C prog, using extern in .h file

MrUmunhum · 05-16-2009, 02:36 PM

Hi group,
This seems like a simple problem but I can't figure it out.

I am trying to simplify the sharing of variables bewteen program modules. I have thse source files:
a.c:

Code:

#define MAIN
#include <a.h>
int main(){
  B = calloc(1,200);
  strcpy( B, "Hello" );
  printf("B: %s\n", B );
  SUB();
  printf("B: %s\n", B );
  return; }

b.c

Code:

#include <a.h>
int SUB(){
  strcpy( B, "Good bye" );
  return; }

a.h:

Code:

#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <malloc.h>
#include <ctype.h>
#include <errno.h>
int SUB();
#ifndef MAIN
extern "C" {
#endif

 char *B;

#ifndef MAIN
}
#endif

Where I compile the sources, I get this error:

Quote:

In file included from b.c:1:
./a.h:15: error: expected identifier or ‘(’ before string constant
b.c: In function ‘SUB’:
b.c:4: error: ‘B’ undeclared (first use in this function)
b.c:4: error: (Each undeclared identifier is reported only once
b.c:4: error: for each function it appears in.)

Any suggestion??

Thanks for your time.

Sergei Steshenko · 05-16-2009, 02:55 PM

Quote:

Originally Posted by MrUmunhum

Hi group,
This seems like a simple problem but I can't figure it out.

I am trying to simplify the sharing of variables bewteen program modules. I have thse source files:
a.c:

Code:

#define MAIN
#include <a.h>
int main(){
  B = calloc(1,200);
  strcpy( B, "Hello" );
  printf("B: %s\n", B );
  SUB();
  printf("B: %s\n", B );
  return; }

b.c

Code:

#include <a.h>
int SUB(){
  strcpy( B, "Good bye" );
  return; }

a.h:

Code:

#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <malloc.h>
#include <ctype.h>
#include <errno.h>
int SUB();
#ifndef MAIN
extern "C" {
#endif

 char *B;

#ifndef MAIN
}
#endif

Where I compile the sources, I get this error:

Any suggestion??

Thanks for your time.

What suggestions do you expect regarding

Code:

./a.h:15: error: expected identifier or ‘(’ before string constant

?

tuxdev · 05-16-2009, 04:15 PM

The immediate problem is that extern "C" is only valid when compiling using a C++ compiler (it indicates that the symbols should not be mangled). You're generally supposed to wrap the line (and its associated closing brace) with #ifdef __cplusplus. What you probably want is just "extern char *B".

However, there's a lot of style issues here. What are you trying to turn C into? Every file should ideally #include exactly what it directly depends on, no more and no less (a.h has that big giant mass of includes). Except for certain special cases, all header files should use a multiple-inclusion guard (a.h lacks one). All upper-case means that the it's a constant (SUB is a function). Though there's disagreement on where to put an opening brace, pretty much every widely used style puts a closing brace on its own line.

paulsm4 · 05-16-2009, 08:01 PM

Hi -

Code:

// a.h
#ifndef _A_H
#define _A_H

#ifdef __cplusplus
extern "C" {
#endif
int SUB();
#ifdef __cplusplus
}
#endif

extern char *B;

#endif /*_A_H */

Code:

// a.cpp
#include <stdio.h>
#include <string.h>
#include <malloc.h>
#include "a.h"

char *B = NULL;

int 
main(int argc, char *argv[])
{
  B = (char *)calloc(1,200);
  strcpy (B, "Hello" );
  printf ("B: %s\n", B);
  SUB ();
  printf ("B: %s\n", B);
  return 0; 
}

Code:

// b.cpp
#include <string.h>
#include "a.h"

int 
SUB()
{
  strcpy (B, "Good bye");
  return 0; 
}

Some notes:
1. To "share a variable between modules", just:
a) Declare the "extern" (EXAMPLE: in "a.h")
b) Define the variable (EXAMPLE: in "a.cpp")

2. You don't need "#define MAIN". That used to be necessary in very, very old C compilers. Most modern compilers now allow you to have the "extern" declaration visible in the same file as your actual definition.

3. Your local header should be quoted:

Code:

//Good
#include "a.h"

//Poor
#include <a.h>

You should only use "<>" with system headers (like "<stdio.h>").

4. You should define a "guard" for each header (EXAMPLE: "#ifndef _A_H/#define _A_H/.../#endif")

5. Like tuxdev suggested, "extern "C" {}" is only needed if you plan on calling a subroutine from both C and C++. You should "guard" your extern "C" with "#ifdef __cplusplus", so it will compile in either/both environments.

6. If you define "int main()",then you need to "return" some integer (else you'll probably get a compiler error).

7. When you define your global pointer, "char *B", you should *always* initialize it to NULL.

8. You should only "#include" those headers actually needed by the source file (in contrast to Microsoft style, which likes to put everything under the sun into one big "stdfx.h" header). For example, in "a.h", you don't need *any* #include's.

'Hope that helps .. PSM

MrUmunhum · 05-17-2009, 12:05 PM

Quote:

Originally Posted by paulsm4

Some notes:
...
2. You don't need "#define MAIN". That used to be necessary in very, very old C compilers. Most modern compilers now allow you to have the "extern" declaration visible in the same file as your actual definition.
...
'Hope that helps .. PSM

This is the answer I needed, thanks.

I'm a very, very old programmer! Just very, very confused.