How does NSFileHandle offsetInFile work?

I am just wanting to know how offsetInFile works? and what is the difference between seekToFileOffSet. and code examples you know of would be helpfull as 开发者_Python百科well :)

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For standard file descriptors, -[NSFileHandle offsetInFile] and -seekToFileOffset: have straightforward POSIX equivalents:

[handle offsetInFile];
    => off_t offset = lseek([handle fileDescriptor], 0, SEEK_CUR);

[handle seekToFileOffset:off];
    => off_t offset = lseek([handle fileDescriptor], off, SEEK_SET);

The difference between them is that the first returns the current offset, while the second changes the offset.