The executing subsystem of the client terminal has an opportunity to save the error code in case it occurs during a MQL4 program run. There is a predefined variable _LastError for each executable MQL4 program.
Before starting the OnInit function, the _LastError variable is reset. In case an erroneous situation occurs during calculations or in the process of internal function calls, the _LastError variable accepts a corresponding error code. The value stored in this variable can be obtained using the GetLastError() function.
There are several critical errors in case of which a program is terminated immediately: