Thread: program execution begins

I have a question about global variable initialization that's been causing me some confusion.

I understand that in C programs, the execution starts from the main function. However, I'm unsure about the difference between initializing a global variable within main versus initializing it before main starts.

Scenario 1: Declaring and Assigning Global Variable in Main

Code:

  #include <stdio.h>

// Declaration of global variable
int x;

int main() {
    // Assigning a value to the global variable within main
    x = 10;

    // Using the global variable
    printf("The value of x is: %d\n", x);

    return 0;
}

Scenario 2: Declaring and Assigning Global Variable Before Main

Code:

#include <stdio.h>

// Declaration and assignment of global variable before main
int x = 20;

int main() {
    // Using the global variable
    printf("The value of x is: %d\n", x);

    return 0;
}

Could you clarify the following points for me?

How does the program flow differ when a global variable is assigned a value within main versus being initialized before main starts?

Originally Posted by Kittu20

I have a question about global variable initialization that's been causing me some confusion.

I understand that in C programs, the execution starts from the main function. However, I'm unsure about the difference between initializing a global variable within main versus initializing it before main starts.

// ...

Could you clarify the following points for me?

How does the program flow differ when a global variable is assigned a value within main versus being initialized before main starts?

Global: int x = 20; // Harcoded "Initialization" by the compiler at compile time.

Global: int x; // Uninitialized global variables are automatically initialized to zero
Local: x = 20; // "Assignment" x receives the value when the line is executed

These are VERY basic concepts that a good up to date book on the C Programming language would teach you! We are not here to teach you the language!

I have posted the following many times!

If you are attempting to learn from painfully inadequate online tutorials, YouTube videos, or some book claiming to teach you in a short time, then THROW THEM AWAY!

Short of taking a course in C Programming from a qualified instructor, you need to study a good book on the C Programming Language, cover to cover, and do all the exercises at the end of each chapter! Choose one of the three listed below:

C Programming, A Modern Approach, 2nd Edition
Author: K. N. King

C Primer Plus, 6th Edition
Stephen Prata

C How to Program, 8/e
Deitel & Deitel

Studying one of these books, and writing code, you will have a much better understanding of the C Programming language.

Originally Posted by rstanley

Global: int x = 20; // Harcoded "Initialization" by the compiler at compile time.

Global: int x; // Uninitialized global variables are automatically initialized to zero
Local: x = 20; // "Assignment" x receives the value when the line is executed
.

This means that the compiler/linker allocates memory address for a global variable and assigns a value to it before the main function is executed.

Originally Posted by Kittu20

This means that the compiler/linker allocates memory address for a global variable and assigns a value to it before the main function is executed.

It's the compiler that initializes x during the compile process. Look at the source file and the assembler code:

Source file:

Code:

int x = 20;

int main(void)
{

  return 0;
}

Compile command: [gcc -s global.c]

Look at the section"x:"

Assembler output:

Code:

        .file   "global.c"
        .text
        .globl  x
        .data
        .align 4
        .type   x, @object
        .size   x, 4
x:
        .long   20
        .text
        .globl  main
        .type   main, @function
main:
.LFB0:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        movl    $0, %eax
        popq    %rbp
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE0:
        .size   main, .-main
        .ident  "GCC: (Debian 12.2.0-14) 12.2.0"
        .section        .note.GNU-stack,"",@progbits

The linker would resolve the definition of the global variable, and it's use in main() if used.

Originally Posted by aghast

In an embedded system, there will be a runtime library function that does whatever initialization is required. However, there is unlikely to be any initialization beyond "set everything to zero". Embedded systems force the coder to set up all the "magically happens before main() is called" parts of the system themselves.

Unless it's a janky/non-standard compiler, there will be a C runtime that initializes variables with values before main() is called even on embedded systems. That's the case with the most popular C compilers for AVR and PIC microcontrollers and with other microcontroller C compilers like SDCC/Z88DK. It's usually just a memcpy() (or often an inlined equivalent) from the read-only program space to RAM. There's no good reason (in my mind) not to properly support initialized global variables—it won't save code space since the programmer would have to do it in main() anyway.