< cpp‎ | io‎ | basic filebuf
Input/output library
I/O manipulators
C-style I/O
(deprecated in C++98)
File I/O
String I/O
Array I/O
(deprecated in C++98)
(deprecated in C++98)
(deprecated in C++98)
Synchronized Output
Error category interface
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

#include <fstream>
#include <iostream>
#include <string>
int main()
        int cnt = 0;
        std::ifstream file;
        char buf[10241];
        file.rdbuf()->pubsetbuf(buf, sizeof buf);"/usr/share/dict/words");
        for (std::string line; getline(file, line); )
        std::cout << cnt << '\n';

See also

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