/BSD/Windows versions and they all take different arguments).

So I wrote a portable qsort_r/qsort_s called sort_r():

void sort_r(void *base, size_t nel, size_t width,
            int (*compar)(const void *a1, const void *a2, void *aarg),
            void *arg);

base is the array to be sorted nel is the number of elements in the array width is the size in bytes of each element of the array compar is the comparison function arg is a pointer to be passed to the comparison function

Add #include "sort_r.h" to the top of your code. Then copy sort_r.h into your working directory, or add -I path/to/sort_r to your compile arguments.

Compile example code (example.c) with:

make

To build using nested functions and qsort instead of qsort_r use

make NESTED_QSORT=1

Nested functions are not permitted under ISO C, they are a GCC extension.

Public Domain. Use as you wish. No warranty. There may be bugs.