重新定义了C宏

Question

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!

Answer 1

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!