Why it's possible to invoke a function declared later in C?

Question

I have this little code to demonstrate it:

#include "stdio.h"

int main() {
    int a = testfunc();  // declared later
    printf("%d", a);
    return 0;
}

int testfunc() {
    return 1;
}

It compiles with no error, and a outputs 1 as expected.

See in action: http://ideone.com/WRF94E

Why there is no error? Is it part of the spec of C or a compiler related thing?

Answer 1

The function testfunc() is implicitly declared. The compiler cannot do any signature checks so you might get a runtime error in case you fail to invoke it corectly.

This is part of the C specification. But the recommendation in the C specification is to declare all the functions you plan to implement and use, at the beginning of the current file or in a header file.

Answer 2

Because, by default the compiler considers the undeclared functions as

int function_name ();

Internally, an undeclared function is considered with return type as int and accepts any number of arguments.It matches with your actual function also. so no issue.