Apps About system directory remount..

[w.s.choi]

Hello,
I have a question about system remount for writing file in /system.
In application, How can I execute remount action. (not adb)
I tried bellow commands, but get errno 1 (EPERM : not super user).

try 1,
... Runtime.exec("mount -oremount,rw /dev/block/mtdblock3 /system);

try 2,
... Runtime.exec("su mount -oremount,rw /dev/block/mtdblock3 /system);

try 3,
env = "LD_LIBRARY_PATH= .....";
dir = "/system/xbin"
... Runtime.exec("mount -oremount,rw /dev/block/mtdblock3 /system, env, dir);

Cannot 3rd party apps execute remount command?
or, Doesn't Android have remount API ?

I expects good reply.
Thanks!