std::basic_filebuf<CharT,Traits>::setbuf
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 readsn-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
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); file.open("/usr/share/dict/words"); for (std::string line; getline(file, line); ) ++cnt; 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 (function) |