No compile error regarding wrong function parameters

Question

I have simple programm in two units:

count_words.c:

int main( int argc, char ** argv )
{
    printf("starting\n");
    int i =  aaa(55555);
    printf("%d",i);
    printf("ending\n");
    return i;
}

clean.c:

int aaa()
{
    printf("aaa\n");
    return 5;
}

Makefile:

count_words:  clean.o count_words.o  -lfl
        gcc count_words.o clean.o -lfl -ocount_words 

%.o:%.c
        gcc -c -o $@ $<

I'm wondering why this code compiles and runs without problem. Function aaa() in clean.c has no parameters, but in count_words.c I pass 55555. Why it compiles and runs. Can I expect unexpected errors in such situations?

UPD:

I have changed int aaa() to int aaa(void), but still have warning and not error.

As you noticed I didn't include header of clean.c in count_words.c and it compiles anyway. Why then I must include headers at all?

You have to declare a function with no parameters as `aaa(void)`. Otherwise it is not standard compliant. — Eugene Sh., Jan 11 '16 at 17:26
And it is pretty much a duplicate of http://stackoverflow.com/questions/15726506/how-to-let-gcc-warn-unmatched-number-of-arguments-when-making-function-calls — Eugene Sh., Jan 11 '16 at 17:27
`int aaa()` does not have "no parameters". It has undefined parameters. — Weather Vane, Jan 11 '16 at 17:34

Vlad from Moscow · Answer 1 · 2016-01-11T17:55:28.030

The program has undefined behaviour.

According to the C Standard (6.5.2.2 Function calls)

..If the number of arguments does not equal the number of parameters, the behavior is undefined.

The function is defined as having no parameters but is called with one argument.

From the C Standard (6.7.6.3 Function declarators (including prototypes))

14....An empty list in a function declarator that is part of a definition of that function specifies that the function has no parameters.

In C++ this code will not compile.

No compile error regarding wrong function parameters

1 Answers1