The following information applies to the client/PC side scripting to invoke actions on your camera from your PC using ptp client applications (see http://chdk.wikia.com/wiki/PTP_Extension) which also use LUA for PC side scripting (not to be confused with using LUA for scripts that run on your camera). (I believe that ->) This information doesn't apply to the (non-GUI) ptpcam client application but does apply to the (GUI) ptpcamgui and chdkptp client applications.
General information about lua execution with PTP
- To execute a Lua script already loaded on your camera (e.g. testprgm.lua) from chdkptp interactive mode, use
con 1 > = loadfile('A/CHDK/SCRIPTS/testprgm.lua')()
- Executing a script while another script is already running can cause crashes and memory leaks.
- To check whether your script has terminated, you can use the script status command (script-status in ptpcam.)
- The function switch_mode_usb() should be used instead of set_record to switch between record and play mode when PTP is connected.
- Lua return values and errors messages are sent using the message interface.
- Simple tables are returned in a limited human readable string format by default, see  for a description. This can be changed by providing your own usb_msg_table_to_string. A replacement that properly serializes lua tables can be found in chdkptp.
- Lua script execution currently leaves CHDK alt mode in a confused state. You may need to press the alt button a couple times to get the alt menu and buttons to respond as expected.
- While scripts in files already on the camera may be used, functions that yield (e.g. sleep(), wait_click() etc.) cannot be used inside require (or other pcall).
Set the camera to play or record mode.
mode is 1 for record or 0 for play.
msg = read_usb_msg([timeout])
Read a message from the CHDK ptp interface.
Returns the next available message as a string, or nil if no messages are available
If timeout is given and not zero, wait until a message is available or timeout expires
status = write_usb_msg(msg,[timeout])
Writes a message to the CHDK ptp interface.
msg may be nil, boolean, number, string or table. Tables will be converted to a string by usb_msg_table_to_string
returns true if the message was queued successfully, otherwise false
If timeout is set and not zero, wait until message is written or timeout expires.
NOTE strings will not include a terminating NULL.
This function is called when a table is returned from a lua script, or sent as a message. Lua code may override this function to provide different serialization formats. The return value should be a string on success. On failure, a lua error should be thrown.