求助下面这个文本流打不开，程序是课本上的，问题出哪了？

#include<iostream.h>
#include<fstream.h>
#include<stdlib.h>
void main()
{
char filename[40],line[80];
ifstream infile;
cout<<"please input a filename:";
cin>>filename;
infile.open (filename);
if(!infile)
{
cout<<"Can not open file:"<<filename<<endl;
exit(1);
}
while(infile.getline(line,80))
cout<<line<<endl;
infile.close();
}