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.
| 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 |
this.
The conditions when this function may be used and the way in which the provided buffer is used is implementation-defined.
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. 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. 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.
#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';
} invokes setbuf() (public member function of std::basic_streambuf) |
|
| sets the buffer and its size for a file stream (function) |
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