shared c constants in a header

Question

I want to share certain C string constants across multiple c files. The constants span multiple lines for readability:

const char *QUERY = "SELECT a,b,c "
                    "FROM table...";

Doing above gives redefinition error for QUERY. I don't want to use macro as backspace '\' will be required after every line. I could define these in separate c file and extern the variables in h file but I feel lazy to do that.

Is there any other way to achieve this in C?

Note that in C++ global `const` implies `static` implicitly: https://stackoverflow.com/questions/12042549/define-constant-variables-in-c-header/53541011#53541011 — Ciro Santilli OurBigBook.com, Nov 29 '18 at 14:22

score 45 · Accepted Answer · answered Mar 31 '11 at 12:04

45

In some .c file, write what you've written. In the appropriate .h file, write

extern const char* QUERY; //just declaration

Include the .h file wherever you need the constant

No other good way :) HTH

answered Mar 31 '11 at 12:04

Armen Tsirunyan

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:( I thought so. Guess I need to stop being so lazy! – Manish Mar 31 '11 at 12:08
@Manish:: Its better than writing the declaration in every `.c` files. And you know `Ctrl+C` -- `Ctrl+V` also works. – Sadique Mar 31 '11 at 12:14
+1 you can have any colour you want as long as it's black LOL – pmg Mar 31 '11 at 12:16
3

@Acme: Ctrl+C -- Ctrl+V is exactly the thing I want to avoid. I know that if I change it in one c file, I'll forget to change it in other which will lead to a debugging nightmare. – Manish Mar 31 '11 at 12:22
Does anyone know if there is a tool to generate the header files? – dykeag Apr 01 '20 at 15:04

score 18 · Answer 2 · edited Aug 10 '17 at 20:14

18

You could use static consts, to all intents and purposes your effect will be achieved.

myext.h:

#ifndef _MYEXT_H
#define _MYEXT_H
static const int myx = 245;
static const unsigned long int myy = 45678;
static const double myz = 3.14;
#endif

myfunc.h:

#ifndef MYFUNC_H
#define MYFUNC_H
void myfunc(void);
#endif

myfunc.c:

#include "myext.h"
#include "myfunc.h"
#include <stdio.h>

void myfunc(void)
{
    printf("%d\t%lu\t%f\n", myx, myy, myz);
}

myext.c:

#include "myext.h"
#include "myfunc.h"
#include <stdio.h>

int main()
{
    printf("%d\t%lu\t%f\n", myx, myy, myz);
    myfunc();
    return 0;
}

edited Aug 10 '17 at 20:14

user812786

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answered Feb 17 '15 at 17:21

tipaye

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1

There is one downside to this approach. If the constant isn't used in a compilation unit, some compilers will throw a warning which cannot be silenced without resorting to compiler specifics. – Sander Mertens Oct 19 '21 at 11:49

score 3 · Answer 3 · answered Mar 31 '11 at 12:05

3

You can simply #define them separate

#define QUERY1 "SELECT a,b,c "
#define QUERY2 "FROM table..."

and then join them in one definition

#define QUERY QUERY1 QUERY2

answered Mar 31 '11 at 12:05

pmg

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1

That is quite creative :) but it looks ugly. – Manish Mar 31 '11 at 12:07
3

Ugly and "backspace '\'" go hand-in-hand :) – pmg Mar 31 '11 at 12:11

score 1 · Answer 4 · answered Mar 31 '11 at 12:12

There are several ways

place your variables in one file, declare them extern in the header and include that header where needed
consider using some external tool to append '\' at the end of your macro definition
overcome your laziness and declare your variables as extern in all your files

shared c constants in a header

4 Answers4

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