xref: /freebsd/lib/libc/stdio/fflush.3 (revision f976241773df2260e6170317080761d1c5814fe5)
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32.\"     @(#)fflush.3	8.1 (Berkeley) 6/4/93
33.\" $FreeBSD$
34.\"
35.Dd January 23, 2020
36.Dt FFLUSH 3
37.Os
38.Sh NAME
39.Nm fflush ,
40.Nm fflush_unlocked ,
41.Nm fpurge
42.Nd flush a stream
43.Sh LIBRARY
44.Lb libc
45.Sh SYNOPSIS
46.In stdio.h
47.Ft int
48.Fn fflush "FILE *stream"
49.Ft int
50.Fn fflush_unlocked "FILE *stream"
51.Ft int
52.Fn fpurge "FILE *stream"
53.Sh DESCRIPTION
54The function
55.Fn fflush
56forces a write of all buffered data for the given output or update
57.Fa stream
58via the stream's underlying write function.
59The open status of the stream is unaffected.
60.Pp
61If the
62.Fa stream
63argument is
64.Dv NULL ,
65.Fn fflush
66flushes
67.Em all
68open output streams.
69.Pp
70The
71.Fn fflush_unlocked
72function is equivalent to
73.Fn fflush ,
74except that the caller is responsible for locking the stream with
75.Xr flockfile 3
76before calling it.
77This function may be used to avoid the overhead of locking the stream and to
78prevent races when multiple threads are operating on the same stream.
79.Pp
80The function
81.Fn fpurge
82erases any input or output buffered in the given
83.Fa stream .
84For output streams this discards any unwritten output.
85For input streams this discards any input read from the underlying object
86but not yet obtained via
87.Xr getc 3 ;
88this includes any text pushed back via
89.Xr ungetc 3 .
90.Sh RETURN VALUES
91Upon successful completion 0 is returned.
92Otherwise,
93.Dv EOF
94is returned and the global variable
95.Va errno
96is set to indicate the error.
97.Sh ERRORS
98.Bl -tag -width Er
99.It Bq Er EBADF
100The
101.Fa stream
102argument
103is not an open stream.
104.El
105.Pp
106The function
107.Fn fflush
108may also fail and set
109.Va errno
110for any of the errors specified for the routine
111.Xr write 2 ,
112except that in case of
113.Fa stream
114being a read-only descriptor,
115.Fn fflush
116returns 0.
117.Sh SEE ALSO
118.Xr write 2 ,
119.Xr fclose 3 ,
120.Xr fopen 3 ,
121.Xr setbuf 3
122.Sh STANDARDS
123The
124.Fn fflush
125function
126conforms to
127.St -isoC .
128