Any utility to test expand C/C++ #define macros?

Question

It seems I often spend way too much time trying to get a #define macro to do exactly what i want. I'll post my current dilemma below and any help is appreciated. But really the bigger question is whether there is any utility someone could recommend, to quickly display what a macro is actually doing? It seems like even the slow trial and error process would go much faster if I could see what is wrong.

Currently, I'm dynamically loading a long list of functions from a DLL I made. The way I've set things up, the function pointers have the same nanes as the exported functions, and the typedef(s) used to prototype them have the same names, but with a prepended underscore. So I want to use a define to simplify assignments of a long long list of function pointers.

For example, In the code statement below, 'hexdump' is the name of a typedef'd function point, and is also the name of the function, while _hexdump is the name of the typedef. If GetProcAddress() fails, a failure counter in incremented.

if (!(hexdump = (_hexdump)GetProcAddress(h, "hexdump"))) --iFail;

So lets say I'd like to replace each line like the above with a macro, like this...

GETADDR_FOR(hexdump )

Well this is the best I've come up with so far. It doesn't work (my // comment is just to prevent text formatting in the message)...

// #define GETADDR_FOR(a) if (!(a = (#_#a)GetProcAddress(h, "/""#a"/""))) --iFail; 

And again, while I'd APPRECIATE an insight into what silly mistake I've made, it would make my day to have a utility that would show me the error of my ways, by simply plugging in my macro

Answer 1

You can just run your code through the preprocessor, which will show you what it will be expanded into (or spit out errors as necessary):

$ cat a.c
#define GETADDR_FOR(a) if (!(a = (#_#a)GetProcAddress(h, "/""#a"/""))) 
GETADDR_FOR(hexdump)

$ gcc -E a.c
# 1 "a.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "a.c"
a.c:1:36: error: '#' is not followed by a macro parameter

GETADDR_FOR(hexdump)

In GCC, it's gcc -E foo.c to only preprocess the file.

Visual Studio uses the /P argument.

Answer 2

You might want to take a look at Boost Wave. Like most of Boost, it's really more a library than a utility, but it does have a driver to act as a complete preprocessor.

Answer 3

You appear to be confused about what the exact syntax is for stringifying or token pasting in C preprocessor macros.

You might find this page about C preprocessor macros in general helpful.

In particular, I think this macro should read like this:

#define GETADDR_FOR(a) if (!(a = (_##a)GetProcAddress(h, #a))) --iFail

The trailing ; should be skipped because you will likely be typing this as GETADDR_FOR(hexdump);, and if you don't it will look very strange in your C code and confuse many syntax highlighters.

And as someone else mentioned gcc -E will run the preprocessor and skip the other compilation steps. This is useful for debugging preprocessor problems.

Answer 4

You might try:

#define GETADDR_FOR(a) if (!(a = (_ ## a)GetProcAddress(h, #a))) --iFail;

The ## operator is used to concatenate two tokens to form a single token; this is what you need to turn hexdump into _hexdump. The # operator is used to get a string representation of an argument (note that its operand must be a parameter name; it can't be some other token). This is what you need to turn hexdump into "hexdump".

As for a good way to test preprocessing, as others have mentioned, both gcc and Visual C++ will allow you to preprocess a source file to an intermediary file. You might also consider using mcpp, which is a standalone implementation of the preprocessor.