When a variable is declared, the process will reserve a certain amount of memory on the stack to store the value of the variable. The process will allocate enough memory to hold the entire possible value. For example, if you declare a string with size 20, the process will allocate 20 bytes of memory on the stack for that string.
If no value is initialized, the initial value of the variable will be undefined. This means that it may have any value, and that value may be different each time the program is executed.
When a value is assigned to a variable, the compiler will store that value at the memory address associated with the variable.
Therefore, it is generally not recommended to rely on the initial value of a variable if it has not been explicitly initialized. This can lead to unpredictable behavior.
It is generally best practice to initialize variables to known values when they are declared.
Code example
#include <stdio.h>
#include <string.h>
int main(void) {
char string1[16]; // create an array of characters with size 16
char string2[16]; // create a second array of characters with size 16
printf("string1 stack address : %p\n", string1);
printf("string2 stack address : %p\n", string2);
printf("%s\n", string1);
printf("%s\n", string2);
// fill the first string with 15 'A's followed by a null character '\0'
memset(string1, 'A', 15);
string1[15] = '\0';
// fill the second string with 15 'B's followed by a null character '\0'
memset(string2, 'B', 15);
string2[15] = '\0';
printf("String 1: %s\n", string1); // print string 1
printf("String 2: %s\n", string2); // print string 2
return 0;
}
This code will declare two arrays of characters, string1 and string2, and set them with the strings "AAAAAAAAAAAAAAA" and "BBBBBBBBBBBBBBB", respectively.
Note that values on the stack aren't initialized to any specific value. If there is already some data in the memory space allocated to the variable, this data will remain until it is overwritten by a new value.
When the string2 variable is declared, the process reserve 16 byte onto the stack to store this variable
The variable string2 is stored above the string1 variable due to LIFO system
Then, the process will do a memset() to each variable to set "A" and "B" 15 times respectively into string1 and string2 and then put a null byte 0x00 at the end of the string