# setbuf

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< c‎ | io

Defined in header `<stdio.h>` |  |
void setbuf( FILE *stream, char *buffer ); |  | (until C99)
void setbuf( FILE *restrict stream, char *restrict buffer ); |  | (since C99)

Sets the internal buffer to use for stream operations. It should be at least `BUFSIZ` characters long.

If `buffer` is not null, equivalent to setvbuf(stream, buffer, _IOFBF, BUFSIZ).

If `buffer` is null, equivalent to setvbuf(stream, NULL, _IONBF, 0), which turns off buffering.

### Parameters

stream | - | the file stream to set the buffer to
buffer | - | pointer to a buffer for the stream to use. If NULL is supplied, the buffering is turned off

### Return value

None.

### Notes

If BUFSIZ is not the appropriate buffer size, setvbuf can be used to change it.

setvbuf should also be used to detect errors, since `setbuf` does not indicate success or failure.

This function may only be used after `stream` has been associated with an open file, but before any other operation (other than a failed call to setbuf/`setvbuf`).

A common error is setting the buffer of stdin or stdout to an array whose lifetime ends before the program terminates:

    int main(void) {
        char buf[BUFSIZ];
        setbuf(stdin, buf);
    } // lifetime of buf ends, undefined behavior

### Example

setbuf may be used to disable buffering on streams that require immediate output.

Run this code

    #include <stdio.h>
    #include <threads.h>

    int main(void)
    {
        setbuf(stdout, NULL); // unbuffered stdout
        putchar('a'); // 'a' appears immediately if stdout is unbuffered
        thrd_sleep(&(struct timespec){.tv_sec=1}, NULL); // sleep 1 sec
        putchar('b');
    }

Output:

    ab

### References

- C11 standard (ISO/IEC 9899:2011):

  - 7.21.5.5 The setbuf function (p: 307-308)

- C99 standard (ISO/IEC 9899:1999):

  - 7.19.5.5 The setbuf function (p: 273)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.9.5.5 The setbuf function

### See also

setvbuf | sets the buffer and its size for a file stream (function)
C++ documentation for setbuf

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