initial value of int array in C

When declaring an array in C like this:

int array[10];

What is the initial value of the integers?? I'm getting different results with different compilers and I want to know if it has something to do with the compiler, or the OS.

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If the array is declared in a function, then the value is undefined. int x[10]; in a function means: take the ownership of 10-int-size area of memroy without doing any initialization. If the array is declared as a global one or as static in a function, then all elements are initialized to zero if they aren't initialized already.

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As set by the standard, all global and function static variables automatically initialised to 0. Automatic variables are not initialised.

int a[10];  // global - all elements are initialised to 0

void foo(void) {
    int b[10];    // automatic storage - contain junk
    static int c[10]; // static - initialised to 0
}

However it is a good practice to always manually initialise function variable, regardless of its storage class. To set all array elements to 0 you just need to assign first array item to 0 - omitted elements will set to 0 automatically:

int b[10] = {0};

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Why are function locals ( auto storage class) not initialized when everything else is?

C is close to the hardware; that's its greatest strength and its biggest danger. The reason auto storage class objects have random initial values is because they are allocated on the stack, and a design decision was made not to automatically clear these (partly because they would need to be cleared on every function call).

On the other hand, the non- auto objects only have to be cleared once. Plus, the OS has to clear allocated pages for security reasons anyway. So the design decision here was to specify zero initialization. Why isn't security an issue with the stack, too? Actually it is cleared, at first. The junk you see is from earlier instances of your own program's call frames and the library code they called.

The end result is fast, memory-efficient code. All the advantages of assembly with none of the pain. Before dmr invented C, "HLL"s like Basic and entire OS kernels were really, literally, implemented as giant assembler programs. (With certain exceptions at places like IBM.)

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