From cppreference.com
std::streamsizesgetn(char_type*s,std::streamsizecount); (1) protected:virtualstd::streamsizexsgetn(char_type*s,std::streamsizecount); (2) 1) Calls xsgetn(s, count) of the most derived class.
2) Reads count characters from the input sequence and stores them into a character array pointed to by s. The characters are read as if by repeated calls to
. That is, if less than count characters are immediately available, the function calls
to provide more until Traits::eof() is returned.
Classes derived from std::basic_streambuf are permitted to provide more efficient implementations of this function.
Parameters
s - pointer to the beginning of a char_type array count - maximum number of characters to read. Return value
The number of characters successfully read. If it is less than count the input sequence has reached the end.
Notes
The rule about "more efficient implementations" permits bulk I/O without intermediate buffering: that's how
simply passes the pointer to the POSIX read() system call in some implementations of iostreams
Example
See also