Why is return 0 optional?

Question

Why, if I write

int main()  {      //...  } 

do I not need to write return 0; at the end of the main function? Does the compiler do it for me?

I use GCC / C99.

Answer 1

C99 and C++ special case the main function to return 0 if control reaches the end without an explicit return. This only applies to the main function.

The relevant bit of the C99 spec is 5.1.2.2.3 for the main special case

5.1.2.2.3 Program termination

If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value
returned by the main function as its argument; reaching the } that terminates the main function returns a value of 0.

6.9.1/12

If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.

You can test this out with gcc:

int foo ( void ) { } int main( void ) { } 

C89 mode ( errors for both functions ):

sandiego:$ gcc src/no_return.c -std=c89 -Wall  src/no_return.c: In function ‘main’: src/no_return.c:2: warning: control reaches end of non-void function src/no_return.c: In function ‘foo’: src/no_return.c:1: warning: control reaches end of non-void function 

C99 mode ( main is a special case ) :

sandiego:$ gcc src/no_return.c -std=c99 -Wall src/no_return.c: In function ‘foo’: src/no_return.c:1: warning: control reaches end of non-void function