std::basic_filebuf<CharT,Traits>::setbuf (3) - Linux Man Pages

std::basic_filebuf<CharT,Traits>::setbuf: std::basic_filebuf<CharT,Traits>::setbuf

NAME

std::basic_filebuf<CharT,Traits>::setbuf - std::basic_filebuf<CharT,Traits>::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 controlled 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::basic_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


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