The function is called in indicators and EAs when the Init event occurs. It is used to initialize a running MQL5 program. There are two function types.

The version that returns the result

int  OnInit(void);

Return Value

int type value, zero means successful initialization.

The OnInit() call that returns the execution result is recommended for use since it not only allows for program initialization, but also returns an error code in case of an early program termination.

The version without a result return is left only for compatibility with old codes. It is not recommended for use

void  OnInit(void);


The Init event is generated immediately after loading an EA or an indicator. The event is not generated for scripts. The OnInit() function is used to initialize an MQL5 program. If OnInit() has a return value of int type, the non-zero return code means failed initialization and generates the Deinit event with the REASON_INITFAILED deinitialization reason code.

OnInit() function of void type always means successful initialization and is not recommended for use.

For optimizing the EA inputs, it is recommended to use values from the ENUM_INIT_RETCODE enumeration as a return code. These values are intended for establishing the optimization process management, including selection of the most suitable test agents. It is possible to request data on agent configuration and resources (number of cores, free memory amount, etc.) using the TerminalInfoInteger() function during the EA initialization before launching the test. Based on the obtained data, you can either allow using the test agent or ban it from optimizing the EA.




Initialization successful, EA test can be continued.

This code means the same as the zero value – the EA initialization in the tester is successful.


Initialization failed. There is no point in continuing the test due to unavoidable errors. For example, it is impossible to create an indicator necessary for the EA operation.

The return of this value means the same as returning the value different from zero – EA initialization in the tester failed.


Designed to denote an incorrect set of input parameters by a programmer. In the general optimization table, the result string with this return code is highlighted in red.

A test for such a set of EA inputs is not performed. The agent is ready to receive a new task.

When this value is received, the strategy tester does not pass this task to other agents for repeated execution.


No program execution errors during initialization. However, for some reasons, the agent is not suitable for conducting a test. For example, there is not enough RAM, no OpenCL support, etc.

After returning this code, the agent no longer receives tasks until the very end of this optimization.

Sample OnInit() function for an EA

//--- input parameters
input int      ma_period=20; // moving average period
//--- handle of the indicator used in the EA
int indicator_handle;   
//| Expert initialization function                                   |
int OnInit()
//--- check ma_period validity
      PrintFormat("Invalid ma_period input value: %d",ma_period);
//--- during optimization
      //--- check available RAM for the agent
      int available_memory_mb=TerminalInfoInteger(TERMINAL_MEMORY_TOTAL);
         PrintFormat("Insufficient memory for the test agent: %d MB",
         return (INIT_AGENT_NOT_SUITABLE);
//--- check for the indicator
      PrintFormat("Failed to generate My_Indicator handle. Error code %d",
      return (INIT_FAILED);
//--- EA initialization successful

See also

OnDeinit, Event handling functions, Program running, Client terminal events, Initialization of variables, Creating and deleting objects