xref: /freebsd/lib/libc/stdio/setbuf.3 (revision 380a989b3223d455375b4fae70fd0b9bdd43bafb)
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36.\"     @(#)setbuf.3	8.1 (Berkeley) 6/4/93
37.\"
38.Dd June 4, 1993
39.Dt SETBUF 3
40.Os BSD 4
41.Sh NAME
42.Nm setbuf ,
43.Nm setbuffer ,
44.Nm setlinebuf ,
45.Nm setvbuf
46.Nd stream buffering operations
47.Sh SYNOPSIS
48.Fd #include <stdio.h>
49.Ft void
50.Fn setbuf "FILE *stream" "char *buf"
51.Ft void
52.Fn setbuffer "FILE *stream" "char *buf" "int size"
53.Ft int
54.Fn setlinebuf "FILE *stream"
55.Ft int
56.Fn setvbuf "FILE *stream" "char *buf" "int mode" "size_t size"
57.Sh DESCRIPTION
58The three types of buffering available are unbuffered, block buffered,
59and line buffered.
60When an output stream is unbuffered, information appears on the
61destination file or terminal as soon as written;
62when it is block buffered many characters are saved up and written as a block;
63when it is line buffered characters are saved up until a newline is
64output or input is read from any stream attached to a terminal device
65(typically stdin).
66The function
67.Xr fflush 3
68may be used to force the block out early.
69(See
70.Xr fclose 3 . )
71.Pp
72Normally all files are block buffered.
73When the first
74.Tn I/O
75operation occurs on a file,
76.Xr malloc 3
77is called,
78and an optimally-sized buffer is obtained.
79If a stream refers to a terminal
80(as
81.Em stdout
82normally does) it is line buffered.
83The standard error stream
84.Em stderr
85is always unbuffered.
86.Pp
87The
88.Fn setvbuf
89function
90may be used to alter the buffering behavior of a stream.
91The
92.Fa mode
93parameter must be one of the following three macros:
94.Bl -tag -width _IOFBF -offset indent
95.It Dv _IONBF
96unbuffered
97.It Dv _IOLBF
98line buffered
99.It Dv _IOFBF
100fully buffered
101.El
102.Pp
103The
104.Fa size
105parameter may be given as zero
106to obtain deferred optimal-size buffer allocation as usual.
107If it is not zero,
108then except for unbuffered files, the
109.Fa buf
110argument should point to a buffer at least
111.Fa size
112bytes long;
113this buffer will be used instead of the current buffer.
114(If the
115.Fa size
116argument
117is not zero but
118.Fa buf
119is
120.Dv NULL ,
121a buffer of the given size will be allocated immediately,
122and released on close.
123This is an extension to ANSI C;
124portable code should use a size of 0 with any
125.Dv NULL
126buffer.)
127.Pp
128The
129.Fn setvbuf
130function may be used at any time,
131but may have peculiar side effects
132(such as discarding input or flushing output)
133if the stream is ``active''.
134Portable applications should call it only once on any given stream,
135and before any
136.Tn I/O
137is performed.
138.Pp
139The other three calls are, in effect, simply aliases for calls to
140.Fn setvbuf .
141Except for the lack of a return value, the
142.Fn setbuf
143function is exactly equivalent to the call
144.Pp
145.Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);"
146.Pp
147The
148.Fn setbuffer
149function
150is the same, except that the size of the buffer is up to the caller,
151rather than being determined by the default
152.Dv BUFSIZ .
153The
154.Fn setlinebuf
155function
156is exactly equivalent to the call:
157.Pp
158.Dl "setvbuf(stream, (char *)NULL, _IOLBF, 0);"
159.Sh RETURN VALUES
160The
161.Fn setvbuf
162function returns 0 on success, or
163.Dv EOF
164if the request cannot be honored
165(note that the stream is still functional in this case).
166.Pp
167The
168.Fn setlinebuf
169function returns what the equivalent
170.Fn setvbuf
171would have returned.
172.Sh SEE ALSO
173.Xr fclose 3 ,
174.Xr fopen 3 ,
175.Xr fread 3 ,
176.Xr malloc 3 ,
177.Xr printf 3 ,
178.Xr puts 3
179.Sh STANDARDS
180The
181.Fn setbuf
182and
183.Fn setvbuf
184functions
185conform to
186.St -ansiC .
187.Sh BUGS
188The
189.Fn setbuffer
190and
191.Fn setlinebuf
192functions are not portable to versions of
193.Bx
194before
195.Bx 4.2 .
196On
197.Bx 4.2
198and
199.Bx 4.3
200systems,
201.Fn setbuf
202always uses a suboptimal buffer size and should be avoided.
203