INFO: Switching from Reading to Writing Files Can Garble Data

• The C Run-Time (CRT), included with:
  • Microsoft C for MS-DOS, versions 6.0ax
  • Microsoft C for OS/2, versions 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
  • Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 2.2, 4.0, 4.1, 5.0, 6.0

SUMMARY

When switching from reading to writing data files, it is necessary to make a call to an fsetpos(), fseek(), or rewind() function. If a call to one of these functions is not made, the file pointer may not be updated and the data could be corrupted. It is also necessary to make a call to one of these functions when switching from writing to reading. This is documented on page 275 of the "Microsoft C Optimizing Compiler Run-Time Library Reference" manual, which shipped with version 5.1 of the Microsoft C Compiler.

MORE INFORMATION

The following program attempts to read in the first character of a file and to write it out as the second character:

Sample Code #1

#include <stdio.h>
void main(void)
{
  FILE *fp;
  char a;

  if (( fp = fopen("text.fil","r+")) != NULL)
  {
    fscanf(fp,"%c",&a);      /* Read one character */ 
    fprintf(fp,"%c",a);     /* Write to the next location */ 
    fclose(fp);
  }
} 

Sample Code #2

#include <stdio.h>
void main(void)
{
  FILE *fp;
  char a;
  fpos_t loc;     /* Storage for the current location */ 

  if (( fp = fopen("text.fil","r+")) != NULL)
  {
    fscanf(fp,"%c",&a);    /* Read one character */ 
    fgetpos(fp,&loc);     /* Get current file pointer pos */ 
    fsetpos(fp,&loc);     /* Set current file pointer pos */ 
    fprintf(fp,"%c",a);   /* Write to next location */ 
    fclose(fp);
  }
} 

Additional query words:

Keywords : kbCRT kbVC kbVC600
Version : MS-DOS:6.00ax,7.0; OS/2:6.0,6.00a; WINDOWS:1.0,1.5; WINDOWS NT:1.0,2.0,2.1,2.2,4.0,4.1,5.0
Platform : MS-DOS NT OS/2 WINDOWS
Issue type : kbinfo