Way to compare strings in C preprocessor? (GCC)

Question

Is there a better way to do this (in GCC C)?

I'm trying to define some symbols representing the hardware platform, to be used for conditional compilation.

But I also want printable strings describing the hardware (for diagnostics).

Ideally I'd like be able to do:

#define HARDWARE "REV4C"

#if (HARDWARE == "REV4C")
    #define LED_RED      // define pin addresses, blah blah...
#endif

printf("HARDWARE %s\n", HARDWARE);

But I don't think that's allowed in C. This works, but it's ugly:

#define REV4C   (403)    // symbols for conditional compilation
#define REV421  (421) 

//#define HARDWARE REV4C // choose hardware platform (just one)
#define HARDWARE REV421

#if (HARDWARE == REV421) // define strings for printing
    #define HARDWARE_ID "REV421"
#elif (HARDWARE == REV4C)
    #define HARDWARE_ID "REV4C"
#else
    #define HARDWARE_ID "unknown"
#endif

#if (HARDWARE == REV421)
    #define LED_RED      // define pin addresses, blah blah...
#endif

/* ... */

printf("HARDWARE_ID %s\n", HARDWARE_ID);

This is ugly because it requires two separate symbols, HARDWARE (an integer, for comparison) and HARDWARE_ID (a string, for printing). And logic to generate the value of HARDWARE_ID.

Is there a better way?

Edit: I don't think this is a duplicate of how to compare string in C conditional preprocessor-directives.

That question (and answer) doesn't address how to get a printable string without ending up with two similar symbols.

(I did look at that answer before posting this question.)

Your second suggestion isn't so bad. Of course, you should not repeat the `#if` blocks for each thing you define, but define everything together. This approach also lets you control the valid values and allows you to issue an `#error` in the `#else` clause when no hardware was specified. (In `#if`s, unknown macros have a value of zero.) — M Oehm, Dec 18 '15 at 21:07
@JosephQuinsey That question doesn't address how to get a printable string. — nerdfever.com, Dec 18 '15 at 21:44
To get a printable string, see Jens Gustedt's answer [Way to compare strings in C preprocessor? (GCC)](//stackoverflow.com/a/34364515). — Joseph Quinsey, Jul 31 '18 at 16:25

score 6 · Accepted Answer · answered Dec 18 '15 at 20:55

6

The usual way of doing it is just to define the required configuration symbol, and then conditionally to define all of the others:

// Select config here
#define REV4C
//#define REV421

#ifdef REV4C
    #define HARDWARE_ID "REV4C"
    #define LED_RED
#elif defined(REV421)
    #define HARDWARE_ID "REV421"
    // .....
#else
//.......
#endif

answered Dec 18 '15 at 20:55

Eugene Sh.

17,802
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Not all my #ifdefs are in one place like that, so that solution degenerates to my original "ugly but works" solution. – nerdfever.com Dec 18 '15 at 21:37
@nerdfever.com reorganize your code so they are in one place – M.M Dec 18 '15 at 23:59
3

@M.M Easy for you to say. The code is *already* organized so that certain other things are together in one place... – nerdfever.com Dec 19 '15 at 00:46

score 2 · Answer 2 · answered Dec 18 '15 at 21:53

2

To do the actual case analysis look at the comment that marks this as duplicate. To actually also have the value as a string you can use stringification, something like

#define STRINGIFY(...) #__VA_ARGS__
#define HARDWARE_STR STRINGIFY(HARDWARE)

answered Dec 18 '15 at 21:53

Jens Gustedt

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score 2 · Answer 3 · answered Dec 18 '15 at 23:26

A completely different approach would be to move the string-dependent logic to the build.

The C pre-processor is quite limited, but make (along with shell commands in rules) is the perfectly flexible string processor. E.g. with GNU make:

ifeq ($(CONFIG),REV4C)
    HARDWARE_ID = \"REV4C\"
else
    HARDWARE_ID = \"DEFAULT\"
endif
CFLAGS += -DHARDWARE_ID=$(HARDWARE_ID)

and call as

gmake CONFIG=REV4C

Way to compare strings in C preprocessor? (GCC)

3 Answers3