Windows下seekg()无法获取tellg()返回的正确位置，macOS(Xcode)运行正常

Hey there! Let's dig into this frustrating seekg() issue you're hitting on Windows—this is a classic text-mode vs binary-mode file stream gotcha that trips up a lot of folks.

Why This Happens #

On Windows, when you open a file in text mode (the default for std::ifstream/std::ofstream), the OS automatically converts newline characters: it turns \n in your code into \r\n when writing, and reverses that when reading. The problem? tellg() returns the byte position of the file pointer after these conversions (so each line adds an extra byte for the \r), but when you use that position with seekg() in text mode, it doesn't map cleanly to the original byte offsets.

On macOS (and all Unix-like systems), text mode and binary mode behave the same—newlines are just \n, so no conversion happens. That's why your code works perfectly there.

The Fixes #

1. Use Binary Mode for File Streams (Recommended)

This is the most reliable solution, especially since you're working with structured parameter data. When opening your files, add the std::ios::binary flag to disable newline conversion:

// For reading
std::ifstream in_file("neural_params.txt", std::ios::binary);
// For writing
std::ofstream out_file("neural_params.txt", std::ios::binary);

Now tellg() will return the exact byte offset in the file, and seekg() will jump directly to that position—no mismatches, and behavior will be consistent across Windows and macOS.

2. Work Around Text Mode (If You Must)

If you absolutely need to stick with text mode, avoid using absolute positions from tellg() for seekg(). Instead:

• Track line numbers instead of byte positions, then re-read lines until you reach the target network's parameters.
• Use relative seeking (e.g., seekg(-10, std::ios::cur) to move back 10 bytes from the current position) instead of absolute offsets.

This is less ideal though, as it's slower and more error-prone for structured data like neural network params.

Quick Debug Tip #

To confirm this is the issue, print the value returned by tellg() right after reading a line on both systems. You'll notice that on Windows, each line's end position is 1 byte larger than on macOS—directly due to the \r\n conversion.

内容的提问来源于stack exchange，提问作者hnqiu