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34 .\" @(#)setbuf.3 8.1 (Berkeley) 6/4/93
44 .Nd stream buffering operations
50 .Fn setbuf "FILE * restrict stream" "char * restrict buf"
52 .Fn setbuffer "FILE *stream" "char *buf" "size_t size"
54 .Fn setlinebuf "FILE *stream"
56 .Fn setvbuf "FILE * restrict stream" "char * restrict buf" "int mode" "size_t size"
58 The three types of buffering available are unbuffered, block buffered,
60 When an output stream is unbuffered, information appears on the
61 destination file or terminal as soon as written;
62 when it is block buffered many characters are saved up and written as a block;
63 when it is line buffered characters are saved up until a newline is
64 output or input is read from any stream attached to a terminal device
67 The default buffer settings can be overwritten per descriptor
71 is the numeric value of the file descriptor represented by the stream, or
74 The environment variable value is a letter followed by an optional numeric
75 value indicating the size of the buffer.
76 Valid sizes range from 0B to 1MB.
78 .Bl -tag -width X -indent
89 may be used to force the block out early.
93 Normally all files are block buffered.
96 operation occurs on a file,
99 and an optimally-sized buffer is obtained.
100 If a stream refers to a terminal
103 normally does) it is line buffered.
104 The standard error stream
106 is initially unbuffered.
111 may be used to alter the buffering behavior of a stream.
114 parameter must be one of the following three macros:
115 .Bl -tag -width _IOFBF -offset indent
126 parameter may be given as zero
127 to obtain deferred optimal-size buffer allocation as usual.
129 then except for unbuffered files, the
131 argument should point to a buffer at least
134 this buffer will be used instead of the current buffer.
142 a buffer of the given size will be allocated immediately,
143 and released on close.
144 This is an extension to ANSI C;
145 portable code should use a size of 0 with any
151 function may be used at any time,
152 but may have peculiar side effects
153 (such as discarding input or flushing output)
154 if the stream is ``active''.
155 Portable applications should call it only once on any given stream,
160 The other three calls are, in effect, simply aliases for calls to
162 Except for the lack of a return value, the
164 function is exactly equivalent to the call
166 .Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);"
171 is the same, except that the size of the buffer is up to the caller,
172 rather than being determined by the default
177 is exactly equivalent to the call:
179 .Dl "setvbuf(stream, (char *)NULL, _IOLBF, 0);"
183 function returns 0 on success, or
185 if the request cannot be honored
186 (note that the stream is still functional in this case).
190 function returns what the equivalent
213 functions are not portable to versions of
223 always uses a suboptimal buffer size and should be avoided.