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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 associated
		  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
	  provide  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';
	}

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		  2022.07.31	 std::basic_filebuf::setbuf(3)

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