1SETBUF(3) Linux Programmer's Manual SETBUF(3)
2
6 setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations
7
9 #include <stdio.h>
10 void setbuf(FILE *stream, char *buf);
12 void setbuffer(FILE *stream, char *buf, size_t size);
14 void setlinebuf(FILE *stream);
16 int setvbuf(FILE *stream, char *buf, int mode, size_t size);
18 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
20 setbuffer(), setlinebuf(): _BSD_SOURCE
22
24 The three types of buffering available are unbuffered, block buffered,
25 and line buffered. When an output stream is unbuffered, information
26 appears on the destination file or terminal as soon as written; when it
27 is block buffered many characters are saved up and written as a block;
28 when it is line buffered characters are saved up until a newline is
29 output or input is read from any stream attached to a terminal device
30 (typically stdin). The function fflush(3) may be used to force the
31 block out early. (See fclose(3).) Normally all files are block
32 buffered. When the first I/O operation occurs on a file, malloc(3) is
33 called, and a buffer is obtained. If a stream refers to a terminal (as
34 stdout normally does) it is line buffered. The standard error stream
35 stderr is always unbuffered by default.
36 The setvbuf() function may be used on any open stream to change its
38 buffer. The mode argument must be one of the following three macros:
39 _IONBF unbuffered
41 _IOLBF line buffered
43 _IOFBF fully buffered
45 Except for unbuffered files, the buf argument should point to a buffer
47 at least size bytes long; this buffer will be used instead of the cur‐
48 rent buffer. If the argument buf is NULL, only the mode is affected; a
49 new buffer will be allocated on the next read or write operation. The
50 setvbuf() function may only be used after opening a stream and before
51 any other operations have been performed on it.
52 The other three calls are, in effect, simply aliases for calls to
54 setvbuf(). The setbuf() function is exactly equivalent to the call
55 setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);
57 The setbuffer() function is the same, except that the size of the buf‐
59 fer is up to the caller, rather than being determined by the default
60 BUFSIZ. The setlinebuf() function is exactly equivalent to the call:
61 setvbuf(stream, (char *) NULL, _IOLBF, 0);
63
65 The function setvbuf() returns 0 on success. It returns nonzero on
66 failure (mode is invalid or the request cannot be honored). It may set
67 errno on failure.
68 The other functions do not return a value.
70
72 The setbuf() and setvbuf() functions conform to C89 and C99.
73
75 The setbuffer() and setlinebuf() functions are not portable to versions
76 of BSD before 4.2BSD, and are available under Linux since libc 4.5.21.
77 On 4.2BSD and 4.3BSD systems, setbuf() always uses a suboptimal buffer
78 size and should be avoided.
79 You must make sure that the space that buf points to still exists by
81 the time stream is closed, which also happens at program termination.
82 For example, the following is invalid:
83 #include <stdio.h>
85 int
87 main(void)
88 {
89 char buf[BUFSIZ];
90 setbuf(stdin, buf);
91 printf("Hello, world!\n");
92 return 0;
93 }
94
96 fclose(3), fflush(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)
97
99 This page is part of release 3.25 of the Linux man-pages project. A
100 description of the project, and information about reporting bugs, can
101 be found at http://www.kernel.org/doc/man-pages/.
102Linux 2008-06-26 SETBUF(3)