Executing code before main()

Question

In object-oriented languages (C++) you can execute code before main() by using a global object or a class static object and have their constructors run the code you want.

Is there any way to do this in C? I don't have any specific problem I'm trying to solve, I'm just curious. One thing this might be useful for is automatically initializing a library.

Answer 1

You can do it with __attribute__ ((constructor)). I've tested the following example with both gcc and clang. That being said, it's not part of the language.

#include <stdio.h>  void __attribute__ ((constructor)) premain() {     printf("premain()\n"); }  int main(int argc, char *argv[]) {     printf("main()\n");     return 0; } 

It does the following:

$ ./test premain() main() 

GCC documents it at: https://gcc.gnu.org/onlinedocs/gcc-8.3.0/gcc/Common-Function-Attributes.html#Common-Function-Attributes