PRB: Bad Results When %lf Is Format Specifier for Long Double

The information in this article applies to:

• The Microsoft C Run-time (CRT), included with:

      - Microsoft C/C++ version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
  

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SYMPTOMS

Unpredictable results occur when %lf is used as a format specifier for a long double in functions such as sprintf() or printf() as in this example:

   sprintf(chr_buffer, "The value of long double variable is %.3lf ",
                       lng_dbl);

CAUSE

The format specifier for a long double is Lf, and the format specifier for a double is lf. The format argument tells the function the size and type of the arguments. A long double is 10 bytes and a double is 8 bytes so they need different format specifiers.

RESOLUTION

Use %Lf as the format specifier for a long double.

STATUS

This behavior is by design.

MORE INFORMATION

Sample Code

/* Compile options needed: none
*/

#include <stdio.h>
void main(void)

     long double lng_dbl = 2.345;
     char ch_arr[100];

 //  Line with the problem resulting from the incorrect specifier
     sprintf(ch_arr, "%.3lf", lng_dbl);
     printf("The value of 2.345 is not %s \n", ch_arr);

 //  Line with the correct format specifier
     sprintf(ch_arr, "%.3Lf ", lng_dbl);
     printf("The value of 2.345 is %s", ch_arr);
 }