popen, pclose \- initiate I/O to/from a process
are pointers to null-terminated strings
containing respectively a shell command line and an I/O
mode, either "r" for reading or "w" for
It creates a pipe between
the command to be executed.
can be used (as appropriate) to write to the standard input
of the command or read from its standard output.
which waits for the associated process to terminate
and returns the exit status of the command.
Because open files are shared, a type "r" command
may be used as an input filter,
and a type "w" as an output filter.
if files or processes cannot be created, or the Shell
is not associated with a `popened' command.
Buffered reading before opening an input filter
may leave the standard input of that filter mispositioned.
Similar problems with an output filter may be
forestalled by careful buffer flushing, e.g. with