What are C macros useful for

cc-preprocessorlanguage-designlanguage-featuresmacros

I have written a little bit of C, and I can read it well enough to get a general idea of what it is doing, but every time I have encountered a macro it has thrown me completely. I end up having to remember what the macro is and substitute it in my head as I read. The ones that I have encountered that were intuitive and easy to understand were always like little mini functions, so I always wondered why they weren't just functions.

I can understand the need to define different build types for debug or cross platform builds in the preprocessor but the ability to define arbitrary substitutions seems to be useful only to make an already difficult language even more difficult to understand.

Why was such a complex preprocessor introduced for C? And does anyone have an example of using it that will make me understand why it still seems to be used for purposes other than simple if #debug style conditional compilations?

Edit:

Having read a number of answers I still just don't get it. The most common answer is to inline code. If the inline keyword doesn't do it then either it has a good reason to not do it, or the implementation needs fixing. I don't understand why a whole different mechanism is needed that means "really inline this code" (aside form the code being written before inline was around). I also don't understand the idea that was mentioned that "if its too silly to be put in a function". Surely any piece of code that takes an input and produces an output is best put in a function. I think I may not be getting it because I am not used to the micro optimisations of writing C, but the preprocessor just feels like a complex solution to a few simple problems.

Best Answer

I end up having to remember what the macro is and substitute it in my head as I read.

That seems to reflect poorly on the naming of the macros. I would assume you wouldn't have to emulate the preprocessor if it were a log_function_entry() macro.

The ones that I have encountered that were intuitive and easy to understand were always like little mini functions, so I always wondered why they weren't just functions.

Usually they should be, unless they need to operate on generic parameters.

#define max(a,b) ((a)<(b)?(b):(a))

will work on any type with an < operator.

More that just functions, macros let you perform operations using the symbols in the source file. That means you can create a new variable name, or reference the source file and line number the macro is on.

In C99, macros also allow you to call variadic functions such as printf

#define log_message(guard,format,...) \
   if (guard) printf("%s:%d: " format "\n", __FILE__, __LINE__,__VA_ARGS_);

log_message( foo == 7, "x %d", x)

In which the format works like printf. If the guard is true, it outputs the message along with the file and line number that printed the message. If it was a function call, it would not know the file and line you called it from, and using a vaprintf would be a bit more work.

Related Solutions

C++ – the difference between #include and #include “filename”

In practice, the difference is in the location where the preprocessor searches for the included file.

For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.

For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.

A more complete description is available in the GCC documentation on search paths.

C++ – Why use apparently meaningless do-while and if-else statements in macros

The do ... while and if ... else are there to make it so that a semicolon after your macro always means the same thing. Let's say you had something like your second macro.

#define BAR(X) f(x); g(x)

Now if you were to use BAR(X); in an if ... else statement, where the bodies of the if statement were not wrapped in curly brackets, you'd get a bad surprise.

if (corge)
  BAR(corge);
else
  gralt();

The above code would expand into

if (corge)
  f(corge); g(corge);
else
  gralt();

which is syntactically incorrect, as the else is no longer associated with the if. It doesn't help to wrap things in curly braces within the macro, because a semicolon after the braces is syntactically incorrect.

if (corge)
  {f(corge); g(corge);};
else
  gralt();

There are two ways of fixing the problem. The first is to use a comma to sequence statements within the macro without robbing it of its ability to act like an expression.

#define BAR(X) f(X), g(X)

The above version of bar BAR expands the above code into what follows, which is syntactically correct.

if (corge)
  f(corge), g(corge);
else
  gralt();

This doesn't work if instead of f(X) you have a more complicated body of code that needs to go in its own block, say for example to declare local variables. In the most general case the solution is to use something like do ... while to cause the macro to be a single statement that takes a semicolon without confusion.

#define BAR(X) do { \
  int i = f(X); \
  if (i > 4) g(i); \
} while (0)

You don't have to use do ... while, you could cook up something with if ... else as well, although when if ... else expands inside of an if ... else it leads to a "dangling else", which could make an existing dangling else problem even harder to find, as in the following code.

if (corge)
  if (1) { f(corge); g(corge); } else;
else
  gralt();

The point is to use up the semicolon in contexts where a dangling semicolon is erroneous. Of course, it could (and probably should) be argued at this point that it would be better to declare BAR as an actual function, not a macro.

In summary, the do ... while is there to work around the shortcomings of the C preprocessor. When those C style guides tell you to lay off the C preprocessor, this is the kind of thing they're worried about.

Best Answer

Related Solutions

C++ – the difference between #include and #include “filename”

C++ – Why use apparently meaningless do-while and if-else statements in macros

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