The predefined Variables
For each executable mql4-program a set of predefined variables is supported, which reflect the state of the current price chart by the moment a mql4-program (Expert Advisor, script or custom indicator) is started.
Values of predefined variables are set by the client terminal before a mql4-program is started. Predefined variables are constant and cannot be changed from a mql4-program. As exception, there is a special variable _LastError, which can be reset to 0 by the ResetLastError function.
Predefined variables cannot be defined in a library. A library uses such variables that are defined in program from which this library is called.