Question

Say I do this (a contrived example):

#include <iostream> #include <fstream>  using namespace std;  int main(int argc, char* argv[]) {     ifstream ifs(argv[1]);      char ch;     while(ifs.read(&ch, 1)) {         cout << ch;     } } 

I assume(hope) that the iostream library does some internal buffering here and doesn’t turn this into gazillions of one-byte file-read operations at the OS level.

Is there a way of:

a) Finding out the size of ifstream’s internal buffer?

b) Changing the size of ifstream’s internal buffer?

I’m writing a file filter that needs to read multi-gigabyte files in small chunks and I’d like to experiment with different buffer sizes to see if it affects performance.

Answer 1

You can use ios::rdbuf() to get a pointer to a streambuf object. This object represents the internal buffer for the stream.

You can call streambuf::pubsetbuf(char * s, streamsize n) to set a new internal buffer with a given size.

See this link for more details.

edit: Here is how it would look in your case:

 #include <iostream> #include <fstream> using namespace std;  int main(int argCount, char ** argList[]) {     ifstream inStream(argList[1]);      char myBuffer [512];     inStream.rdbuf()->pubsetbuf(myBuffer, sizeof(myBuffer));      char ch;     while(inStream.read(&ch, 1))     {         cout << ch;     } }

edit: as pointed out by litb, the actual behavior of streambuf::pubsetbuf is ‘implementation-defined’.

If you really want to play around with the buffers, you may have to roll your own buffering class that inherits from streambuf.