read bytes to a buffer from C++ std::istream

ReadBytesFromIStream.cpp

#include <iostream>

std::size_t read_bytes( std::istream& in, char* buf, std::size_t len ) {
	std::size_t n = 0;
	
	while( len > 0 && in.good() ) {
		in.read( &buf[n], len );
		int i = in.gcount();
		n += i;
		len -= i;
	}
	
	return n;
}

int main( int argc, char** argv ) {
	std::istringstream in( "this is a test stream");
	char buf[100];
	
	std::size_t n = read_bytes( in, buf, sizeof( buf ));
	assert( n < 100 );
}

You can use rdbuf->sgetn method:

std::size_t read_bytes( std::istream& in, char* buf, std::size_t len ) {
	return in.rdbuf()->sgetn(buf, len);
}