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ftellTextModeFile.md

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ftellModeTextFile

Message: According to Microsoft, the value returned by ftell may not reflect the physical byte offset for streams opened in text mode, because text mode causes carriage return-line feed translation. See also 7.21.9.4 in C11 standard.
Category: Portability
Severity: Style
Language: C/C++

Description

This checker detects the use of ftell() on a file open in text (or translate) mode. The text mode is not consistent in between Linux and Windows system and may cause ftell() to return the wrong offset inside a text file.

This warning helps improve code quality by:

  • Making the intent clear that the use of ftell() in "t" mode may cause portability problem.

Motivation

This checker improves portability accross system.

How to fix

According to C11, the file must be opened in binary mode 'b' to prevent this problem.

Before:

     FILE *f = fopen("Example.txt", "rt");
     if (f)
     {
        fseek(f, 0, SEEK_END);
        printf( "Offset %d\n", ftell(f);
        fclose(f);
     }

After:

     FILE *f = fopen("Example.txt", "rb");
     if (f)
     {
        fseek(f, 0, SEEK_END);
        printf( "Offset %d\n", ftell(f);
        fclose(f);
     }

Notes

See https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/fopen-wfopen?view=msvc-170