[SOLVED] confused with compile result

Wim Sturkenboom · 08-04-2009, 04:39 AM

Code:

int myinteger;

int myfunction();

int main(int argc, char *argv[])
{

        myinteger=33;

        myfunction();

        printf("myinteger is now %d\n",myinteger);
        return 0;


}

Code:

#include <stdio.h>

extern int myinteger;

int myfunction()
{
        printf("Hallo;\n myinteger is %d\n",myinteger);
        myinteger=44;
        return 0;
}

when I compile this code with

Code:

gcc -Wall test1.c test2.c -o myapp

it is happy

If I omit the extern and recompile, it's still happy? Question is why that would be

From my turboC days I seem to remember that 'extern' was required in situations like above. Does the compiler nowadays take care of this? Or is there something else that I'm missing?

Sergei Steshenko · 08-04-2009, 04:59 AM

Quote:

Originally Posted by Wim Sturkenboom

Code:

int myinteger;

int myfunction();

int main(int argc, char *argv[])
{

        myinteger=33;

        myfunction();

        printf("myinteger is now %d\n",myinteger);
        return 0;


}

Code:

#include <stdio.h>

extern int myinteger;

int myfunction()
{
        printf("Hallo;\n myinteger is %d\n",myinteger);
        myinteger=44;
        return 0;
}

when I compile this code with

Code:

gcc -Wall test1.c test2.c -o myapp

it is happy

If I omit the extern and recompile, it's still happy? Question is why that would be

From my turboC days I seem to remember that 'extern' was required in situations like above. Does the compiler nowadays take care of this? Or is there something else that I'm missing?

Why shouldn't the compiler be happy ?

Without the "extern" you declare your global 'myinteger' in the same file as 'myfunction', so what ? Inside 'myfunction' is known the first time it is used.

The linker might be unhappy because it sees two global entities with the same name, but probably you need to supply linker with a command line flag that would enable such warnings.

...

Maybe you are wondering about a good way to declare/initialize global variables. If it's the case I've found a solution which suits me well. Let me know if you are interested.

Wim Sturkenboom · 08-04-2009, 06:09 AM

Sorry, my mistake, it should have read linker.

Further I would expect running the program without 'extern' should give me an uninitialized 'myinteger' in the second file and therefore random results.

Always interested in solutions that others have found. My usual solution is to have an define __SOMETHING__ in my 'main' source file and not in the other source files and the include file will contain ifdef __SOMETHING__ to determine if a variable must be declared as external or normal.

The code posted earlier was a quick and dirty example that I gave to someone and I was surprised that it compiled/linked/worked correctly in both cases.

WimS