main() function in C

Question

I've been learning C programming in a self-taught fashion for some weeks, and there are some questions that I have concerning the main() function.

1. All functions must be declared in their function prototype, and later on, in their defintion. Why don't we have to declare the main() function in a prototype first?

2. Why do we have to use int main() instead of void main()?

3. What does return 0 exactly do in the main() function? What would happen if I wrote a program ending the main() function with return 1;, for example?

Answer 1

1. A declaration of a function is needed only before a function is used. The definition is itself a declaration, so no prior prototype is required. (Some compilers and other tools may warn if a function is defined without a prior prototype. This is intended as a helpful guideline, not a rule of the C language.)
2. Because the C standard says so. Operating systems pass the return value to the calling program (usually the shell). Some compilers will accept void main, but this is a non-standard extension (it usually means "always return zero to the OS").
3. By convention, a non-zero return value signals that an error occurred. Shell scripts and other programs can use this to find out if your program terminated successfully.