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.Dd April 2, 2022
.Dt FERROR 3
.Os
.Sh NAME
.Nm clearerr ,
.Nm clearerr_unlocked ,
.Nm feof ,
.Nm feof_unlocked ,
.Nm ferror ,
.Nm ferror_unlocked ,
.Nm fileno ,
.Nm fileno_unlocked
.Nd check and reset stream status
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft void
.Fn clearerr "FILE *stream"
.Ft void
.Fn clearerr_unlocked "FILE *stream"
.Ft int
.Fn feof "FILE *stream"
.Ft int
.Fn feof_unlocked "FILE *stream"
.Ft int
.Fn ferror "FILE *stream"
.Ft int
.Fn ferror_unlocked "FILE *stream"
.Ft int
.Fn fileno "FILE *stream"
.Ft int
.Fn fileno_unlocked "FILE *stream"
.Sh DESCRIPTION
The function
.Fn clearerr
clears the end-of-file and error indicators for the stream pointed
to by
.Fa stream .
.Pp
The function
.Fn feof
tests the end-of-file indicator for the stream pointed to by
.Fa stream ,
returning non-zero if it is set.
The end-of-file indicator may be cleared by explicitly calling
.Fn clearerr ,
or as a side-effect of other operations, e.g.\&
.Fn fseek .
.Pp
The function
.Fn ferror
tests the error indicator for the stream pointed to by
.Fa stream ,
returning non-zero if it is set.
.Pp
The function
.Fn fileno
examines the argument
.Fa stream
and returns its integer descriptor.
.Pp
The
.Fn clearerr_unlocked ,
.Fn feof_unlocked ,
.Fn ferror_unlocked ,
and
.Fn fileno_unlocked
functions are equivalent to
.Fn clearerr ,
.Fn feof ,
.Fn ferror ,
and
.Fn fileno
respectively, except that the caller is responsible for locking the stream
with
.Xr flockfile 3
before calling them.
These functions may be used to avoid the overhead of locking the stream
and to prevent races when multiple threads are operating on the same stream.
.Sh ERRORS
These functions, except
.Fn fileno ,
should not fail and do not set the external variable
.Va errno .
.Pp
On error,
.Fn fileno
returns \-1 and sets
.Va errno
to one of the following values:
.Bl -tag -width Er
.It Bq Er EBADF
The stream is not associated with a file.
.It Bq Er EBADF
The file descriptor underlying stream is not a valid file descriptor.
.Pp
Note that detection of this condition is not reliable, the error might
be not reported.
.El
.Sh SEE ALSO
.Xr open 2 ,
.Xr fdopen 3 ,
.Xr flockfile 3 ,
.Xr stdio 3
.Sh STANDARDS
The functions
.Fn clearerr ,
.Fn feof ,
and
.Fn ferror
conform to
.St -isoC .
.Sh HISTORY
The functions
.Fn clearerr ,
.Fn feof ,
.Fn ferror ,
and
.Fn fileno
first appeared in
.At v7 .