How do I concatenate two string macros in C?

Question

I am trying to implement VERSION macro for my program, that is to be changed under certain circumstances.

macro VERSION is defined via Makefile (git info is put there) and is a string. Now I have a set of #define'd switches and I want VERSION to reflect which of them are on. This looks now like the follows (main.h):

#define COMPLEX_DEPOSITION // This is switch. later in code it is used in #ifdef...#endif construction.

#ifdef COMPLEX_DEPOSITION
#define CD "_COMP_DEP" // this is the string I want to put in the end of VERSION
#define VERSION_ VERSION CD

#undef VERSION // this is to suppress 'macro redefinition' warning
#define VERSION VERSION_
#undef VERSION_
#endif

Well, I get a lot of errors, most of which make me think that C preprocessor works with lines in file in random order:(

Later I have an even more complex thing that is intended to make VERSION -> VERSION_WLT_GAP_2

#define WIRESLIFETIMES

#ifdef WIRESLIFETIMES
#define GAP 2
#define VERSION_ (VERSION ## "_WLT_GAP_" ## #GAP)
#define VERSION VERSION_
#undef VERSION_
#endif

and I got no idea what to do and if this is even possible

I suspect the easiest solution is #else #define CD ""- always concatenate the same macro's, but just define them to empty strings if not needed. — MSalters
– MSalters, Commented Jun 24, 2019 at 11:16
@SirJoBlack Thank you, I have seen this question and its solution does not work for me. — Xtotdam
– Xtotdam, Commented Jun 24, 2019 at 11:33
@MSalters Thank you, this solves the first problem but does not solve the second one — Xtotdam
– Xtotdam, Commented Jun 24, 2019 at 11:38
The problem with the second example appears to be circular logic. VERSION_ is defined in terms of VERSION and VERSION is then defined as VERSION_`. I don't even understand the real intent there. — MSalters
– MSalters, Commented Jun 24, 2019 at 11:55

Petr Skocik · Accepted Answer · 2019-06-24 11:51:19Z

6

String literals concatenate naturally when placed next to each other

"foo" "bar" is the same as "foobar".

As for the second example, you probably want:

#define CAT_(A,B) A##B
#define CAT(A,B) CAT_(A,B)

#define GAP 2
#define VERSION CAT(VERSION_WLT_GAP_ , GAP)

VERSION //expands to VERSION_WLT_GAP_2

I recommend playing with gcc -E/clang -E a bit, to learn how macros work, before trying to compose anything complex with them.

answered Jun 24, 2019 at 11:51

Petr Skocik

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2 Comments

Xtotdam Over a year ago

Thank you, but point is that VERSION macro holds a value that comes from Makefile

Petr Skocik Over a year ago

@Xtotdam Conditionally (#if) #undef it, then? I'm not proposing you make it as simple as this. I'm just presenting the core technique you could use.

Xtotdam · Accepted Answer · 2019-06-24 11:54:45Z

Well, the answer seems to be the following:

// https://stackoverflow.com/questions/5256313/c-c-macro-string-concatenation
// Concatenate preprocessor tokens A and B without expanding macro definitions (however, if invoked from a macro, macro arguments are expanded).
#define PPCAT_NX(A, B) A ## B

// Concatenate preprocessor tokens A and B after macro-expanding them.
#define PPCAT(A, B) PPCAT_NX(A, B)

// Turn A into a string literal without expanding macro definitions (however, if invoked from a macro, macro arguments are expanded).
#define STRINGIZE_NX(A) #A

// Turn A into a string literal after macro-expanding it.
#define STR(A) STRINGIZE_NX(A)


#define COMPLEX_DEPOSITION

#ifdef COMPLEX_DEPOSITION
#define CD "_COMPDEP"
#else
#define CD ""
#endif


#define WIRESLIFETIMES

#ifdef WIRESLIFETIMES
#define GAP 2
#define WLT STR(PPCAT(_WLT:G, GAP))
#define DISABLE_METROPOLIS
#else
#define WLT ""
#endif

#define VERSION VERSIONX CD WLT

which produces V008.1-11-g68a9c89cb4-dirty_COMPDEP_WLT:G2 and I am happy with it.

Must be noted that I changed -DVERSION=... to -DVERSIONX=... inside Makefile

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