std::basic_filebuf::setbuf
| protected: virtual std::basic_streambuf<CharT, Traits>* setbuf( char_type* s, std::streamsize n ) |
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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.
Contents |
[edit] Parameters
| s | - | pointer to the first byte in the user-provided buffer or null |
| n | - | the number of bytes in the user-provided buffer or zero |
[edit] Return value
*this, cast to the base class std::basic_streambuf.
[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
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'; }
[edit] See also
| invokes setbuf() (public member function of std::basic_streambuf) | |
| sets the buffer and its size for a file stream (function) | |
