FILE macro will be concatenated to previous string accidentally in C

Question

This is the simplest example of my question:

#include<stdio.h>
int main() {
        printf("%s:%s\n", "I am in file" __FILE__);
}

When I forget to insert a comma before "__FILE__" macro, I expect to get a compile error, but just get a warning. So the output will be:

I am in filetest.c:[C

1st string will be : "I am in file" concatenate with __FILE__ macro
2nd string will be undefined

Could someone tell me why it is not a compile error?

To the people marking this as a duplicate of questions about string concatenation, the question is *not* about the string concatenation but why the compiler doesn't give an error for passing to few arguments to a varargs function. — Some programmer dude, Jul 26 '17 at 07:17
As Some programmer dude said: He already got how string concatenation works. OP saids: `2nd string will be undefined` because of `%s:%s`. What he want's to know is why this is not a compiler error. — Andre Kampling, Jul 26 '17 at 07:19
@Someprogrammerdude so what should I close this as? brightshine: because you're using a bad compiler (and you didn't disclose which one you're using). — Antti Haapala -- Слава Україні, Jul 26 '17 at 07:23
You say that you _just get a warning_. What warning? `gcc` gives: `warning: format ‘%s’ expects a matching ‘char *’ argument`, which seems pretty clear to me. — Hans Lub, Jul 26 '17 at 07:24
@AnttiHaapala Of what? Of questions about string-concatenations? But the question *isn't* about that, therefore such questions aren't duplicates. If you find a question about why missing arguments to vararg functions isn't an error, then I'll gladly vote to close this question, but not otherwise. — Some programmer dude, Jul 26 '17 at 07:34
The title of the question states that the concatanation is "accidental". The duplicate quenstion makes clear that such concatenations _always_ happen. — Hans Lub, Jul 26 '17 at 07:42
@HansLub: With accidental he wants to express that fact that he might forget the comma: `When I forget to insert a comma before [...]`. I think this is clear to the OP. — Andre Kampling, Jul 26 '17 at 07:43
@HansLub The title is a little misleading and should probably be edited. The question itself is IMO quite clear. — Some programmer dude, Jul 26 '17 at 07:43
@Someprogrammerdude a duplicate of "why no error of wrong number of arguments to printf" — Antti Haapala -- Слава Україні, Jul 26 '17 at 07:53
@AnttiHaapala I can't find that question on SO. Can you provide a link? — Some programmer dude, Jul 26 '17 at 07:56
For example this: https://stackoverflow.com/q/28194227/918959 even simpler. Why conflate it with string concatenation. — Antti Haapala -- Слава Україні, Jul 26 '17 at 07:56
The string concatenation is just a reasoning why the error could happen to a careful user, but at the core the question is about "why doesn't compiler give a diagnostic message when too few arguments are provided to printf when the format is known". — Antti Haapala -- Слава Україні, Jul 26 '17 at 08:00

score 5 · Answer 1 · answered Jul 26 '17 at 07:18

While it is possible for the compiler to do it in e.g. printf and scanf (because it knows about the format-strings for those functions), it can't be generally done for variable-argument functions.

If you create a vararg function, how would the compiler know what number of arguments is the correct? The answer is that it can't. Therefore the C specification doesn't say it has to be an error.

That compilers (some, not all) give warnings for printf and scanf is just because the creators of the compiler are nice enough to add it. It's not required.

__FILE__ macro will be concatenated to previous string accidentally in C

1 Answers1

FILE macro will be concatenated to previous string accidentally in C