DOCUMENT:Q43072  25-JUL-2001  [visualc]
TITLE   :INFO: Switching from Reading to Writing Files Can Garble Data
PRODUCT :Microsoft C Compiler
PROD/VER: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,
OPER/SYS:
KEYWORDS:kbCRT kbVC kbVC600

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-------------------------------------------------------------------------------
The information in this article applies to:

 - 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);
     }
   }

The above program fails because there is no fseek, fsetpos, or rewind between the
fscanf and fprintf to change the pointer position. The following program
performs the desired operation:

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 
Technology        : kbVCsearch kbAudDeveloper kbCRT
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
Issue type        : kbinfo

=============================================================================

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