为什么在cmd行（-D）中重新定义malloc（）时，gcc为什么不检查函数的隐式声明？

Question

Here is my original function (test.c):

#include <stdlib.h>    
int main()
    {
        void *p = malloc(1);
        free(p);
        return 0;
    }

If I re-define malloc in gcc cmd line with -D, gcc compiles it fine. (why it doesn't complain at my_malloc()?)

gcc -c -Wall -D malloc=my_malloc test.c

If I change the malloc name in my code to malloc2:

#include <stdlib.h>
int main()
{
    void *p = malloc2(1);
    free(p);
    return 0;
}

And then re-define it, gcc now finally complains:

$ gcc -c -Wall -D malloc2=my_malloc test.c 
test.c: In function ‘main’:
test.c:5:5: warning: implicit declaration of function ‘my_malloc’ [-Wimplicit-function-declaration]
     void *p = malloc2(1);

Could someone help me explain why this is happening?

Answer 1

This has to do with how the preprocessor is working.

Assuming you #include <stdlib.h> at the top of your file, the first thing that happens is that this include file is basically imported into your source file. Next, the define passed in to the command line gets processed.

In the first case, this changes not only the call to malloc in your code but also the declaration of malloc imported from stdlib.h. So no implicit declaration. In the second case, the malloc2 call gets changed, but there's no corresponding declaration that gets swapped out as well. So you get the warning.

This is under gcc 4.1.2.

Answer 2

The warning you get in the third case is because you have used a function malloc2() (or my_malloc() in case you use the -Dmalloc2=my_malloc define) without a prototype. In this case, the <stdlib.h> defined prototype doesn't match the define you make in the command line, and you don't get a prototype for it anymore.

As standard behaviour, if you don't include a prototype for a function before it's first use, it defaults to return int and to use an undefined number of parameters. This means that the compiler is assuming you have a int my_malloc(); function defined elsewhere and as you are trying to assign an integer value to a pointer variable, there's something wrong in your code.

If you want the compiler assume your malloc implementation is like the standard library one, just define it with void *malloc(const size_t); before that first use.