Commit ce65317

For streams opened in text mode, **fseek** and **_fseeki64** have limited use, because carriage return-linefeed translations can cause **fseek** and **_fseeki64** to produce unexpected results. The only **fseek** and **_fseeki64** operations guaranteed to work on streams opened in text mode are:

**ftell** and **_ftelli64** return the current file position. The value returned by **ftell** and **_ftelli64** 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](fseek-fseeki64.md) or **_ftelli64** with [_fseeki64](fseek-fseeki64.md) to return to file locations correctly. On error, **ftell** and **_ftelli64** invoke the invalid parameter handler, as described in [Parameter Validation](../../c-runtime-library/parameter-validation.md). If execution is allowed to continue, these functions return -1L and set **errno** to one of two constants, defined in ERRNO.H. The **EBADF** constant means the *stream* argument is not a valid file pointer value or does not refer to an open file. **EINVAL** means an invalid *stream* argument was passed to the function. On devices incapable of seeking (such as terminals and printers), or when *stream* does not refer to an open file, the return value is undefined.

The **fwrite** function writes up to *count* items, of *size* length each, from *buffer* to the output *stream*. The file pointer associated with *stream* (if there is one) is incremented by the number of bytes actually written. If *stream* is opened in text mode, each linefeed is replaced with a carriage-return - linefeed pair. The replacement has no effect on the return value.

**_read** returns the number of bytes read, which might be less than *buffer_size* if there are fewer than *buffer_size* bytes left in the file, or if the file was opened in text mode. In text mode, each carriage return-line feed pair `\r\n` is replaced with a single linefeed character `\n`. Only the single linefeed character is counted in the return value. The replacement does not affect the file pointer.

If the file is opened in text mode, each linefeed character is replaced with a carriage return - linefeed pair in the output. The replacement does not affect the return value.

`_O_TEXT` | Opens file in text (translated) mode. Carriage return - linefeed (CR-LF) combinations are translated into a single linefeed (LF) on input. Linefeed characters are translated into CR-LF combinations on output. Also, CTRL+Z is interpreted as an end-of-file character on input. In files opened for reading and reading/writing, `fopen` checks for CTRL+Z at the end of the file and removes it, if possible. This is done because using the `fseek` and `ftell` functions to move within a file ending with CTRL+Z may cause `fseek` to behave improperly near the end of the file.

- Carriage return - linefeed (CR-LF) translation. This translation occurs before the MBCS - Unicode conversion (for Unicode stream input functions) and after the Unicode - MBCS conversion (for Unicode stream output functions). During input, each carriage return - linefeed combination is translated into a single linefeed character. During output, each linefeed character is translated into a carriage return - linefeed combination.

This error can be caused by missing linefeed and carriage return characters on the last line of a text file.