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POPortable nested functions in C
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copied!<p>Is it possible to write portable C code using nested functions/blocks?</p> <p>I understand that gcc only supports nested functions as an non-standard extension, and clang only supports blocks - but is there a way to write code that will compile on both using standard C with MACROS?</p> <p>If it is not possible - what is the best work around? As an example, how would one implement a portable version of the following sort that takes a parameter? Trivial example in GCC:</p> <pre><code>int main(int argc, char*[] argv) { char reverse = 0; int cmp_func(const void *a, const void *b) { const int* aa = (const int)a; const int* bb = (const int)b; return (reverse) ? aa - bb : bb - aa; } int list[8] = {1,2,3,4,5,20,100,200}; qsort(list, 8, sizeof(int), &cmp_func); } </code></pre> <p>A similar example could be put together using Blocks in Clang. Ideally the solution should be thread-safe (so avoid global variables).</p> <p><strong>Edit:</strong> For clarity, lets assume "standard" means C99. The above is a trivial example. What I'm after is a C99 approach to a sort that requires some parameters. Here it just uses a char as a boolean, but I'm after a solution that would take multiple integers etc. It looks like this might not be possible without global variables. </p> <p><strong>Edit 2:</strong> I realised that passing a void pointer along with a function pointer enables you to do everything that can be done with nested functions. Thanks to @Quuxplusone for suggesting <code>qsort_r</code> and <code>qsort_s</code>. I've tried to put together a portable wrapper on <code>qsort_r</code> and <code>qsort_s</code>. It takes a comparator function and a void pointer to store state in, thus removing the dependency on nested functions for intricate sorting algorithms -- so you can compile with both GCC and Clang. </p> <pre><code>typedef struct { void *arg; int (*compar)(const void *a1, const void *a2, void *aarg); } SortStruct; int cmp_switch(void *s, const void *aa, const void *bb) { SortStruct *ss = (SortStruct*)s; return (ss->compar)(aa, bb, ss->arg); } void sort_r(void *base, size_t nel, size_t width, int (*compar)(const void *a1, const void *a2, void *aarg), void *arg) { #if (defined _GNU_SOURCE || defined __GNU__ || defined __linux__) qsort_r(base, nel, width, compar, arg); #elif (defined __APPLE__ || defined __MACH__ || defined __DARWIN__ || \ defined __FREEBSD__ || defined __BSD__ || \ defined OpenBSD3_1 || defined OpenBSD3_9) SortStruct tmp = {arg, compar}; qsort_r(base, nel, width, &tmp, &cmp_switch); #elif (defined _WIN32 || defined _WIN64 || defined __WINDOWS__) SortStruct tmp = {arg, compar}; qsort_s(*base, nel, width, &cmp_switch, &tmp); #else #error Cannot detect operating system #endif } </code></pre> <p>Note: I haven't tested this on many platforms, so please let me know if you see a bug / this doesn't work on your machine. </p> <p>As an example of usage, I've implemented the same sort as in the chosen answer:</p> <pre><code>int sort_r_cmp(const void *aa, const void *bb, void *arg) { const int *a = aa, *b = bb, *p = arg; int cmp = *a - *b; int inv_start = p[0], inv_end = p[1]; char norm = (*a < inv_start || *a > inv_end || *b < inv_start || *b > inv_end); return norm ? cmp : -cmp; } int arr[18] = {1, 5, 28, 4, 3, 2, 10, 20, 18, 25, 21, 29, 34, 35, 14, 100, 27, 19}; int p[] = {20, 30}; sort_r(arr, 18, sizeof(int), sort_r_cmp, p); </code></pre>

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