SUMMARY
Using the Microsoft C run-time library function fscanf() with the
format specifier %x.xf to read floating-point values usually produces
undesirable results. This happens especially if the file contains
floating point information that was not recorded using the same %x.x
specifier that is being used to read.
The sample code below can be used as an example of expected behavior.
The first output line shows the input values, the second shows what an
incorrect fscanf() format specifier reads, and the third is the correct
data read by fscanf(). The output from the program is as follows:
Values: 3.104000, 34.235340, 834.343201, 5968.394043
After Read: 3.100000, 419561472.000000, 0.000003, 4.227506
After Rewind: 3.104000, 34.235340, 834.343201, 5968.394043
The output of the second line (After Read) may differ between compilers
and may depend upon the compiler options, but is still not the expected
values.
This is not a problem with the fscanf() function; it is due to using
a different floating point specifier to read the value than was used
to write the value.
In the sample code below, the first value is read correctly. After the
first value is read, the file pointer points to the second 0 (zero) in
3.104000. When the second value is read, the information in the file
does not conform to floating-point format, and zeros are read after that.
The compiler has no way of knowing how far to move the file pointer to
get to the next value.
Read floating point values with the same specifier that was used to
write them. Once a value has been read correctly, it may be truncated
or formatted as desired.
For more information on the scanf() family of functions, please refer
to the Visual C++ Books Online.
The following sample code demonstrates the importance of reading
floating point values with the same type specifier that was used in
writing them. Refer to the SUMMARY above for the expected program
output.
Sample Code
/* Compile options needed: none
*/
#include<stdio.h>
void main( void )
{
float a = 3.104f, b = 34.23534f, c = 834.3432f, d = 5968.394f;
float ar, br, cr, dr;
FILE *stream;
if( (stream = fopen( "file.tst", "a" )) == NULL )
printf( "ERROR: Unable to open output file" );
else
fprintf( stream, "%f,%f,%f,%f\n", a, b, c, d );
fclose( stream );
if( (stream = fopen( "file.tst", "r" )) == NULL )
printf( "ERROR: Unable to open input file" );
else
/* NOTE: %x.x format specifier */
{ /* | */
printf("Values: %f, %f, %f, %f\n", a, b, c, d );
fscanf( stream, "%4f,%4f,%4f,%4f", &ar, &br, &cr, &dr );
printf("After Read: %f, %f, %f, %f\n", ar, br, cr, dr );
rewind(stream);
fscanf( stream, "%f,%f,%f,%f", &ar, &br, &cr, &dr );
printf("After Rewind: %f, %f, %f, %f\n", ar, br, cr, dr );
}
}