path: valid or not in `#include`?

Question

Today I read this question Any rules about underscores in filenames in C/C++?, and I found it very interesting that the standard seems to not allow what is usually seen in many libraries (I also do it in my personal library this way):

For example, in opencv we can see this:

// File: opencv/include/opencv2/opencv.hpp

#include "opencv2/opencv_modules.hpp"

But the standard says:

§ 6.10.2 Source file inclusion

Semantics

5 The implementation shall provide unique mappings for sequences consisting of one or more nondigits or digits (6.4.2.1) followed by a period (.) and a single nondigit. The first character shall not be a digit. The implementation may ignore distinctions of alphabetical case and restrict the mapping to eight significant characters before the period.

nondigit means letters (A-Z a-z) and underscore _.

It says absolutely nothing about / which would imply that it is forbidden to use a path, not to mention dots or hyphens in file names.

To test this first, I wrote a simple program with a source file test.c and a header file _1.2-3~a.hh in the same directory tst/:

// File: test.c

#include "./..//tst//./_1.2-3~a.hh"

int main(void)
{
    char    a [10]  = "abcdefghi";
    char    b [5]   = "qwert";

    strncpy(b, a, 5 - 1);
    printf("b: \"%c%c%c%c%c\"\n", b[0], b[1], b[2], b[3], b[4]);
    /* printed: b: "abcdt" */
    b[5 - 1] = '\0';
    printf("b: \"%c%c%c%c%c\"\n", b[0], b[1], b[2], b[3], b[4]);
    /* printed: b: "abcd" */

    return  0;
}

// File: _1.2-3~a.hh

#include <stdio.h>
#include <string.h>

Which I compiled with this options: $ gcc -std=c11 -pedantic-errors test.c -o tst with no complain from the compiler (I have gcc (Debian 8.2.0-8) 8.2.0).

• Is it really forbidden to use a relative path in an include?

Answer 1

Ah; the standard is really talking about the minimum character set of the filesystem supporting the C compiler.

Anything in the "" (or <> with some preprocessing first) is parsed as a string according to normal C rules and passed from there to the OS to do whatever it wants with it.

This leads to compiler errors on Windows when the programmer forgets to type \\ instead of '\' when writing a path into the header files. On modern Windows we can just use '/' and expect it to work but on older Windows or DOS it didn't.

For extra fun, try

#include "/dev/tty"

Really nice one. It wants you to type C code while compiling.

Answer 2

I'd would say it's not forbidden but not recommanded since it will not compile in some of cases there.

For example:

• if you clone this directory into your root (so you'd have C:\test\).
• if you try to run it in a virtual environment online, you may face issues.

Is it really forbidden to use a path in an include?

Not sure what you mean here: relative paths are commonly used, but using absolute path would be foolish.