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

54
votes

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 let's 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.

5
c++macrosc-preprocessor
what editor are u using? – Anycorn

5 Answers

82
votes

enter image description hereGo to https://godbolt.org/. Enter your code in the left pane and select compiler as gcc put the argument as -E in the right pane. Your pre-processed code will appear on the right.

26
votes

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.

3
votes

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.

2
votes

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.