setbuf

 

C

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headers

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Thread support (C11)

Atomic operations (C11)

Technical Specifications

 

File input/output

Functions
File access
fopenfopen_s (C11) freopenfreopen_s (C11) fwide (C95)	setbuf setvbuf fclose fflush
Direct input/output
fread
fwrite
Unformatted input/output
fgetc fgets fputc fputs getchar getsgets_s (until C11)(since C11) putchar puts ungetc	fgetwcgetwc (C95)(C95) fgetws (C95) fputwcputwc (C95)(C95) fputws (C95) getwchar (C95) putwchar (C95) ungetwc (C95)
Formatted input
scanffscanfsscanfscanf_sfscanf_ssscanf_s (C11)(C11)(C11) wscanffwscanfswscanfwscanf_sfwscanf_sswscanf_s (C95)(C95)(C95)(C11)(C11)(C11)	vscanfvfscanfvsscanfvscanf_svfscanf_svsscanf_s (C99)(C99)(C99)(C11)(C11)(C11) vwscanfvfwscanfvswscanfvwscanf_svfwscanf_svswscanf_s (C99)(C99)(C99)(C11)(C11)(C11)
Formatted output
printffprintfsprintfsnprintfprintf_sfprintf_ssprintf_ssnprintf_s (C99)(C11)(C11)(C11)(C11) wprintffwprintfswprintfwprintf_sfwprintf_sswprintf_ssnwprintf_s (C95)(C95)(C95)(C11)(C11)(C11)(C11)	vprintfvfprintfvsprintfvsnprintfvprintf_svfprintf_svsprintf_svsnprintf_s (C99)(C11)(C11)(C11)(C11) vwprintfvfwprintfvswprintfvwprintf_svfwprintf_svswprintf_svsnwprintf_s (C95)(C95)(C95)(C11)(C11)(C11)(C11)
File positioning
ftell
fgetpos
fseek
fsetpos
rewind
Error handling
clearerr
feof
ferror
perror
Operations on files
remove
rename
tmpfiletmpfile_s (C11)
tmpnamtmpnam_s (C11)

 

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