22.6 The main Function

Every complete executable program requires at least one function, called main, which is where execution begins. You do not have to explicitly declare main, though GNU C permits you to do so. Conventionally, main should be defined to follow one of these calling conventions:

int main (void) {…}
int main (int argc, char *argv[]) {…}
int main (int argc, char *argv[], char *envp[]) {…}

Using void as the parameter list means that main does not use the arguments. You can write char **argv instead of char *argv[], and likewise for envp, as the two constructs are equivalent.

You can call main from C code, as you can call any other function, though that is an unusual thing to do. When you do that, you must write the call to pass arguments that match the parameters in the definition of main.

The main function is not actually the first code that runs when a program starts. In fact, the first code that runs is system code from the file crt0.o. In Unix, this was hand-written assembler code, but in GNU we replaced it with C code. Its job is to find the arguments for main and call that.

• Returning Values from main
• Accessing Command-line Parameters
• Accessing Environment Variables