Tell gcc to not include a header more than once?

Hi there,
here's a really quick question:

say I have some files:

linalg.h

Code:

---

typedef struct {
    ...
} evector;
void copyMatrix2D( double **dst, double **src, int nrows, int ncols );
...

---

electron.h

Code:

---

#include linalg.h
typedef struct {
        evector r;
        ...
} diffuser;
void moveDmcElectron( evector *epos, gsl_rng *r, const double sigma );
...

---

test.c

Code:

---

#include electron.h
extern inline void moveDmcElectron( evector *epos, gsl_rng *r, const double sigma );
#include linalg.h
extern inline void copyMatrix2D( double **dst, double **src, int nrows, int ncols );
...

---

Basically when I try to compile (I compiled the various bits seperately, then try to link when compiling test.c) - it complains that I have two definitions of 'evector' (a typedef'd struct).

Is there a way to get around this? for eg telling the compiler not to include a header that it's already included (even if it was indirectly?)

There has to be a way around this, because people would do this all the time surely? (ie include two headers, one of which also includes the other one, but for different reasons.

cheers :)

In your header files, use include guards:

Code:

---


 #ifndef MYHEADER
  #define MYHEADER
 
  /*
  Prototypes etc here
  */
 
  #endif

---

Sample here: (yes, it c++, but you'll get the idea)
http://faq.cprogramming.com/cgi-bin/...&id=1043284392

Without really reading your code, I'm gonna give you 2 suggestions:

1. Add:

PHP Code:

---

#pragma once 


---

above the header.

or...

2. Add a

PHP Code:

---

#ifndef __HEADER_FILE_HERE_
#define __HEADER_FILE_HERE_ 


---

at the top of the header file, and a

PHP Code:

---

#endif __HEADER_FILE_HERE_ 


---

as the very last line of the header file.
** you can change the text after define's **

oh! I actually tried that but it didn't seem to help - I got the exact same error message :confused:

Perhaps I did it wrong somehow...

cheers for the help :D

does that pragma statement only apply to the line directly beneath it?

Quote:

---

Originally Posted by eccles

does that pragma statement only apply to the line directly beneath it?

---

No, it goes for the whole file from top to bottom.

It just basically says "include this file only once in file XXX.c(pp)"

>#ifndef __HEADER_FILE_HERE_
Names with leading underscores are reserved for the implementation. Your code is dangerous at best.

>#pragma once
Last I heard, that was an obsolescent feature.

Keep in mind that pragma statement is compiler specific. It's best to use the include guards as mentioned above.

Quote:

---

Originally Posted by Prelude

>#ifndef __HEADER_FILE_HERE_
Names with leading underscores are reserved for the implementation. Your code is dangerous at best.

>#pragma once
Last I heard, that was an obsolescent feature.

---

For #1, I had a feeling it was "wrong", but I was doing it off the top of my head since I haven't done C/++ in quite a while.

As for #2, I knew it was obsolete, but it does work.

aaahhh wait a sec - now I get what you mean...
hehe I had been trying to put an include statement in the ifndef...

cool! so once it's loaded once, any further attempts will do nothing. brilliant I love it! thanks so much :D

Sample:

Code:

---

    /* main.c */'
    #include <stdio.h>
   
    #include "head1.h"
    #include "head2.h"
   
    int main(void)
    {
      return(0);
    }

---

Code:

---

    /* head1.h */
    #ifndef HEAD1_H
    #define HEAD1_H
   
    typedef struct mystruct
    {
      int i;
    } mystruct_t;
   
    #endif

---

Code:

---

    /* head2.h */
    #ifndef HEAD2_H
    #define HEAD2_H
   
    #include "head1.h"
   
   
   
    #endif

---

>>I had a feeling it was "wrong"
If you're not sure of what you're saying, then I'd suggest you be mindful of what you post.

>>I knew it was obsolete, but it does work.
Not my compiler.

yay! I can't get over how elegant this solution is. I love it. thank you all so much once more :D