A UNIX Path Name Library for C++ - Path(C++)

Implementation

We conclude this tutorial with a note regarding implementation. The function expand_wildcards might have been implemented as follows:

       void Path::expand_wildcards(List<Path> & ret) {
           const char *tmp = tmpnam();
           ostrstream o;
           o << "ls " << *this << " > " << tmp <<ends;
           system(o.str());
           ifstream f(tmp);
           String s;
           ret.make_empty();
           while (f >> s)
               ret.put(s);
       }

First the shell command ls pattern>tmp\fR is sent to the shell to perform the actual pattern expansion, then the file containing the results is read and the resulting list is constructed. This implementation has several problems:

It is slow. Invoking system requires starting up an entire subshell.
It is unreliable. It requires that ls be somewhere along the current search path, and further that the ls found have the appropriate behavior. If the user has installed a local ls, there is no guarantee the above function will work correctly.
It uses /bin/sh, which is the C shell on BSD systems, and the Bourne shell on most other systems. Using different shells on different machines means that the programmer cannot be guaranteed that patterns will be expanded in the same way across all machines. Also, the pattern matching capabilities of both the C shell and the Bourne shell are inferior to those of the Korn shell.

To overcome these problems, the actual implementation of expand_wildcards steals code from the Korn shell. The resulting implementation is fast, reliable, and (of course) Korn shell-based.

Acknowledgement Special thanks to Dave Korn for his patient explanations of the inner workings of the Korn shell.