NAME

setvbuf — assign buffering to a stream

SYNOPSIS

#include <stdio.h>

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


DESCRIPTION

^[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The setvbuf() function may be used after the stream pointed to by stream is associated with an open file but before any other operation (other than an unsuccessful call to setvbuf()) is performed on the stream. The argument type determines how stream shall be buffered, as follows:

• {_IOFBF} shall cause input/output to be fully buffered.

• {_IOLBF} shall cause input/output to be line buffered.

• {_IONBF} shall cause input/output to be unbuffered.

If buf is not a null pointer, the array it points to may be used instead of a buffer allocated by setvbuf() and the argument size specifies the size of the array; otherwise, size may determine the size of a buffer allocated by the setvbuf() function. The contents of the array at any time are unspecified.

For information about streams, see 2.5 Standard I/O Streams.

RETURN VALUE

Upon successful completion, setvbuf() shall return 0. Otherwise, it shall return a non-zero value if an invalid value is given for type or if the request cannot be honored, ^[CX]  and may set errno to indicate the error.

ERRORS

The setvbuf() function may fail if:

[EBADF]

^[CX] The stream is not a memory stream and the file descriptor underlying the stream is not valid.

EXAMPLES

None.

APPLICATION USAGE

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.

With setvbuf(), allocating a buffer of size bytes does not necessarily imply that all of size bytes are used for the buffer area.

Applications should note that many implementations only provide line buffering on input from terminal devices.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

2.5 Standard I/O Streams, fopen, setbuf

XBD <stdio.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

Extensions beyond the ISO C standard are marked.

The setvbuf() prototype is updated for alignment with the ISO/IEC 9899:1999 standard.

Issue 8

Austin Group Defect 1144 is applied, changing the [EBADF] error condition.