do you know what exactly startup code does ?

let me explain what happens before the main()

Most C programmers believe that a program starts executing from the main() function. Surprisingly, that is not true. Before main() is ever called, another piece of code runs automatically. This hidden code is known as startup code.

Startup code is provided by the compiler and the C library(runtime). Although programmers never write it themselves, every C program depends on it. Its job is simple but essential—it prepares the program so that main() can execute correctly.

The first thing startup code does is initialize memory. It loads the values of initialized global and static variables into memory and clears the "_uninitialized variables by setting them to zero. _".This ensures that every global and static variable begins with the correct value.

Next, startup code prepares the execution environment. It sets up the stack, initializes the runtime library, and, in operating systems like Linux and Windows, prepares the command-line arguments (argc and argv) that are passed to the main() function.
like:

#include <stdio.h>

int main(int argc, char *argv[])
{
    printf("Number of arguments = %d\n", argc);

    for (int i = 0; i < argc; i++)
    {
        printf("argv[%d] = %s\n", i, argv[i]);
    }

    return 0;
}

Once everything is ready, startup code transfers control to main(), where the programmer's code finally begins execution. When main() finishes, startup code performs the final cleanup by calling the necessary exit routines and returning the program's exit status(collects the value and check either succseed or failed) to the operating system.

The execution flow of a C program can be summarized as follows:

Program Starts
      ↓
Startup Code
      ↓
Initialize Memory
      ↓
Setup Runtime Environment
      ↓
Call main()
      ↓
Cleanup and Exit

Although startup code remains invisible to most programmers, it is one of the most important parts of a C program. Without it, the program would not have a properly initialized memory or execution environment, making reliable execution impossible. Understanding startup code gives us a clearer picture of what really happens before our own code begins to run.

Can We See the Source Code of Startup Code ?