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

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


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


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.


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



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.


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);

          for (std::string line; getline(file, line); )
          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)