ftello

ftello is a function in the C standard library (POSIX) that returns the current file position indicator for a given stream. The value represents the offset, in bytes, from the beginning of the file and is returned as an off_t type. It serves as a large-file–capable alternative to ftell, which historically uses a long int and may be unable to represent large offsets on some platforms.

Compared with ftell, ftello uses the off_t offset type, allowing it to represent larger file positions on systems that support large files. On some platforms, off_t may be 64 bits, enabling offsets beyond 2 GB or 4 GB depending on architecture. Because ftello only reads the current position, it is often used in conjunction with fseeko, the counterpart that moves the file position using an off_t offset.

Return value and errors: On success, ftello returns the current offset as an off_t value. On error, it returns (off_t) -1 and sets errno to indicate the error. Common error conditions include EBADF if the stream is not a valid FILE*, ESPIPE if the stream is not seekable (for example, a pipe), and other stream-related errors.

Usage notes: To reposition within a file, programs commonly pair ftello with fseeko, or use ftello to obtain the position and then adjust it with fseeko as needed. For portable code that must handle large files, ftello/fseeko are preferred over ftell/fseek, especially when _FILE_OFFSET_BITS is configured for 64-bit offsets.

See also: ftell, fseek, fseeko, off_t, fgetpos.