What does the # operator do before a parameter in the definition of a macro?

Question

One of the possible macro definitions assert(), as I found in Mingw’s code,.

#define assert(_Expression) \
 (void) \
 ((!!(_Expression)) || \
  (_assert(#_Expression,__FILE__,__LINE__),0))

I don’t understand what the excerpt #_Expression in the fourth line makes. How it works?

Answer 1

It is a preprocessing operator formally known as "Stringification Operator" or "Stringfication Operator".

It is used in the body of a macro for the purpose of informing the preprocessor that the macro parameter must be converted into a string.

Basically, the preprocessor will put the macro parameter in double quotes after being replaced in the macro body.

Follow a practical example as a demonstration:

#include <stdio.h>

#define debug_ptr(x)     printf("%s => %p\n", #x, x )
#define debug_int(x)     printf("%s => %d\n", #x, x )
#define debug_bool(x)    printf("%s => %s\n", #x, (x) ? "TRUE" : "FALSE" )

int foobar(int x)
{
    return x * 2;
}

int main(void)
{
    int a = 123;
    int b = 321;

    debug_ptr(&a);
    debug_int(b);
    debug_bool(a > b);
    debug_int(foobar(a));

    return 0;
}

After the pre-processing step, code macros would be expanded to something like:

#include <stdio.h>

int foobar(int x)
{
    return x * 2;
}

int main(void)
{
    int a = 123;
    int b = 321;

    printf("%s => %p\n", "&a", &a )
    printf("%s => %d\n", "b", b )
    printf("%s => %s\n", "a > b", (a > b) ? "TRUE" : "FALSE" )
    printf("%s => %d\n", "foobar(a)", foobar(a) )

    return 0;
}

Exit:

&a => 0x7ffee3284a98
b => 321
a > b => FALSE
foobar(a) => 246

Answer 2

He’s a kind of toString() of macro. He takes that content and turns it into a string, so he quotes there on that expression.

So let’s say that you call the (macro) function of the question like this:

assert(x == 0);

After preprocessing the code will look something like this:

(void)((!!(x == 0)) || (_assert("x == 0", __FILE__, __LINE__), 0))

This way there in function _assert() can be used with the exact message of the expression you used in the code to print or log in some information.

Documentation.