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Top Forums Programming File * Post 16428 by Perderabo on Friday 1st of March 2002 08:36:11 AM
Old 03-01-2002
Once you have unbuffered the file every read will get the data from the file system cache, invoking a physical read if necessary. This the best that your program can do.

But if another program is writing to the file, it may be using buffered i/o. In this case, you will not see new data until the other program flushes its internal buffer.
 
Test Your Knowledge in Computers #989
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OpenBSD includes no third-party components.
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SETBUF(3S)																SETBUF(3S)

NAME
setbuf, setbuffer, setlinebuf - assign buffering to a stream SYNOPSIS
#include <stdio.h> setbuf(stream, buf) FILE *stream; char *buf; setbuffer(stream, buf, size) FILE *stream; char *buf; int size; setlinebuf(stream) FILE *stream; DESCRIPTION
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 newline is encountered or input is read from stdin. Fflush (see fclose(3S)) may be used to force the block out early. Normally all files are block buffered. A buffer is obtained from malloc(3) upon the first getc or putc(3S) on the file. If the standard stream stdout refers to a terminal it is line buffered. The standard stream stderr is always unbuffered. Setbuf is used after a stream has been opened but before it is read or written. The character array buf is used instead of an automati- cally allocated buffer. If buf is the constant pointer NULL, input/output will be completely unbuffered. A manifest constant BUFSIZ tells how big an array is needed: char buf[BUFSIZ]; Setbuffer, an alternate form of setbuf, is used after a stream has been opened but before it is read or written. The character array buf whose size is determined by the size argument is used instead of an automatically allocated buffer. If buf is the constant pointer NULL, input/output will be completely unbuffered. Setlinebuf is used to change stdout or stderr from block buffered or unbuffered to line buffered. Unlike setbuf and setbuffer it can be used at any time that the file descriptor is active. A file can be changed from unbuffered or line buffered to block buffered by using freopen (see fopen(3S)). A file can be changed from block buffered or line buffered to unbuffered by using freopen followed by setbuf with a buffer argument of NULL. SEE ALSO
fopen(3S), getc(3S), putc(3S), malloc(3), fclose(3S), puts(3S), printf(3S), fread(3S) BUGS
The standard error stream should be line buffered by default. The setbuffer and setlinebuf functions are not portable to non-4.2BSD versions of UNIX. On 4.2BSD and 4.3BSD systems, setbuf always uses a suboptimal buffer size and should be avoided. Setbuffer is not usually needed as the default file I/O buffer sizes are optimal. 4th Berkeley Distribution May 12, 1986 SETBUF(3S)

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