Notes on Writing Macros in C: Lessons from a Debugging Session

Here's the code I had:

The macro was defined like this:

#define ERROR_RET(x) error_code=x;return false;

And later I expected to call it somewhere like this:

void fun()
{
    if(sth) ERROR_RET(eno);
    ....
}

But the compiled result never turned out as expected — the logic just wasn't what I had in mind... I spent a long time checking other code, convinced there was something wrong with how the data was being processed elsewhere.

Finally it was this spot's turn, and I fixed it in tears.

The fix was easy, as shown below:

-#define ERROR_RET(x) error_code=x;return false;
+#define ERROR_RET(x) {error_code=x;return false;}

Macros and inline functions are not the same — a macro simply substitutes the relevant code at compile time. So in the original code, the result of fun should have been something like this:

void fun()
{
    if(sth)
        error_code=x;
    return false;
    ....
}

So every place that called ERROR_RET() was returning an error unconditionally.

If I had just looked at the warning more carefully, I could have spotted it right away.