PRB: printf() Appears to Print Incorrect Results for Floats

ID: Q44816

The information in this article applies to:

The C Run-Time (CRT), included with:
- Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax
- Microsoft C for OS/2, versions 5.1, 6.0, 6.0a
- Microsoft C/C++ for MS-DOS, version 7.0
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0

SYMPTOMS

The printf() in the sample code below does not print out the correct values; the float is printed with a hexadecimal specifier before the float specifier.

CAUSE

The problem is that a 2-byte format specifier is being used for an 8-byte value.

RESOLUTION

To correct this problem, replace "%x" and "%lx" with "%lx %lx".

MORE INFORMATION

Sample Code


/* Compile options needed: none
*/ 

#include <stdio.h>
main()
{
   float flt  = 1.701411e+038;
   double dbl = 1.701411e+038;

   printf("\nFLOAT         %x   %e", flt,flt);
   printf("\nDOUBLE        %lx  %le",dbl,dbl);
}

The above code produces incorrect output. However, the source code is incorrect. When printing, the above code is using a 2-byte hex format specifier %x or a 4-byte hex format specifier %lx with an 8-byte double argument (the float is also passed as a double). This produces the incorrect results. To solve this problem, use two %lx format specifiers to remove 8-bytes off the stack before printing the second double. For example:


printf("\nFloat  %lx %lx %e",flt,flt);
printf("\nDouble %lx %lx %le",dbl,dbl);

This works correctly for Microsoft C under MS-DOS or OS/2; however, this code may not be portable under other systems that support types of different sizes.

Eight bytes are passed, independent of the fact that one argument is a float and the other a double, because the float is being promoted to a double. This is because all floats are passed as doubles unless they are specified as floats in the prototype. Because printf() has a variable number of parameters, the arguments are not prototyped and therefore all floats are promoted up to doubles.

Additional query words:

Keywords : kbCRT kbVC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Version : 5.1 6.0 6.0a 6.0ax 7.0 1.
Platform : MS-DOS NT WINDOWS
Issue type : kbprb