C-Preprocessor: Combine chars into a string

Question

I have a list of character definitions, like this:

#define MYCHAR_1 0xC3
#define MYCHAR_2 0xB6

(These are the two bytes that form the UTF-8 character "ö")

Is it possible to create a macro that takes the prefix (in this example "MYCHAR") and yields the string "\xC3\xB6"? (i.e. "ö")

In other words can the C-preprocessor create a static string (or array) out of static array-elements?

The end result should be usable by a function that has a string as parameter, for example:

printf(MY_MAGIC_MACRO(MYCHAR));

should print "ö".

You could of course do char ouml[] = { MYCHAR_1, MYCHAR_2, 0 }; — Magnus Hoff
– Magnus Hoff, Commented Oct 2, 2014 at 14:01
OK, but how can I use that in a function? - I added a clarification to the question. (BTW, you would need unsigned for UTF-8 characters) — Roland Seuhs
– Roland Seuhs, Commented Oct 2, 2014 at 14:19
You don't have to use unsigned char for UTF-8 characters; either signed char or plain char is also correct, usable, etc. However, in all respects except standard library support, unsigned char will be easier than a possibly (or definitely) signed char representation. — Jonathan Leffler
– Jonathan Leffler, Commented Oct 2, 2014 at 14:41

Jonathan Leffler · Accepted Answer · 2014-10-02 14:34:31Z

With numeric definitions as shown, it is moderately hard. However, assuming you have a C99 or later compiler, you can use a compound literal:

#define MYCHAR_1 0xC3
#define MYCHAR_2 0xB6

printf("%s", (char []){ MYCHAR_1, MYCHAR_2, '\0' });

If you're stuck with C89, then you probably don't have any good options other than to define an array variable and pass that to the function.

If you want a MY_MAGIC_MACRO(MYCHAR) to work, then you have to know how many names there are (2 per prefix in this example):

#define MY_MAGIC_MACRO(x) ((char []){ x##_1, x##_2, '\0' })

printf("%s\n", MY_MAGIC_MACRO(MYCHAR));

#include <stdio.h>

#define MYCHAR_1 0xC3
#define MYCHAR_2 0xB6

#define MY_MAGIC_MACRO(x) ((char []){ x##_1, x##_2, '\0' })

int main(void)
{
    printf("%s\n", (char[]){ MYCHAR_1, MYCHAR_2, '\0' });
    printf("%s\n", MY_MAGIC_MACRO(MYCHAR));
    return 0;
}

The output on a UTF-8 terminal is ö twice on separate lines.

Lundin · Accepted Answer · 2014-10-02 14:03:44Z

1

The closest thing to what you want to achieve that I can come up with:

#define MYCHAR_1 "\xC3"
#define MYCHAR_2 "\xB6"

const char STR [] = "ABC" MYCHAR_1 "DEF" MYCHAR_2;

answered Oct 2, 2014 at 14:03

Lundin

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