fseeko

fseeko is a function in the C standard library used to set the file position indicator for a given stream using a potentially large offset. It is designed to support large files by using the off_t type for the offset, rather than a long int.

The prototype is int fseeko(FILE *stream, off_t offset, int whence); The offset is interpreted according to the whence parameter, which can be SEEK_SET, SEEK_CUR, or SEEK_END, similar to fseek. On success, fseeko returns 0; on error, it returns a nonzero value and sets errno to indicate the error.

Compared with fseek, fseeko uses off_t for the offset, enabling repositioning within files that exceed the range of long. The corresponding function to query the current position is ftello, which returns an off_t representing the current file offset.

Portability and availability vary by platform. fseeko is defined in POSIX and is widely supported on modern Unix-like systems and many C libraries. Some environments may provide alternative or legacy variants such as fseek for small offsets, or platform-specific 64-bit variants like fseeko64 or ftello64. When targeting very large files on platforms without fseeko, developers may need to use those alternatives or rely on off_t width.

Usage scenarios include working with large data files, binary formats, or file-based databases where offsets exceed 32-bit ranges. As with fseek, error handling should consider that the stream must be seekable, the offset must be valid, and the operation must succeed for subsequent I/O to proceed correctly.