CODEWITHSUNDEEP
cfunctiongcclinkerglobal-variables
Plouff
Plouff

Reputation: 31

What happened if a global variable and a function have the same name?

It is known that a function and a variable can't share the same name.
like this :

#include<stdio.h>
int main;
int main()
{
    return 0;
}

This casued by ‘main’ redeclared as different kind of symbol.
But following two files can be compiled successfuly and make me confused. main.c:

void print_main(void);

int main() {
    print_main();
    return 0;
}

print_main.c

#include <stdio.h>

char main;

void print_main() {
    printf("Output: 0x%x\n", main);    //Output: ffffffe9
}

Confusion

What I've tryed

main.c

void print_main(void);

int main() {
    printf("%p\n", &main);   // 003D12BC 
    print_main();            
    return 0;
}

print_main.c

#include <stdio.h>

char main;

void print_main() {
    printf("Output: %x\n", main);   //ffffffe9
    printf("%p\n", &main);          //003D12BC
}

Upvotes: 1

Views: 494

Answers (1)

John Bollinger
John Bollinger

Reputation: 180181

It seems like I answered substantially the same question a few days ago. Oh wait, I did. I would close the present question as a dupe if the other's OP had accepted an answer.

Summary:

  • A conforming compiler must diagnose incompatible declarations of the same identifier in one translation unit, but it is not required to diagnose that issue for declarations in different translation units.

  • A conforming compiler is not required to reject the program in either case, whether it emits a diagnostic or not, but if it accepts it then the program has undefined behavior.

  • I wonder why compiler didn't remind me ‘main’ redeclared as different kind of symbol?

It is not obligated to do so (see above), and separate compilation makes it challenging to do so. Ultimately, this is a quality of implementation issue.

  • why I get a strange number "ffffffe9" and what is it ?

Two words: undefined behavior.

  • why the strange number changed when I run these files on another IDE?

Again: undefined behavior.

It is not much useful to try to dig into the various ways the program's undefined behavior manifests in different C implementations. Instead, write programs whose behavior is well defined. Especially do not write programs whose behavior you know to be undefined.

Upvotes: 6

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