setbuf.3s

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 3S 代码 · 共 172 行

.\" SCCSID: @(#)setbuf.3s	8.2	1/29/91
.TH setbuf 3s 
.SH Name
setbuf, setbuffer, setlinebuf, setvbuf \- assign buffering to a stream
.SH Syntax
.B #include <stdio.h>
.PP
.B void setbuf(\fIstream, buf\fP)
.br
.SM
.B FILE
.B *\fIstream\fP;
.br
.B char *\fIbuf\fP;
.PP
.B void setbuffer(\fIstream, buf, size\fP)
.br
.SM
.B FILE
.B *\fIstream\fP;
.br
.B char *\fIbuf\fP;
.br
.B int \fIsize\fP;
.PP
.B void setlinebuf(\fIstream\fP)
.br
.SM
.B FILE
.B *\fIstream\fP;
.PP
.B int setvbuf(\fIstream, buf, type, size\fP)
.br
.B FILE *\fIstream\fP;
.br
.B char *\fIbuf\fP;
.br 
.B int \fItype\fP;
.B size_t \fIsize\fP; 
.SH Description
.NXR "setbuf subroutine"
.NXR "setbuffer subroutine"
.NXR "setlinebuf subroutine"
.NXR "setvbuf subroutine"
.NXR "stream" "assigning buffering to"
.NXR "buffering" "types of"
The three types of buffering available are unbuffered, block buffered,
and line buffered.
When an output stream is unbuffered, information appears on the
destination file or terminal as soon as written;
when it is block buffered many
characters are saved up and written as a block;
when it is line buffered characters are saved up until a new line is
encountered or input is read from stdin.
The routine
.PN fflush ,
may be used to force the block out early.
Normally all files are block buffered.
For further information, see 
.MS fclose 3s .
A buffer is obtained from 
.MS malloc 3
upon the first
.PN getc
or
.PN putc
on the file.
If the standard stream
.B stdout
refers to a terminal it is line buffered.
The standard stream
.B stderr
is always unbuffered.
.PP
The
.PN setbuf
routine is used after a stream has been 
opened but before it is read or written.
The character array
.I buf
is used instead of an automatically allocated buffer.  If
.I buf
is the constant pointer NULL,
input/output will be completely unbuffered.
A manifest constant BUFSIZ
tells how big an array is needed:
.EX
char buf[BUFSIZ];
.EE
.PP
The
.PN setbuffer
routine, an alternate form of 
.PN setbuf ,
is used after a stream has been opened but before it is read or written.
The character array
.I buf
whose size is determined by the 
.I size
argument is used instead of an automatically allocated buffer.  If
.I buf
is the constant pointer NULL,
input/output will be completely unbuffered.
.PP
The
.PN setlinebuf
routine is used to change
.B stdout
or
.B stderr
from block buffered or unbuffered to line buffered.
Unlike
.PN setbuf
and
.PN setbuffer
it can be used at any time that the file descriptor is active.
.PP
The
.PN setvbuf
routine may be used after a stream has been opened but
before it is read or written.
.I Type
determines how \fIstream\fP will be buffered.
Legal values for \fItype\fP, defined in stdio.h are:
.TP 15
 _IOFBF
causes input/output to be fully buffered.
.TP 15
 _IOLBF
causes output to be line buffered; the buffer will be
flushed when a new line is written, the buffer is full, 
or input is requested.
.TP 15
 _IONBF
causes input/output to be completely unbuffered.
.PP
If \fIbuf\fP is not the NULL pointer, the array it points to 
will be used for buffering,
instead of an automatically allocated buffer.
The
.I size
specifies the size of the buffer to be used.  The constant
BUFSIZ in <stdio.h> is suggested as a good buffer size.
If input/output is unbuffered, \fIbuf\fP and \fIsize\fP
are ignored.
.PP
By default, output to a terminal is line buffered
and all other input/output is fully buffered.
.PP
A file can be changed from unbuffered or line buffered to block buffered
by using
.PN freopen .
For further information, see 
.MS fopen 3s .
A file can be changed from block buffered or line buffered to unbuffered
by using
.PN freopen
followed by
.PN setbuf
with a buffer argument of NULL.
.SH Restrictions
The standard error stream should be line buffered by default.
.PP
The
.PN setbuffer
and
.PN setlinebuf
functions are not portable to non 4.2 BSD versions of UNIX.
.SH See Also
malloc(3), fclose(3s), fopen(3s), fread(3s), 
getc(3s), printf(3s), putc(3s), puts(3s).