Why do programs in C compile even when the return statement is missing? [duplicate]

Answer 1

The return type of a function is optional and is considered "int" if not specified.

Answer 2

The function will not return 0 as the answer above say. In fact, printf returns a value: the number of chars printed. In your case printf, which is the last expression in your test function, will returns 3. Thus the function test will return 3.

Answer 3

I haven't seen the why addressed well in any of the answers so far. In C, it's perfectly legal for a function with a non-void return type to end without a return statement/value, as long as the caller does not attempt to use the return value. For instance (slightly nontrivial example):

#include <stdio.h>
int foo(int want_result)
{
    puts("hello");
    if (want_result) return 42;
}

int main()
{
    foo(0);
    printf("%d\n", foo(1));
}

This example is somewhat contrived, but it could actually become meaningful if the return type is a huge structure that would take non-trivial time to return.

Answer 4

It's because in C, any variable / function is implicitly int.

This is the same reason that you can use register instead of register int, or unsigned instead of unsigned int, auto instead of auto int, and static instead of static int. I personally always explicitly qualify my variables with int, but whether you do or not is your option.

Answer 5

C is an old language and at the time it was introduced, returning integers was common enough to be the default return type of a function. People later started realizing that with more complicated return types, it was best to specify int to be sure you are not forgetting the return type, but in order to maintain backwards compatibility with old code, C could not remove this default behavior. Instead most compilers issue warnings.

If the function reaches the end without a return statement an undefined value is returned except in the main function, 0 is returned. This has the same reason as above.

/* implicit declaration of printf as: int printf(int); */

/* implicit int type */
main()
{
    printf("hello, world\n");
} /* implicit return 0; */