Page History

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However, the file position indicator returned by ftell() with a file opened in text mode is only useful in calls to fseek(). As such, the value of file_size may not necessarily be a meaningful measure of the number of characters in the file, and consequently, the amount of memory allocated may be incorrect, leading to a potential vulnerability.

Implementation Details

The Visual Studio documentation for ftell() [MSDN]

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states:

The value returned by ftell may not reflect the physical byte offset for streams opened in text mode, because text mode causes carriage return-linefeed translation. Use ftell with fseek to return to file locations correctly.

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#include <stdio.h>
#include <stdlib.h>

int main(void) {

  FILE *fp;
  char a[11];
  long offset;

  /* The contents of foo.txt are: 0123456789 repeated 10 times separated by line feeds (ie \n) */
  fp = fopen( "foo.txt", "r" );
  if (fp == NULL) {
    perror("fopen() error");
    exit(EXIT_FAILURE);
  }

  /* Read 10 (n-1) bytes */
  if (fgets(a, 11, fp) == NULL) {
    perror("fgets() error");
    fclose(fp);
    exit(EXIT_FAILURE);
  }

  offset = ftell(fp);
  if (offset == -1) {
    perror("ftell() error");
    fclose(fp);
    exit(EXIT_FAILURE);
  }

  printf("offset = %ld\n", offset);   /* Prints out 0.*/ 

  return 0;
}

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However, 0 is incorrect; the correct value should be 10, as evident when the file is opened in binary mode rather than text mode.

Compliant Solution

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This compliant solution uses fstat() instead to the size of the text file.

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ISO/IEC 9899:2011 Section 7.21.9.2, "The fseek function", Section 7.21.3, "Files," and Section 7.21.9.4, "The ftell function"

Bibliography

[MSDN] "ftell"

[Open Group 2008]

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