I have set a shortcut, <Super>l, to execute slock, which was /usr/bin/slock (and suid root), and all was fine and good.

Recently I have uninstalled slock, and changed the command to lock, which is ~/bin/lock (in path, not suid root but setcap CAP_DAC_READ_SEARCH+ep).  executing lock in terminal, or as a panel plugin, works just fine, but the above shortcut just makes the monitor blink for a second.

I have then changed the command to ~/tst, which is a simple wraper:

~ $ cat tst.c
#include <stdio.h>
#include <stdlib.h>

int
main(void)
{
    system("echo starting... > dbg");
    int i = system("lock 2>> dbg");
    FILE *fp = fopen("dbg", "a");
    fprintf(fp, "%d",i);
    return i;
}

~/dbg is indeed created, and its content is:
~ $ cat dbg
starting...
256

How do I debug such an issue?

XFCE 4.10 on Arch Linux.

Thanks.