NAME

setbuf, setvbuf, setbuffer, setlinebuf − assign buffering to a stream

SYNOPSIS

#include <stdio.h>

void setbuf (stream, buf)
FILE ∗stream;
char ∗buf;

SYNOPSIS (AT&T System V)

int setvbuf (stream, buf, type, size)
FILE ∗stream;
char ∗buf;
int type, size;

SYNOPSIS (4.2 BSD)

void setbuffer (stream, buf, size)
FILE ∗stream;
char ∗buf;
int size;

void 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(3C) 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 may be used after a stream has been opened but before it is read or written.  It causes the array pointed to by buf to be used instead of an automatically allocated buffer.  If buf is the NULL pointer input/output will be completely unbuffered. 

A constant BUFSIZ, defined in the <stdio.h> header file, tells how big an array is needed:

char buf[BUFSIZ];

Setvbuf may be used after a stream has been opened but before it is read or written.  Type determines how stream will be buffered.  Legal values for type (defined in stdio.h) are:

_IOFBF causes input/output to be fully buffered. 

_IOLBF causes output to be line buffered; the buffer will be flushed when a newline is written, the buffer is full, or input is requested. 

_IONBF causes input/output to be completely unbuffered. 

If buf is not the NULL pointer, the array it points to will be used for buffering, instead of an automatically allocated buffer.  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, buf and size are ignored. 

Setbuffer, an alternate form of setbuf, may be 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 may be 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. 

DIAGNOSTICS

If an illegal value for type or size is provided, setvbuf returns a non-zero value.  In programs linked in one of the ANSI C compilation modes (see hc(1)), and in programs linked in the 88open OCS-compliant mode, setvbuf also returns a non-zero value when the request cannot otherwise be honored.  In all other cases, the value returned will be zero. 

SEE ALSO

fopen(3S), getc(3S), malloc(3C), putc(3S), stdio(3S), hc(1). 

NOTE

A common source of error is allocating buffer space as an ”automatic” variable in a code block, and then failing to close the stream in the same block. 

The order of the buf and type parameters to setvbuf was reversed in System V Release 3.0 to be consistent with the System V Interface Definition. Source code written for AT&T System V Release 2 or CX/UX Release 3 must be changed before linking.

CX/UX Programmer’s Reference Manual