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 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¶

Provides 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';
}

Possible output:¶

356010

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 173 C++98 the type of n was misspecified as corrected to std::streamsize
int

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
(function)