BUG: istream::readsome Does Not Read Correctly on File Stream (246934)




This article was previously published under Q246934

SYMPTOMS

The istream::readsome function does not read correct data from a file.

CAUSE

The basic_istream::readsome function has some limitations when used on certain streams. Moreover, the MSDN documentation for basic_istream::readsome states that internally basic_istream::readsome calls the read function. However, in the basic_istream::readsome implementation, the read function is never called.

RESOLUTION

Give preference to basic_istream::read.

STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.

MORE INFORMATION

Here is a code sample that illustrates the problem and provides a workaround:
#include <iostream>
#include <fstream>

using namespace std;

int main()
{
	ifstream file("c:\\config.sys",ios::in|ios::binary);

	static char buffer[10];
	buffer[10] = 0;

	if ( !file )
		cout << "unable to open file" << endl;
	else
	{
		file.readsome(buffer,10); // It does not work properly.
		//file.read( buffer, 10 ); // Uncomment this line for work around.
		cout << buffer << endl;

		cout << endl << "File contents are:" << endl;
		file.seekg( 0 );	// Rewind file.
		while( file.good() )
		{
			file.getline( buffer, 10 );
			cout << buffer << endl;
		}
	}
	return 0;
}
Modification Type: Major Last Reviewed: 7/30/2001
Keywords: kbbug KB246934
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