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, versions 1.0, 1.5 - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 2.2, 4.0,

       4.1, 5.0
  

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);
     }
   }

Keywords          : CRTIss kbfasttip
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