Initialize Variables of a Header File

[Ramon]

Is there a way to initialize the variables (or other data) of a header
file by using a function (similar to main() function)??

Thankx

 

[William Pursell]

Is there a way to initialize the variables (or other data) of a header
file by using a function (similar to main() function)??

I'm not sure I understand your question. IMO, variables
should only be declared in a header, and not defined,
so it makes little sense to attempt to initialize them.
What prevents you from doing:

int x;
int main( void )
{
initialize_x();
...


I get the impression that you are trying instead to do:
/* myfile.h */
int x = initialize_x();

and this is a very bad idea, indeed, since header
files should be able to be included in multiple
translation units...do you want the initialize_x()
function to run multiple times?

What exactly are you trying to do?

 

[Ben Pfaff]

Is there a way to initialize the variables (or other data) of a header
file by using a function (similar to main() function)??

Sure. Write a function to initialize your variables, then call
it.

 

[Ramon]

I'm not sure I understand your question. IMO, variables
should only be declared in a header, and not defined,
so it makes little sense to attempt to initialize them.
What prevents you from doing:

int x;
int main( void )
{
initialize_x();
...


I get the impression that you are trying instead to do:
/* myfile.h */
int x = initialize_x();

and this is a very bad idea, indeed, since header
files should be able to be included in multiple
translation units...do you want the initialize_x()
function to run multiple times?

What exactly are you trying to do?

Sorry for not being clear.

I've tried to initialize variables in a header file by doing something
like this:


#ifndef HANDSHAKING_H_
#define HANDSHAKING_H_
..
..
..
int nextHS = 0; // location of next element of handshakeList
..
..
..
#endif /*HANDSHAKING_H_*/


By doing this, the linker complains and apparently it is not acceptable
in gcc (version 4.1.1-21).

My question is:
Can I write a function that is called /automatically/ by the compiler
and that initializes all my variables that are in the header file
(somewhat similar to main() function of a normal C program) ?

And if I can write such a function, how can I implement it?

Thankx in advance!

 

[Ben Pfaff]

I've tried to initialize variables in a header file by doing something
like this:


#ifndef HANDSHAKING_H_
#define HANDSHAKING_H_
.
.
.
int nextHS = 0; // location of next element of handshakeList
.
.
.
#endif /*HANDSHAKING_H_*/

This is not the way you do it. Instead, write "extern int
nextHS;" in the header file and "int nextHS = 0;" in exactly one
C source file (not header file).

 

[Harald van Dijk]

Sorry for not being clear.

I've tried to initialize variables in a header file by doing something
like this:


#ifndef HANDSHAKING_H_
#define HANDSHAKING_H_
.
.
.
int nextHS = 0; // location of next element of handshakeList .
.
.
#endif /*HANDSHAKING_H_*/

So when you have two files that include that header, you have two files
that define nextHS.

By doing this, the linker complains and apparently it is not acceptable
in gcc (version 4.1.1-21).

You should usually post the exact error message. In this case, the
problem is apparent from the code itself, but when it's not, the error
message can clarify a lot.

My question is:
Can I write a function that is called /automatically/ by the compiler
and that initializes all my variables that are in the header file
(somewhat similar to main() function of a normal C program) ?

No. But you don't need one, see below.

And if I can write such a function, how can I implement it?

In your handshaking.h, put

extern int nextHS;

In _one_ source file, put

int nextHS = 0;

The linker tells you that you have multiple definitions of nextHS, so
this way, you make sure you only have one definition.

 

[William Pursell]

Sorry for not being clear.

I've tried to initialize variables in a header file by doing something
like this:

#ifndef HANDSHAKING_H_
#define HANDSHAKING_H_
.
.
.
int nextHS = 0; // location of next element of handshakeList
.
.
.
#endif /*HANDSHAKING_H_*/

By doing this, the linker complains and apparently it is not acceptable
in gcc (version 4.1.1-21).

My question is:
Can I write a function that is called /automatically/ by the compiler
and that initializes all my variables that are in the header file
(somewhat similar to main() function of a normal C program) ?

And if I can write such a function, how can I implement it?

AFAIK, you cannot do that in standard C. However, gcc
allows you to identify functions which are called prior
to main():

void __attribute__((constructor))
initialize (void)
{
/* code that executes before main */
}

IMO, you are better off just calling the initialization
function yourself...it makes for clearer code and there's
no gain whatsoever in using tricks like this.

However, you keep using the phrase "variables ... in
the header file". Only declarations should appear in the
header file; the definition should not be in the header.
(eg, in the header, you should have the word "extern"
on each variable declaration.)

There is a common style to use headers simply
as textual replacement to minimize the amount
of cruft at the start of the .c file, but that's
not really what they're for. If you have a lot
of global variables and you are trying to hide
the declarations/definitions, consider something
like:

a.h:
extern int x;

a.c:
#include "a.h"
int main(void) { /* use x */ }

x.c:
int x;

$ gcc -c x.c && gcc -c a.c && gcc a.o x.o