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### 4.8 `ferror`, `ferror_unlocked`—test whether read/write error has occurred

Synopsis

    #include <stdio.h>
    int ferror(FILE *fp);

    #define _BSD_SOURCE
    #include <stdio.h>
    int ferror_unlocked(FILE *fp);

Description
The `stdio` functions maintain an error indicator with each file pointer fp, to record whether any read or write errors have occurred on the associated file or stream. Use `ferror` to query this indicator.

See `clearerr` to reset the error indicator.

`ferror_unlocked` is a non-thread-safe version of `ferror`. `ferror_unlocked` may only safely be used within a scope protected by flockfile() (or ftrylockfile()) and funlockfile(). This function may safely be used in a multi-threaded program if and only if they are called while the invoking thread owns the (FILE *) object, as is the case after a successful call to the flockfile() or ftrylockfile() functions. If threads are disabled, then `ferror_unlocked` is equivalent to `ferror`.

Returns
`ferror` returns `0` if no errors have occurred; it returns a nonzero value otherwise.

Portability
ANSI C requires `ferror`.

`ferror_unlocked` is a BSD extension also provided by GNU libc.

No supporting OS subroutines are required.