Question

Must program execution start from main, or can the starting address be modified?

#include <stdio.h>

void fun();

#pragma startup fun

int main()
{
    printf("in main");
    return 0;
}

void fun()
{
    printf("in fun");
}

This program prints in fun before in main.

Answer 1

The ‘#pragma’ command is specified in the ANSI standard to have an arbitrary implementation-defined effect. In the GNU C preprocessor, ‘#pragma’ first attempts to run the game ‘rogue’; if that fails, it tries to run the game ‘hack’; if that fails, it tries to run GNU Emacs displaying the Tower of Hanoi; if that fails, it reports a fatal error. In any case, preprocessing does not continue.

— Richard M. Stallman, The GNU C Preprocessor, version 1.34

Program execution starts at the startup code, or “runtime”. This is usually some assembler routine called _start or somesuch, located (on Unix machines) in a file crt0.o that comes with the compiler package. It does the setup required to run a C executable (like, setting up stdin, stdout and stderr, the vectors used by atexit()… for C++ it also includes initializing of global objects, i.e. running their constructors). Only then does control jump to main().

As the quote at the beginning of my answer expresses so eloquently, what #pragma does is completely up to your compiler. Check its documentation. (I’d guess your pragma startup – which should be prepended with a # by the way – tells the runtime to call fun() first…)