Syntax of C Macro Declaration

[kreagan]

Hi everyone,

I found this in macro and wanted clearification of the syntax. In other words, I understand what it does but do not understand why it is written this way. Just so you understand, when a variable called status is assigned a value less than zero, that function goes to a catch statement instead of completing the rest of the function.

//if an error occurs then jump to the label
//catch to perform the clean up operation
#define OnErrDoAction(X) status = (X);      \
    if(status < 0)                               \
    {                                                \
        goto CATCH;                               \
    }

1.) Why isn't this written like a typical function? Functionname( params ) { code } The syntax seems wierd.

2.) Why are there slashes after each line?

3.) Would #define OnErrDoAction(status);
if(status < 0){....... }
work?

[weaknessforcats]

Look at how it is expanded:

In your code: OnErrDoAction(25);

In the expanded code:

 status = 25;
 if(status < 0)
{ 
   goto CATCH;   
}
 

The backslashes are line continuation characters.

Probably this is a debug-only macro. Later, when the program is released the macro might become:

#define OnErrDoAction(X)

so that nothing is expanded inthe code sent ot the compiler.

Overall, this macro is really bad. Especially, that goto stuck in there and the fact that CATCH may also be another macro.

[JosAH]

Hi everyone,

I found this in macro and wanted clearification of the syntax. In other words, I understand what it does but do not understand why it is written this way. Just so you understand, when a variable called status is assigned a value less than zero, that function goes to a catch statement instead of completing the rest of the function.

//if an error occurs then jump to the label
//catch to perform the clean up operation
#define OnErrDoAction(X) status = (X);      \
    if(status < 0)                               \
    {                                                \
        goto CATCH;                               \
    }

1.) Why isn't this written like a typical function? Functionname( params ) { code } The syntax seems wierd.

2.) Why are there slashes after each line?

3.) Would #define OnErrDoAction(status);
if(status < 0){....... }
work?

That macro is so incredibly is broken; consider these (sick) examples:

if (<some_condition>)
   OnErrDoAction(42);
else
   printf("everything's fine here\n");
 

It looks quite reasonable but wait 'till the preprecessor has mutilated that beyond
recognition.

And this one:

OnErrDoAction(42) else printf("neener, neener, neener\n");
 

It expands to something one wouldn't expect given the un-preprocessed code.

kind regards,

Jos