Why Does `tellg()` Sometimes Return an Incorrect File Size?

Why Does tellg() Return an Inaccurate File Size?

In the context of file handling, the tellg() function is commonly utilized to determine the current position within a file. However, certain implementations of tellg() may return a value that exceeds the actual amount of data read from the file.

Understanding tellg()

Contrary to popular belief, tellg() does not directly report the file size. Instead, it provides a token value that enables the user to later navigate to the same location within the file. The specific interpretation of this value varies across operating systems.

Unix vs. Windows

On Unix systems, tellg() typically returns the offset in bytes from the beginning of the file. In contrast, on Windows, the returned value represents the offset only for files opened in binary mode. For text mode files, tellg() may report an arbitrary value that is greater than the actual byte count.

Determining File Size

To accurately ascertain the file size, consider using the following approach:

#include <limits>

file.ignore(std::numeric_limits<std::streamsize>::max());
std::streamsize length = file.gcount();
file.clear();  // Reset eof flag set by ignore
file.seekg(0, std::ios_base::beg);

Additional Considerations

• Buffer Management: Ensure that the buffer used to store file data is appropriately sized. The example code allocates memory statically, which may not be optimal. Consider using dynamic memory allocation or standard containers (e.g., std::vector).
• Loop Condition: Adjust the loop condition in the main function to iterate over the correct number of characters.
• Error Handling: Implement error handling to gracefully handle file opening failures and report appropriate messages to the caller.

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