setvbuf / setbuf / setbuffer / setlinebuf - 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 setvbuf(FILE *stream, char *buf, int mode, size_t size); void setbuf(FILE *stream, char *buf); void setbuffer(FILE *stream, char *buf, size_t size); void setlinebuf(FILE *stream);

Modes – _IOFBF full, _IOLBF line, _IONBF unbuffered. Call after opening and before any other operation on the stream (manual, Controlling Buffering). If buf is NULL, glibc mallocs the buffer of size and frees it at fclose; a caller-provided buffer must stay alive (static or heap, not a dead stack frame) for the stream’s lifetime and must not be read or written by the program while the stream uses it.

Equivalences – setbuf(s, buf) is setvbuf(s, buf, buf ? _IOFBF : _IONBF, BUFSIZ); setbuffer is the same with a caller-chosen size; setlinebuf selects _IOLBF with an allocated buffer. BUFSIZ is the historic default chunk; the truly optimal size is st_blksize from fstat on the descriptor (manual). Nonzero return from setvbuf signals an invalid mode or a refused request.