CODEWITHSUNDEEP
cgccprogram-entry-point
Khamidulla
Khamidulla

Reputation: 2975

Is it possible to redefine main function in c

I saw redefine function here using macro in c. So I am interesting is it possible to redefine main function?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv)
{
    printf("Original main function\n");
    return 0;
}

int _main(int argc, char **argv)
{
    printf("New Original main function\n");
    return main(argc, argv);
}

#ifdef DEBUG
#define main(argc, argv) _main(argc, argv)
#endif

Code compiled with out any problem but I am getting:

Original main function

So I am wondering why it does not work? When I use same techniques for malloc and free functions it works perfect. So what is wrong?

Why I want to do something like this? I want to do some code before main function will be executed. Is it possible in this way? if not is there are some other way?

P.S.: Sorry I did not mention in question. I am using gcc in Ubuntu OS. If you are down voting please give a reason in comments. You comments is very useful to my further development.

Upvotes: 0

Views: 3283

Answers (3)

mjs
mjs

Reputation: 3004

If your question is really: "can i execute code before main?" Then the answer is an emphatic YES.

Since you are using GCC, you can use function attributes (http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html) to mark a function as a constructor.

void pre_main_function (void) __attribute__ ((constructor));

A useful example can be found at http://www.geeksforgeeks.org/functions-that-are-executed-before-and-after-main-in-c/

EDIT

The following syntax can also be used:

__attribute__ (( constructor(n) ))

where n specifies the priority, allowing you to mark multiple functions to be executed before main whilst giving you control over the execution order ( the lower the value of n, the earlier the function is executed.

Upvotes: 1

Valeri Atamaniouk
Valeri Atamaniouk

Reputation: 5163

If you want to change entry point of your program, you don't need play with defines. You can use linker's -e option for that:

gcc -Wl,-e,__main ...

Please note extra underscore. Depending on some options, the symbol name can be different.

Upvotes: 2

vogomatix
vogomatix

Reputation: 5041

Your #define does not change the main function at all - it is a macro preprocessor.

The only effect of your #define will be to change the call to main in _main into a recursive call to _main(). But since _main is not called, this is dead code. This is what your code looks like after the preprocessor has run...

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv)
{
    printf("Original main function\n");
    return 0;
}

int _main(int argc, char **argv)
{
    printf("New Original main function\n");
    return _main(argc, argv); /* recursive call due to macro replace */
}

This then leads to the next question - which is why redefine main at all? If you want some entirely different code to run on debug simply declare main as

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv)
{
#ifdef DEBUG
    return debugApp( argc, argv);
#else
    return productionApp( argc, argv);
#endif
}

N.B Just because you can do something doesn't mean you should do it. :-)

Upvotes: 1

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