Article: Q44816
Product(s): Microsoft C Compiler
Version(s): 5.1 6.0 6.0a 6.0ax 7.0 1.
Operating System(s):
Keyword(s): kbCRT kbVC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 22-JUL-2001
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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
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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:
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Keywords : kbCRT kbVC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Technology : kbVCsearch kbAudDeveloper kbCRT
Version : 5.1 6.0 6.0a 6.0ax 7.0 1.
Issue type : kbprb
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