CODEWITHSUNDEEP
cgccmacros
Matias Casado
Matias Casado

Reputation: 35

C macro redefined

I want to replace all the instances of a wrapper function around free() called myfree() with myfree2(). Unfortunately, I cannot get it to work because the second macro redefines the first. Why is the second macro redefining the first if it has no argument?

// I must delete this function or the macro will replace it as well and cause a syntax error!
void myfree(void *p)
{
    if(p != NULL)
    free(p);
}

void myfree2(void *p)
{
    if(p != NULL)
    free(p);
}

#define myfree(p) do { myfree2(p); p = (void *)0xdeadbeef; } while (0);
#define myfree myfree2

myfree(p); // Wrapper around free().

afunc(arg, myfree); // Wrapper is used as a function argument!

Upvotes: 0

Views: 1116

Answers (1)

Chris Dodd
Chris Dodd

Reputation: 126203

The C preprocessor does not allow overloading of macros based on the number of arguments -- you can only have a single macro of a given name. You can get around this problem in your case by using redundant parentheses in the declaration of myfree:

#define myfree(p) do { myfree(p); p = (void *)0xdeadbeef; } while (0)

void (myfree)(void *p)
{
    if(p != NULL)
    free(p);
}

myfree(p); // Wrapper around free().

afunc(arg, myfree); // Wrapper is used as a function argument!

Upvotes: 1

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