C function defined as int but having no return statement in the body still compiles

Say you have a C code like this:

#include <stdio.h>

int main(){
    printf("Hello, world!\n");
    printf("%d\n", f());    
}

int f(){

}

It compiles fine with gcc, and the output 开发者_JAVA百科(on my system) is:

Hello, world!

14

But.. but.. how is that possible? I thought that C won't let you compile something like that because f() doesn't have a return statement returning an integer. Why is that allowed? Is it a C feature or compiler omission, and where did 14 come from?

The return value in this case, depending on the exact platform, will likely be whatever random value happened to be left in the return register (e.g. EAX on x86) at the assembly level. Not explicitly returning a value is allowed, but gives an undefined value.

In this case, the 14 is the return value from printf.

compile with -Wall to enable more sanity checking in the compiler.

gcc -Wall /tmp/a.c
/tmp/a.c: In function ‘main’:
/tmp/a.c:5: warning: implicit declaration of function ‘f’
/tmp/a.c:6: warning: control reaches end of non-void function
/tmp/a.c: In function ‘f’:
/tmp/a.c:10: warning: control reaches end of non-void function

Note how it flags up the missing return statements - as "control reaches end of non-void function"?

Always compile using -Wall or similar - you will save yourself heartache later on.

I can recall several occasions when this exact issue has caused hours or days of debugging to fix - it sorta works until it doesn't one day.

14 is exactly the return value of the first printf, also. Probably, the compiler optimized out the call to f() and then we're left with 14 on EAX.

Try compiling your code with different optimization levels (-O0, -O1, -O2, -O3, -Os) and see if the output changes.

18 is the return value of the first print statement. (the number of characters printed)

This value is stored in stack memory.

In second printf function the stack value is 'returned' by function f. But f just left the value that printf created in that slot on the stack.

for example, in this code:

#include <stdio.h>

int main(){
    printf("Hello, world!1234\n");
    printf("%d\n", f());    
}

int f(){

}

Which compiles fine with gcc, and the output (on my system) is:

Hello, world!

18

for details of printf() returning value mail me.

You probably have the warning level of your compiler set very low. So it is allowed, even if the result is undefined.

The default return value from a function is int. In other words, unless explicitly specified the default return value by compiler would be integer value from function.

So, the ommiting of return statement is allowed, but undefined value will be returned, if you try to use it.

I compile with -Werror=return-type to prevent this. GCC will give an error if you don't return from a function (unless it's void return).