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


std::basic_filebuf::setbuf - std::basic_filebuf::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);"/usr/share/dict/words");

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

See also

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