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Name

setbuf, setvbuf - assign buffering to a stream

Synopsis

#include <stdio.h>

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

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

MT-Level

MT-Safe

Description

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. The constant BUFSIZ , defined in the <stdio.h> header, indicates how large the array pointed to by buf should be.

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. If input/output is unbuffered, buf and size are ignored.

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

Return Values

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

See Also

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

Notes

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.

When using setbuf(), buf should always be sized using BUFSIZ . If the array pointed to by buf is larger than BUFSIZ , a portion of buf will not be used. If buf is smaller than BUFSIZ , other memory may be unexpectedly overwritten.

Parts of buf will be used for internal bookkeeping of the stream and, therefore, buf will contain less than size bytes when full. It is recommended that stdio(3S) be used to handle buffer allocation when using setvbuf().

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