INFO: ftell() May Return an Invalid Value with a Text File (68337)

SUMMARY

When ftell() is used on a file opened in text mode that contains only linefeeds (0x0A) with no carriage returns (0x0D), ftell() may return an incorrect value on the first call, causing all subsequent return values to be wrong as well. Opening the file in binary mode eliminates this problem. A text file, by definition, contains CR-LF pairs that are condensed to single LF (linefeed) characters on input. A file that contains LF characters with no CR (carriage return) characters is an ill-formed text file and should be processed in binary mode.

Sample Code

/* Compile options needed: none
*/ 

#include <stdio.h>

void main( void)
{
   FILE *ptr;
   char a[80];
   fpos_t offset;
   int row;
   char buffer[42] = "0123456789012345678901234567890123456789\n";
                   /* |    10   |    10   |    10   |   10   | */ 

   /* Create a file called TESTFILE.TXT that contains 12 lines of
      40 chars each with linefeeds only. */ 
   ptr = fopen( "TESTFILE.TXT", "wb" );

   for( row = 0; row < 12; row++ )
      fprintf( ptr, buffer );

   fclose( ptr );

   /* Open the file in text mode. */ 
   ptr = fopen( "TESTFILE.TXT", "r" );

   /* Positions (or should position) the file pointer at an offset.
      of 41 */ 
   fgets( a, 79, ptr );

   offset = ftell( ptr );
   printf( "offset = %ld\n", offset );   /* Prints out 30. */ 
   fclose( ptr );
}