From version 1.6 of Zabbix, zabbix_sender has changed, and it doesn't read config file from the command line anymore. I've strace-ed zabbix_sender, and there is no open call for anything I put after -c parameter... Here are the examples:

1. /usr/sbin/zabbix_sender -z myserver.mydomain.com -c /etc/zabbix/zabbix_agentd.conf -k mysql.bytesreceived -o 45940
   Info from server: "Processed 1 Failed 0 Total 1 Seconds spent 0.004097"
   sent: 1; skipped: 0; total: 1

1. /usr/sbin/zabbix_sender -c /etc/zabbix/zabbix_agentd.conf -k mysql.bytesreceived -o 45940
   usage: zabbix_sender [-Vhv] {[-zpsI] -ko | [-zpI] -i <file>}

   [-c <file>]

1. strace /usr/sbin/zabbix_sender -c /etc/zabbix/zabbix_agentd.conf -k mysql.bytesreceived -o 45940 2>&1 | grep open | grep etc
   open("/etc/ld.so.cache", O_RDONLY) = 3

Problem is that function that parses options from command line has wrong check... It only checks if ZABBIX_SERVER is on the command line ('-z' argument), and if it's not, it exits with error. That is not the correct behaviour because ZABBIX_SERVER can be stated in the config file too. Here is the patch....

— src/zabbix_sender/zabbix_sender.c 2008-12-19 16:59:18.428357863 +0100
+++ src/zabbix_sender/zabbix_sender.c 2008-12-19 17:07:56.309106954 +0100
@@ -360,7 +360,7 @@
break;
}

• if(ZABBIX_SERVER == NULL)
  + if((ZABBIX_SERVER == NULL) && (CONFIG_FILE == NULL))
  {
  usage();
  exit(FAIL);