Includes work in .c file, but not in .h file

Question

I'm able to compile the following with no warnings using gcc -c program.c -o program.o

In program.h

#include<math.h>

In program.c

#include<sys/types.h>
#include"program.h"

But not this:

In program.h

#include<sys/types.h>
#include<math.h>

In program.c

#include"program.h"

If done the latter way, I get implicit declarations of all functions called from any libraries included after sys/types.h. For example:

program.c:352:55: warning: incompatible implicit declaration of built-in function ‘pow’
program.c:352:55: note: include ‘<math.h>’ or provide a declaration of ‘pow’

Why is this?

Don't describe the error messages, *show* them. Copy/paste **exactly** what the compiler outputs. Also, show enough code to reproduce the issue. — dbush, Dec 12 '20 at 17:31
it's an almost [universal convention](https://github.com/isocpp/CppCoreGuidelines/issues/769) that includes are written with a space, for example `#include `, rather than `#include`. — jarmod, Dec 12 '20 at 17:32
Smells like something in the environment is broken. Please share your environment, what you are compiling and what flags you are passing. — Acorn, Dec 12 '20 at 17:32
Is this really all the code you have in `program.h` and `.c`? — meaning-matters, Dec 12 '20 at 17:33
"get implicit declarations of all functions called from any libraries included after sys/types.h" --> But there are no function calls posted, no line 352. A [mcve] would be more useful rather than only a description of code. — chux - Reinstate Monica, Dec 12 '20 at 17:34
@GHOSTHUNT system headers can be included multiple times. That can't be the problem. — meaning-matters, Dec 12 '20 at 17:35
@meaning-matters I'll get you a more code. This is part of a behemoth of a code base. — Evan Hendler, Dec 12 '20 at 17:36
Something else must be wrong somewhere in the few hundred lines you're having. Please delete all code from these files until the issue disappears and you should be close to solving this yourself. — meaning-matters, Dec 12 '20 at 17:37
@meaning-matters got a minimum reproducible example. Standby. — Evan Hendler, Dec 12 '20 at 17:41
You need to add `-lm` flag. `gcc -c program.c -o program.o -lm`. — Krishna Kanth Yenumula, Dec 12 '20 at 17:46
Check this https://stackoverflow.com/questions/19372317/c-failing-to-compile-cant-find-math-h-functions — Krishna Kanth Yenumula, Dec 12 '20 at 17:48
@KrishnaKanthYenumula I had seen that before posting. Not the answer. — Evan Hendler, Dec 12 '20 at 17:51
Guess: In editing `program.h` back and forth, file not saved as expected prior to compilation. — chux - Reinstate Monica, Dec 12 '20 at 17:54
@KrishnaKanthYenumula The error is a compilation error. `-lm` solves a link time error. — chux - Reinstate Monica, Dec 12 '20 at 17:56
@KrishnaKanthYenumula: Adding `-lm` solves a linkng issue. The error messages above occur during compilation. — M Oehm, Dec 12 '20 at 17:56
@chux-ReinstateMonica I'm baffled at this point. I got to a minimum reproducible result, and it compiled... — Evan Hendler, Dec 12 '20 at 17:56
EvanHendler See [this](https://stackoverflow.com/questions/65267805/include-sys-types-h-in-header-file#comment115387573_65267805). — chux - Reinstate Monica, Dec 12 '20 at 17:57
Check this https://stackoverflow.com/questions/17475268/c-c-header-file-not-found/17475422 — Krishna Kanth Yenumula, Dec 12 '20 at 18:07

score 1 · Accepted Answer · answered Dec 12 '20 at 18:09

So, the error was caused by a file program.h.gch within the compilation directory. These are precompiled headers. gcc looks for these prior to actually compiling. The compiler was using a precompiled header without #include sys/types.h, vice program.h which had the correct includes.

See What is a .h.gch file? for more information.

Includes work in .c file, but not in .h file

1 Answers1