fileposition

File position, sometimes called the file offset or file pointer, is the current location within an open file or data stream from which the next read or write operation will begin. It is usually measured in bytes from the start of the file, though some systems expose platform-specific representations for large offsets.

In many programming languages, the file position can be queried, advanced, or set explicitly. In C, the file position indicator is manipulated with fseek to move the position, ftell to query it, and fgetpos/fsetpos to save and restore a position, with fpos_t representing the position type. C++ streams offer similar controls via tellg and seekg for input, and tellp and seekp for output. In Python, file objects provide tell() to obtain the current position and seek(offset, whence) to reposition, with whence indicating the reference point (start, current, or end). Java exposes position control through FileChannel.position() and RandomAccessFile.getFilePointer()/seek. Go exposes Seek on os.File to move the position.

Behavior varies with the data source. Some streams are non-seekable, such as pipes or network streams, where the position cannot be queried or changed. Buffered I/O can also affect the apparent position, since data may be held in buffers before the underlying file is updated; flushing or syncing may be required to make position changes visible.

Text vs. binary mode matters on some platforms. In text mode, especially on Windows, translating newline characters can alter the byte position relative to character counts, so byte-based positions may not align with character positions without considering encoding.

Common uses include random access reads and writes, implementing parsers, and seeking to start or end of files for appending or truncation.