General Unix/C question

[Doug Gwyn]

In article <794 at philmds.UUCP> leo at philmds.UUCP (Leo de Wit) writes:
-|Below is the program I use to run a unix command ...
-            2) when you start the program the input/output redirection is
-done, perhaps by your shell. This means that these input/output streams
-must be readable/writable by the current uid/gid; when you setuid &
-setgid the process may not be allowed anymore to write these
-descriptors.

On UNIX, the permissions are checked when the files are opened,
not on each read and write.  If the files got opened okay then
they should remain usable after the set*id() calls.