/*
Unhide shows hidden processes.

Basically it's a tiny little program which sends signal 0 
to each possible PID. It could be easily written in PERL 
or sh.  All it does is give you a sorted list of PIDs, 
which you have to correlate. On a busy system all you have 
to do is minimize the time between running the commands: 
$ ./hide > /tmp/foo; ps -ax > /tmp/bar 

Then use your standard Unix tools for extracting the list of processes: 
$ tail -n +2 /tmp/bar | awk -- '{ print $1; }' | sort > /tmp/baz 

Note that the files must be sorted lexically, not numerically, for comm 
to work: 
$ sort /tmp/foo > /tmp/bar

Then show the pids found only by unhide: 
$ comm -2 -3 /tmp/bar /tmp/baz 

Note that I do all postprocessing after I've saved the values; you really 
don't want awk and sort and tail to show up in the listings. 
Ideally you would use it on a quiescent system, but sometimes that's 
not possible.  Happy hunting!

Idea from Thomas Ptacek.
*/

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <errno.h>
#include <sys/types.h>

#define PID_MAX 30000   /* 4.4BSD, YMMV */

int main(int argc, char **argv) {
        int i = 0;
        
        for(i = 1; i < PID_MAX + 1; i++) 
                if(kill(i, 0) != -1 || errno != ESRCH)
                        printf("%d\n", i);

        exit(0);
}