std::basic_filebuf<CharT,Traits>::setbuf - cppreference.com (original) (raw)

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.

[edit] Parameters

[edit] Return value

this

[edit] 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.

[edit] Example

Provides a 10k buffer for reading. On linux, the strace utility may be used to observe the actual number of bytes read.

#include #include #include   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:

[edit] Defect reports

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

[edit] See also

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