IBC#003: The main function

Where everything starts

As we discovered in Coding #103, the entry point of every C++ project is the main() function; of course, the same can be said for Bitcoin Core.

The main() function of Bitcoin Core can be found in the bitcoind.cpp file¹, inside the src folder².

Here we are. We are ready to dive Inside Bitcoin Code!

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Main(): overall function

Here, the main() function is reported:

MAIN_FUNCTION
{
#ifdef WIN32
    common::WinCmdLineArgs winArgs;
    std::tie(argc, argv) = winArgs.get();
#endif

    NodeContext node;
    int exit_status;
    std::unique_ptr<interfaces::Init> init = interfaces::MakeNodeInit(node, argc, argv, exit_status);
    if (!init) {
        return exit_status;
    }

    SetupEnvironment();

    // Connect bitcoind signal handlers
    noui_connect();

    return (AppInit(node, argc, argv) ? EXIT_SUCCESS : EXIT_FAILURE);
}

The first thing that can be noticed is that there isn’t the classical main:

int main(int argc, char* argv[])

Where argc represents the number of inputs and argv an array of arrays of char that contains the inputs.

However, here there is something more compact:

MAIN_FUNCTION

This is a preprocessor definition that can be found in compact.h³, and allows to prevent a bug that occurs when building using the mingw-w64 compiler and exporting for a 32 bit windows machine.

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Main(): step-by step

Command line arguments

First of all, the code checks if the target is a 32-bit Windows machine:

#ifdef WIN32
    common::WinCmdLineArgs winArgs;
    std::tie(argc, argv) = winArgs.get();
#endif

This is done by using the preprocessor directive #ifdef. This directive checks if a variable (WIN32 in our case), is defined; if so, the functions inside the block are called, otherwise they are skipped. #endif closes the block.

This blocks takes the main() arguments form command line from a tuple (a special container that holds a collection of elements) and fills the argc and argv variables.

Node context initialization

After that, a NodeContext, a structure containing references to chain state and connection state, is created and initialized:

NodeContext node;
int exit_status;
std::unique_ptr<interfaces::Init> init = 
    interfaces::MakeNodeInit(node, argc, argv, exit_status);
if (!init) {
    return exit_status;
}

The exit_status variable is declared in order to hold the return status of the initialization, in case it fails.

A std::unique_ptr (a special type of smart pointer, we will talk about it in Learn2Code) that holds the node initialization is then declared and is defined by the MakeNodeInit funtcion.

Finally, if initialization fails, the main() function returns reporting the exit_status.

Setting up the environment

Later, a function to set up the environment is called:

SetupEnvironment();

This functions sets up the environemnt according to the target machine and it can be found in the system.cpp file⁴. It sets up things like the input/output default charset.

Event handlers connection

Once the environment is set up, Bitcoin Core creates and connects all the events not connected to the Graphic User Interface (GUI).

noui_connect();

These handlers are used for logging and printing messages.

Launching the application

Once everything is set up, it is finally possible to launch the application:

return (AppInit(node, argc, argv) ? EXIT_SUCCESS : EXIT_FAILURE);

Here, the AppInit function takes as input the node and the command line arguments.

The return value of the main() function is chosen through a so-called ternary operator, which presents the following syntax:

condition ? val1 : val2

if condition evaluates to TRUE, the value taken into consideration will be val1 else it will be val2. Thus, if AppInit returns a TRUE value, the return value of the main() will be EXIT_SUCCESS, else it would be EXIT_FAILURE. These values are defined inside stdlib.h, the C standard library.

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If you got here, good job again! You now know what happens inside the main() functions of Bitcoin Core.

Thank you again for reading Inside Bitcoin Code, your support is truly appreciated. Consider sharing this post and this newsletter with anyone you think might be interested and let me know in the comments section what you think.

See you next week!

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1. bitcoind.cpp

2. See IBC#002 for folder structure

3. compat.h

4. system.cpp