How to concat macros in C

Question

I have a macro that displays the month from __DATE__ as a numeric value string literal:

#define MONTH (\
  __DATE__[2] == 'n' ? (__DATE__[1] == 'a' ? "01" : "06") \
: __DATE__[2] == 'b' ? "02" \
: __DATE__[2] == 'r' ? (__DATE__[0] == 'M' ? "03" : "04") \
: __DATE__[2] == 'y' ? "05" \
: __DATE__[2] == 'l' ? "07" \
: __DATE__[2] == 'g' ? "08" \
: __DATE__[2] == 'p' ? "09" \
: __DATE__[2] == 't' ? "10" \
: __DATE__[2] == 'v' ? "11" \
: "12")

This works fine and produces the month I'm looking for. printf("%s", MONTH);. However, I'm trying to add this to a larger group of macros and am having trouble:

#define MYMONTH "M" MONTH

should produce the string: M11 right now. But instead, it produces the following compiler error:

error: called object is not a function or function pointer
#define MYMONTH   "M" MONTH

I know you can combine string literals with a macro by appending them:

#define COMMA ","
#define EXCLA "!"
#define MYSTR "Hello" COMMA " world" EXCLA

But why won't it work when there is logic involved?

The preprocessor can concatenate string literals only. The evaluation of that logic is not done at preprocessing time, but later on—probably at compile time, because it’s a constant expression. E.g., if __DATE__ expands to "Oct 31 2016" then MONTH expands to the expression ("Oct 31 2016"[2] == 'n' ? …), not a literal. — Jon Purdy
– Jon Purdy, Commented Nov 1, 2016 at 22:26

mascoj · Accepted Answer · 2016-11-01 22:06:55Z

1

Macros are just a string copy done in preprocessing.

If we copy your MONTH macro into MYMONTH we get:

#define MYMONTH "M" (\
  __DATE__[2] == 'n' ? (__DATE__[1] == 'a' ? "01" : "06") \
: __DATE__[2] == 'b' ? "02" \
: __DATE__[2] == 'r' ? (__DATE__[0] == 'M' ? "03" : "04") \
: __DATE__[2] == 'y' ? "05" \
: __DATE__[2] == 'l' ? "07" \
: __DATE__[2] == 'g' ? "08" \
: __DATE__[2] == 'p' ? "09" \
: __DATE__[2] == 't' ? "10" \
: __DATE__[2] == 'v' ? "11" \
: "12")

Which to the compiler looks like you are trying "M"(<some string here>)

answered Nov 1, 2016 at 22:06

mascoj

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3 Comments

linsek Over a year ago

I'm not sure I follow. What is the problem with that? You can concat string literals together with macros as I illustrated in the question. MONTH will evaluate to be a string literal ("11" right now). So what is wrong with #define MYMONTH "M" MONTH?

mascoj Over a year ago

MONTH is evaluated to ("11") so your MYMONTH evaluates to "M"("11")

rici Over a year ago

@linsek: you can concatenate string literals. You cannot concatenate expressions.

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