ferror

ferror is a function in the C standard library that reports whether the error indicator of a FILE stream is set. It is declared in stdio.h and takes a pointer to a FILE object as its argument. The function is used to determine if a recent I/O operation on the stream failed.

The function returns a nonzero value if the stream’s error indicator is set, and zero if it is not. The error indicator is set by a read or write operation that fails, or by library routines that signal an error on the stream. This indicator is separate from the end-of-file indicator.

Importantly, the error indicator is not automatically cleared by successful I/O. To reset the stream’s error and end-of-file indicators, you can call clearerr on the stream. After an I/O sequence, ferror and feof can be used together to distinguish between an error and reaching end-of-file; feof reports end-of-file, while ferror reports an error if one has occurred.

Usage typically involves checking ferror after a loop that reads from or writes to a file, especially if the loop exits without an explicit error return. If ferror indicates an error, the program can handle or report it; if feof is set and ferror is not, the loop ended due to end-of-file.

Relation to errno: some implementations may set errno on an I/O error, but the C standard does not require errno to reflect the stream’s error state. Therefore, diagnosing an I/O problem should rely on the stream’s error indicators and any system-specific error information rather than errno alone.