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### 4.58 `setvbuf`—specify file or stream buffering

Synopsis

         #include <stdio.h>
         int setvbuf(FILE *fp, char *buf,
             int mode, size_t size);

Description
Use `setvbuf` to specify what kind of buffering you want for the file or stream identified by fp, by using one of the following values (from `stdio.h`) as the mode argument:

`_IONBF`
  Do not use a buffer: send output directly to the host system for the file or stream identified by fp.
`_IOFBF`
  Use full output buffering: output will be passed on to the host system only when the buffer is full, or when an input operation intervenes.
`_IOLBF`
  Use line buffering: pass on output to the host system at every newline, as well as when the buffer is full, or when an input operation intervenes.

Use the size argument to specify how large a buffer you wish. You can supply the buffer itself, if you wish, by passing a pointer to a suitable area of memory as buf. Otherwise, you may pass `NULL` as the buf argument, and `setvbuf` will allocate the buffer.

Warnings
You may only use `setvbuf` before performing any file operation other than opening the file.

If you supply a non-null buf, you must ensure that the associated storage continues to be available until you close the stream identified by fp.

Returns
A `0` result indicates success, `EOF` failure (invalid mode or size can cause failure).

Portability
Both ANSI C and the System V Interface Definition (Issue 2) require `setvbuf`. However, they differ on the meaning of a `NULL` buffer pointer: the SVID issue 2 specification says that a `NULL` buffer pointer requests unbuffered output. For maximum portability, avoid `NULL` buffer pointers.

Both specifications describe the result on failure only as a nonzero value.

Supporting OS subroutines required: `close`, `fstat`, `isatty`, `lseek`, `read`, `sbrk`, `write`.