diff --git a/include/stdio.h b/include/stdio.h
index b7c412c81da..0b6d9f22709 100644
--- a/include/stdio.h
+++ b/include/stdio.h
@@ -497,10 +497,6 @@ extern int __isthreaded;
 #define	ferror(p)	(!__isthreaded ? __sferror(p) : (ferror)(p))
 #define	clearerr(p)	(!__isthreaded ? __sclearerr(p) : (clearerr)(p))
 
-#if __POSIX_VISIBLE
-#define	fileno(p)	(!__isthreaded ? __sfileno(p) : (fileno)(p))
-#endif
-
 #define	getc(fp)	(!__isthreaded ? __sgetc(fp) : (getc)(fp))
 #define	putc(x, fp)	(!__isthreaded ? __sputc(x, fp) : (putc)(x, fp))
 
diff --git a/lib/libc/stdio/ferror.3 b/lib/libc/stdio/ferror.3
index 1529c002fb7..7c4a3ee58b2 100644
--- a/lib/libc/stdio/ferror.3
+++ b/lib/libc/stdio/ferror.3
@@ -112,9 +112,25 @@ 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 should not fail and do not set the external
-variable
+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 ,
diff --git a/lib/libc/stdio/fileno.c b/lib/libc/stdio/fileno.c
index 3adf8dcb959..cb5bfef8409 100644
--- a/lib/libc/stdio/fileno.c
+++ b/lib/libc/stdio/fileno.c
@@ -36,6 +36,7 @@
 static char sccsid[] = "@(#)fileno.c	8.1 (Berkeley) 6/4/93";
 #endif /* LIBC_SCCS and not lint */
 #include "namespace.h"
+#include <errno.h>
 #include <stdio.h>
 #include "un-namespace.h"
 #include "libc_private.h"
@@ -43,14 +44,29 @@ static char sccsid[] = "@(#)fileno.c	8.1 (Berkeley) 6/4/93";
 #undef fileno
 #undef fileno_unlocked
 
+static int
+__fileno_impl(FILE *fp)
+{
+	int fd;
+
+	fd = fp->_file;
+	if (fd == -1)
+		errno = EBADF;
+	return (fd);
+}
+
 int
 fileno(FILE *fp)
 {
 	int fd;
 
-	FLOCKFILE(fp);
-	fd = __sfileno(fp);
-	FUNLOCKFILE(fp);
+	if (__isthreaded) {
+		FLOCKFILE(fp);
+		fd = __fileno_impl(fp);
+		FUNLOCKFILE(fp);
+	} else {
+		fd = __fileno_impl(fp);
+	}
 
 	return (fd);
 }
@@ -58,6 +74,5 @@ fileno(FILE *fp)
 int
 fileno_unlocked(FILE *fp)
 {
-
-	return (__sfileno(fp));
+	return (__fileno_impl(fp));
 }