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std::basic_filebuf::setbuf(3) C++ Standard Libarystd::basic_filebuf::setbuf(3)

NAME
       std::basic_filebuf::setbuf - std::basic_filebuf::setbuf

Synopsis
	  protected:
	  virtual  std::basic_streambuf<CharT,	Traits>* setbuf( char_type* s,
       std::streamsize
	  n )

	  If s is a null pointer and n is zero,	the filebuf becomes unbuffered
       for output,
	  meaning pbase() and pptr() are null and any  output  is  immediately
       sent to file.

	  Otherwise, a call to setbuf()	replaces the internal buffer (the con-
       trolled character
	  sequence) with the user-supplied character array whose first element
       is pointed to
	  by  s	and allows this	std::basic_filebuf object to use up to n bytes
       in that array
	  for buffering.

	  This function	is protected virtual, it may only  be  called  through
       pubsetbuf() or
	  from	member functions of a user-defined class derived from std::ba-
       sic_filebuf.

Parameters
	  s - pointer to the first CharT in the	user-provided buffer or	null
	  n - the number of CharT elements in the user-provided	buffer or zero

Return value
	  this

Notes
	  The conditions when this function may	be used	and the	way  in	 which
       the provided
	  buffer is used is implementation-defined.

	    * GCC 4.6 libstdc++

	  setbuf() may only be called when the std::basic_filebuf is not asso-
       ciated with a
	  file (has no effect otherwise). With a user-provided buffer, reading
       from file reads
	  n-1 bytes at a time.
	    * Clang++3.0 libc++

	  setbuf()  may	 be  called after opening the file, but	before any I/O
       (may crash
	  otherwise). With a user-provided buffer,  reading  from  file	 reads
       largest multiples
	  of 4096 that fit in the buffer.
	    * Visual Studio 2010

	  setbuf()  may	be called at any time, even after some I/O took	place.
       Current contents
	  of the buffer, if any, are lost.

	  The standard does not	define any behavior for	this  function	except
       that setbuf(0, 0)
	  called  before any I/O has taken place is required to	set unbuffered
       output.

Example
	  Provides a 10k buffer	for reading. On	linux, the strace utility  may
       be used to
	  observe the actual number of bytes read.

       // Run this code

	#include <fstream>
	#include <iostream>
	#include <string>

	int main()
	{
	    int	cnt = 0;
	    std::ifstream file;
	    char buf[10241];

	    file.rdbuf()->pubsetbuf(buf, sizeof	buf);
	    file.open("/usr/share/dict/words");

	    for	(std::string line; getline(file, line);)
		++cnt;
	    std::cout << cnt <<	'\n';
	}

Possible output:
	356010

	  Defect reports

	  The following	behavior-changing defect reports were applied retroac-
       tively to
	  previously published C++ standards.

	    DR	   Applied  to	      Behavior as published		  Cor-
       rect behavior
	  LWG 173 C++98	     the type of n was misspecified as	  corrected to
       std::streamsize
			     int

See also
	  pubsetbuf invokes setbuf()
		    (public    member	 function    of	    std::basic_stream-
       buf<CharT,Traits>)
	  setvbuf   sets the buffer and	its size for a file stream
		    (function)

http://cppreference.com		  2024.06.10	 std::basic_filebuf::setbuf(3)

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