fseeko / ftello (fseek, ftell, rewind, fgetpos, fsetpos) - glibc

Split from the chapter glibc: stdio and Formatted I/O into the per-function layout; see the method page and the glibc ledger for coverage status.

Signature – int fseeko(FILE *stream, off_t offset, int whence); off_t ftello(FILE *stream); void rewind(FILE *stream); int fgetpos(FILE *stream, fpos_t *pos); int fsetpos(FILE *stream, const fpos_t *pos);

Semantics – fseek/ftell take long int; fseeko/ftello take off_t, which is the type that actually matches the descriptor layer (manual, File Positioning). SEEK_SET/SEEK_CUR/SEEK_END match lseek. A successful seek flushes or repositions buffered output, discards ungetc pushback, and clears the EOF indicator; rewind is fseek(stream, 0L, SEEK_SET) that additionally resets the error indicator and reports nothing. fgetpos/fsetpos traffic in the opaque fpos_t, which on glibc also carries the wide-conversion state (mbstate_t) – the only fully portable pair, and required to preserve multibyte state on wide streams (manual, Portable Positioning).

64-bit interplay – with _FILE_OFFSET_BITS == 64 the o names become the 64 twins on 32-bit hosts; ftell’s long int can be too narrow for large files even when off_t is not, one more reason to prefer ftello. Failure convention: fseek/fseeko return nonzero and set errno; fgetpos/fsetpos return nonzero and set errno to an implementation-defined positive value.