setbuf, setvbuf -- assign buffering to a stream


   #include <stdio.h>

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

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


setbuf may be used after a stream [see intro(3)] 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.

setbuf can also be used after a stream has been opened and written to if the buffer has been flushed with fflush prior to calling setbuf. [See fopen(3S)]. This will prevent loss of data in the existing buffer.

To use setbuf after a read, make sure that the application has read all of the characters in one buffer and make sure that it will not receive any more characters until after calling setbuf.

While there is no limitation on the size of the buffer, the constant BUFSIZ, defined in the stdio.h header file, is typically a good buffer size:

   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. Valid values for type (defined in stdio.h) are:

causes input/output to be fully buffered.

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

causes input/output to be completely unbuffered.

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

For a further discussion of buffering, see Intro(3S).


If an invalid value for type is provided, setvbuf returns a non-zero value. Otherwise, it returns zero.


fopen(3S), getc(3S), Intro(3S) malloc(3C), putc(3S),


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.

Parts of buf are used for internal bookkeeping of the stream and, therefore, buf contains less than size bytes when full. It is recommended that the automatically allocated buffer is used when using setvbuf.

© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 25 April 2004