Can the C preprocessor detect if a header file cannot be found and issue a specific warning?

Question

I have a C file that needs a specific header file. If that header file does not exist, I want the preprocessor to issue a specific warning. Something like:

#if !(#include <special.h>)
#warning "Don't worry, you can fix this."
#warning "You just need to update this other repo over here:"
#endif

Is this possible with the C preprocessor?

possible duplicate of [Can the C preprocessor be used to tell if a file exists?](http://stackoverflow.com/questions/142877/can-the-c-preprocessor-be-used-to-tell-if-a-file-exists) — Ciro Santilli OurBigBook.com, Aug 02 '15 at 08:35

score 20 · Answer 1 · answered Mar 28 '14 at 10:04

20

I know this is a old question, but I also needed a way to do this and I was disappointed when I read these answers. It turns out that clang has a nice __has_include("header name") directive that works flawlessly to do what you need :) Maybe you want to check if the directive is available first by calling #if defined(__has_include)

answered Mar 28 '14 at 10:04

Vik

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[gcc also has this](https://gcc.gnu.org/gcc-5/changes.html). It says gcc 5, but 4.9 seems to work fine for me. – Henry Gomersall Jan 09 '15 at 14:05

score 9 · Answer 2 · answered Feb 22 '12 at 21:01

No, this is not possible. All you can do is try to #include the header, and if it doesn't exist, the compiler will give you an error.

An alternative would be to use a tool like GNU autoconf. autoconf generates a shell script called configure which analyzes your build system and determines things like whether or not you have certain header files installed, and it generates a header file consisting of macros indicating that information.

For example, it might define HAVE_SYS_TIME_H if your build system includes the header <sys/time.h>, so you can then write code such as:

#if HAVE_SYS_TIME_H
#include <sys/time.h>
#else
#warning "Don't worry, you can fix this."
#endif

yepp, but it makes more sense to issue the warning/error from the configure script... — Karoly Horvath, Feb 22 '12 at 21:02

score 8 · Accepted Answer · edited Dec 13 '16 at 00:41

8

You could always check if the #define X macro, from the included file, is ... defined :)

edited Dec 13 '16 at 00:41

Chad

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answered Feb 22 '12 at 20:57

IProblemFactory

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@asaelr Not sure if this is true for everyone, but in my build setup the PP/CC will bravely press on if it can't find an include file. So `#include ` followed by `#ifndef __SPECIAL_H__` works for me – Robert Martin Feb 22 '12 at 21:15
If I have cross-platform code, I want to see if `platform-specific.h` exists before I import it. Using this approach with clang, I have to import it to see if something is defined, but if it doesn't exist, attempting to import it is a compiler/linker error – Ky - Jun 11 '20 at 16:49
This won't work in all compilers because they'll fail the compile on the missing include. – Azeroth2b Jan 11 '21 at 15:36

score -5 · Answer 4 · answered Feb 23 '12 at 00:08

-5

If all you need is a warning that the header file is not there (and no custom code), then most compilers will already give you one.

$ LC_MESSAGES=en_US gcc -c bar.c
bar.c:1:17: fatal error: foo.h: No such file or directory
compilation terminated.

answered Feb 23 '12 at 00:08

jørgensen

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So, a fatal error and a warning are pretty much the same thing for you, aren't they? – Armen Tsirunyan Feb 23 '12 at 00:11

Can the C preprocessor detect if a header file cannot be found and issue a specific warning?

4 Answers4