HOWTO: Use #define Constants in printf() Format Strings

• The C Run-time (CRT) included with: - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5 - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

Using #define constants to specify precision or width directly within the printf() format string does not work as expected. The constant identifier is printed literally.

For example, at times, it is convenient to define a maximum output precision or width for printing or scanning values within a given program. The following printf() statement truncates the output string at 20 characters:

   printf( "This is the string: %.20s\n",szBuf );

   #define MAXSIZE 20
   printf( "This is the string: %.MAXSIZEs\n",szBuf );

MORE INFORMATION

A couple of steps are needed to get the behavior you want:

   #define MAXSIZE 20
   #define STRX(y) #y            // "Stringizes" parameter y.
   #define STR(x) STRX(x)        //  Evaluates x

   printf( "This is the string: %."STR(MAXSIZE)"s\n", szBuf );

MORE INFORMATION

Below is a simple program that includes the code described above.

Sample Code

   /* Compile option needed: none
    */

   #include <stdio.h>

   #define MAXSIZE 20
   #define STRX(y) #y            // "Stringizes" parameter y.
   #define STR(x) STRX(x)        //  Evaluates x

   void main( void )
   {
      char szBuf[17] = { "Howdy, stranger!" };


      printf( "This is the string: %.20s\n",szBuf );

      printf( "This is the string: %.MAXSIZEs\n",szBuf );
      printf( "This is the string: %."STR(MAXSIZE)"s\n", szBuf );
   }

Keywords          : CRTIss
Version           : MS-DOS:7.0;WIN3X:1.0,1.5;WINNT:1.0,2.0,2.1,4.0,5.0;
Platform          : MS-DOS NT WINDOWS
Issue type        : kbhowto