Article ID: 44816
Article Last Modified on 12/12/2003
APPLIES TO
- The C Run-Time (CRT), when used with:
- Microsoft Visual C++ 1.0 Professional Edition
- Microsoft Visual C++ 1.5 Professional Edition
- Microsoft Visual C++ 1.51
- Microsoft Visual C++ 1.52 Professional Edition
- Microsoft Visual C++ 1.0 Professional Edition
- Microsoft Visual C++ 2.0 Professional Edition
- Microsoft Visual C++ 2.1
- Microsoft Visual C++ 4.0 Standard Edition
- Microsoft Visual C++ 5.0 Standard Edition
- Microsoft Visual C++ 6.0 Service Pack 5
This article was previously published under Q44816
SUMMARY
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. The problem is that a 2-byte format specifier is being used for an 8-byte value. To correct this problem, replace "%x" and "%lx" with "%lx %lx".
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.
Keywords: kbcrt kbprb KB44816
