fflush - 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 fflush(FILE *stream);

Semantics – delivers buffered output to the kernel. fflush(NULL) flushes all open output streams – the strongest per-process synchronization point between stdio and write(2), and the standard brace before fork() so parent and child do not duplicate buffered data. fflush on an input stream discards buffered input on seekable files but leaves the descriptor offset unpredictable (manual, Cleaning Streams). Returns 0, or EOF with errno from the underlying write.

fsync interplay – fflush moves data from the FILE buffer to the page cache only; durability against crashes needs fsync(fd) on fileno(stream) afterwards (manual, Synchronizing I/O operations: write returning does not mean the media got the data). The two calls are complementary: stdio-to-kernel, then kernel-to-device.

Related – _flushlbf (GNU, stdio_ext.h) flushes only line buffered streams; __fpurge discards an input buffer without delivering it.