C header file include error

Question

Hopefully this is a straightforward question... Here's my process to reproduce this issue. First I create my source file:

bash $ cat t.c
#include "t.h"

int main()
{
  ABC abc;
}

Then I create my corresponding header file:

bash $ cat t.h
#ifdef _T_H
#define _T_H

#ifdef __cplusplus
extern "C" {
#endif

typedef struct abc { 
  int a;
} ABC;

#ifdef __cplusplus
}
#endif

#endif

Then, I try to compile it:

bash $ gcc -o t t.c
t.c: In function ‘main’:
t.c:5: error: ‘ABC’ undeclared (first use in this function)
t.c:5: error: (Each undeclared identifier is reported only once
t.c:5: error: for each function it appears in.)
t.c:5: error: expected ‘;’ before ‘abc’

What is going on? If I use 'struct abc' instead of 'ABC' as the type in t.c, then it does compile. Why aren't the typedefs working?

Greg Hewgill · Accepted Answer · 2010-12-11 03:03:47Z

9

Try:

#ifndef _T_H
#define _T_H

I happened to notice this because the _T_H didn't line up, and my subconscious brain knew it should.

answered Dec 11, 2010 at 3:03

Greg Hewgill

1.0m192 gold badges1.2k silver badges1.3k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Scott Over a year ago

Bingo. Wow, yeah, that one little 'n' screwing everything up. Well hopefully somebody else who has that problem will see this thread :)

Karl Knechtel Over a year ago

BTW, do not use things starting with underscore, or containing a double underscore, for your header guards. Such names are reserved for use by the standard library implementation. Just T_H does the trick just fine.

Collectives™ on Stack Overflow

C header file include error

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related