Stringification of a macro value

Question

I faced a problem - I need to use a macro value both as string and as integer.

 #define RECORDS_PER_PAGE 10

 /*... */

 #define REQUEST_RECORDS \
      "SELECT Fields FROM Table WHERE Conditions" \
      " OFFSET %d * " #RECORDS_PER_PAGE \
      " LIMIT " #RECORDS_PER_PAGE ";"

 char result_buffer[RECORDS_PER_PAGE][MAX_RECORD_LEN];

 /* ...and some more uses of RECORDS_PER_PAGE, elsewhere... */

This fails with a message about "stray #", and even if it worked, I guess I'd get the macro names stringified, not the values. Of course I can feed the values to the final method ( "LIMIT %d ", page*RECORDS_PER_PAGE ) but it's neither pretty nor efficient. It's times like this when I wish the preprocessor didn't treat strings in a special way and would process their content just like normal code. For now, I cludged it with #define RECORDS_PER_PAGE_TXT "10" but understandably, I'm not happy about it.

How to get it right?

Answer 1

Try double escaping your quotes

#define RECORDS_PER_PAGE 10
#define MAX_RECORD_LEN 10

 /*... */
#define DOUBLEESCAPE(a) #a
#define ESCAPEQUOTE(a) DOUBLEESCAPE(a)
#define REQUEST_RECORDS \
      "SELECT Fields FROM Table WHERE Conditions" \
      " OFFSET %d * " ESCAPEQUOTE(RECORDS_PER_PAGE)       \
      " LIMIT " ESCAPEQUOTE(RECORDS_PER_PAGE) ";"

 char result_buffer[RECORDS_PER_PAGE][MAX_RECORD_LEN];

int main(){
  char * a = REQUEST_RECORDS;
}

compiles for me. The token RECORDS_PER_PAGE will be expanded by the ESCAPEQUOTE macro call, which is then sent into DOUBLEESCAPE to be quoted.

Answer 2

#include <stdio.h>

#define RECORDS_PER_PAGE 10

#define TEXTIFY(A) #A

#define _REQUEST_RECORDS(OFFSET, LIMIT)                 \
        "SELECT Fields FROM Table WHERE Conditions"     \
        " OFFSET %d * " TEXTIFY(OFFSET)                 \
        " LIMIT " TEXTIFY(LIMIT) ";"

#define REQUEST_RECORDS _REQUEST_RECORDS(RECORDS_PER_PAGE, RECORDS_PER_PAGE)

int main() {
        printf("%s\n", REQUEST_RECORDS);
        return 0;
}

Outputs:

SELECT Fields FROM Table WHERE Conditions OFFSET %d * 10 LIMIT 10;

Note the indirection to _REQUEST_RECORDS to evaluate the arguments before stringifying them.

Answer 3

The xstr macro defined below will stringify after doing macro-expansion.

#define xstr(a) str(a)
#define str(a) #a

#define RECORDS_PER_PAGE 10

#define REQUEST_RECORDS \
    "SELECT Fields FROM Table WHERE Conditions" \
    " OFFSET %d * " xstr(RECORDS_PER_PAGE) \
    " LIMIT " xstr(RECORDS_PER_PAGE) ";"