fileno / feof / ferror / clearerr - 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 fileno(FILE *stream); int feof(FILE *stream); int ferror(FILE *stream); void clearerr(FILE *stream);

Semantics – fileno exposes the descriptor behind a stream (STDIN_FILENO etc. are 0/1/2, manual, Descriptors and Streams); it returns -1 on an invalid or non-file stream. feof/ferror test the stream’s two sticky indicators; they are mutually exclusive for narrow streams, and positioning calls clear the EOF flag. clearerr resets both. Because the indicators are sticky, testing them before an operation is meaningless – test after failure.

Edge cases – clearing the error flag and retrying a failed stream operation is wrong: a partially failed write may have committed an unknown amount, and a failed read can leave the offset misplaced; seek to a known position before retrying (manual, Error Recovery). Most stream errors are not retryable anyway; the notable exception is EINTR, avoidable by installing handlers with SA_RESTART. errno after a stream failure carries the underlying read/write value (manual, EOF and Errors).