setbuf(3)

1setbuf(3)                  Library Functions Manual                  setbuf(3)
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NAME

6       setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations
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LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <stdio.h>
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14       int setvbuf(FILE *restrict stream, char buf[restrict .size],
15                   int mode, size_t size);
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17       void setbuf(FILE *restrict stream, char *restrict buf);
18       void setbuffer(FILE *restrict stream, char buf[restrict .size],
19                   size_t size);
20       void setlinebuf(FILE *stream);
21
22   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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24       setbuffer(), setlinebuf():
25           Since glibc 2.19:
26               _DEFAULT_SOURCE
27           glibc 2.19 and earlier:
28               _BSD_SOURCE
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DESCRIPTION

31       The  three types of buffering available are unbuffered, block buffered,
32       and line buffered.  When an output stream  is  unbuffered,  information
33       appears on the destination file or terminal as soon as written; when it
34       is block buffered, many characters are saved up and written as a block;
35       when  it  is  line buffered, characters are saved up until a newline is
36       output or input is read from any stream attached to a  terminal  device
37       (typically  stdin).   The  function  fflush(3) may be used to force the
38       block out early.  (See fclose(3).)
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40       Normally all files are block buffered.  If a stream refers to a  termi‐
41       nal (as stdout normally does), it is line buffered.  The standard error
42       stream stderr is always unbuffered by default.
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44       The setvbuf() function may be used on any open  stream  to  change  its
45       buffer.  The mode argument must be one of the following three macros:
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47              _IONBF unbuffered
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49              _IOLBF line buffered
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51              _IOFBF fully buffered
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53       Except  for unbuffered files, the buf argument should point to a buffer
54       at least size bytes long; this buffer will be used instead of the  cur‐
55       rent buffer.  If the argument buf is NULL, only the mode is affected; a
56       new buffer will be allocated on the next read or write operation.   The
57       setvbuf()  function  may be used only after opening a stream and before
58       any other operations have been performed on it.
59
60       The other three calls are, in  effect,  simply  aliases  for  calls  to
61       setvbuf().  The setbuf() function is exactly equivalent to the call
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63           setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);
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65       The  setbuffer() function is the same, except that the size of the buf‐
66       fer is up to the caller, rather than being determined  by  the  default
67       BUFSIZ.  The setlinebuf() function is exactly equivalent to the call:
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69           setvbuf(stream, NULL, _IOLBF, 0);
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RETURN VALUE

72       The  function  setvbuf()  returns  0 on success.  It returns nonzero on
73       failure (mode is invalid or the request cannot be honored).  It may set
74       errno on failure.
75
76       The other functions do not return a value.
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ATTRIBUTES

79       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
80       tributes(7).
81
82       ┌────────────────────────────────────────────┬───────────────┬─────────┐
83       │Interface                                   │ Attribute     │ Value   │
84       ├────────────────────────────────────────────┼───────────────┼─────────┤
85       │setbuf(), setbuffer(), setlinebuf(),        │ Thread safety │ MT-Safe │
86       │setvbuf()                                   │               │         │
87       └────────────────────────────────────────────┴───────────────┴─────────┘
88

STANDARDS

90       setbuf()
91       setvbuf()
92              C11, POSIX.1-2008.
93

HISTORY

95       setbuf()
96       setvbuf()
97              C89, POSIX.1-2001.
98

CAVEATS

100       POSIX notes that the value of errno is unspecified after a call to set‐
101       buf() and further notes that, since the value of errno is not  required
102       to  be  unchanged  after  a  successful  call to setbuf(), applications
103       should instead use setvbuf() in order to detect errors.
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BUGS

106       You must make sure that the space that buf points to  still  exists  by
107       the  time  stream is closed, which also happens at program termination.
108       For example, the following is invalid:
109
110       #include <stdio.h>
111
112       int
113       main(void)
114       {
115           char buf[BUFSIZ];
116
117           setbuf(stdout, buf);
118           printf("Hello, world!\n");
119           return 0;
120       }
121