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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



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);


The setbuf() function may be used after the stream pointed to by stream (see Intro(3)) is 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 the size of the array pointed to by buf.

The setvbuf() function may be used after a stream is opened but before it is read or written. The type argument determines how stream will be buffered. Legal values for type (defined in < stdio.h>) are:


Input/output to be fully buffered.


Output to be line buffered; the buffer will be flushed when a NEWLINE is written, the buffer is full, or input is requested.


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. The size argument 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(3C).

Return Values

If an illegal value for type is provided, setvbuf() returns a non-zero value. Otherwise, it returns 0.


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(3C) be used to handle buffer allocation when using setvbuf().


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

malloc(3C), fopen(3C), getc(3C), putc(3C), stdio(3C), attributes(7), standards(7)