std::setbuf

 

C++

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Thread support library (C++11)

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Technical Specifications

 

Input/output library

I/O manipulators

C-style I/O

Buffers

basic_streambuf

basic_filebuf

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Abstractions

ios_base

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istrstream (deprecated)

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Types

streamoff

streamsize

fpos

Error category interface

iostream_category (C++11)

io_errc (C++11)

 

C-style I/O

Functions
File access
fopen freopen fclose fflush	fwide setbuf setvbuf
Direct input/output
fread
fwrite
Unformatted input/output
fgetc fgets fputc fputs getchar gets (until C++14) putchar puts ungetc	fgetwc fgetws fputwc fputws getwchar putwchar ungetwc
Formatted input/output
scanffscanfsscanf vscanfvfscanfvsscanf (C++11)(C++11)(C++11) printffprintfsprintfsnprintf (C++11) vprintfvfprintfvsprintfvsnprintf (C++11)	wscanffwscanfswscanf vwscanfvfwscanfvswscanf (C++11)(C++11)(C++11) wprintffwprintfswprintf vwprintfvfwprintfvswprintf
File positioning
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Error handling
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remove
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tmpfile
tmpnam

 

Defined in header <cstdio>
void setbuf( std::FILE* stream, char* buffer );

Sets the internal buffer to use for I/O operations performed on the C stream stream.

If buffer is not null, equivalent to std::setvbuf(stream, buffer, _IOFBF, BUFSIZ)

If buffer is null, equivalent to std::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. If not null, must be able to hold at least BUFSIZ characters

Return value

(none)

Notes

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

std::setvbuf should also be used to detect errors, since std::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 std::setbuf/std::setvbuf).

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

int main() {
    char buf[BUFSIZ];
    std::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 <cstdio>
#include <thread>
#include <chrono>
 
int main()
{
    using namespace std::chrono_literals;
 
    std::setbuf(stdout, NULL); // unbuffered stdout
    std::putchar('a'); // appears immediately on unbuffered stream
    std::this_thread::sleep_for(1s);
    std::putchar('b');
}

Output:

ab

See also

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