Is un-initialized integer always default to 0 in c?

External and static variables are initialized to zero by default, this is guaranteed. Automatic and register variables that do not have en explicit initializer will have an indeterminate value (either an unspecified value or a trap representation).

From The Standard:

C89

6.5.7:

If an object that has static storage duration is not initialized explicitly, it is initialized implicitly as if every member that has arithmetic type were assigned 0 and every member that has pointer type were assigned a null pointer constant. If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.

C99

6.2.4, §5:

For such an object that does not have a variable length array type, its lifetime extends from entry into the block with which it is associated until execution of that block ends in anyway.(Entering an enclosed block or calling a function suspends, but does not end, execution of the current block.) If the block is entered recursively,anew instance of the object is created each time. The initial value of the object is indeterminate. If an initialization is specified for the object, it is performed each time the declaration is reached in the execution of the block; otherwise, the value becomes indeterminate each time the declaration is reached.

6.7.8, §10:

If an object that has automatic storage duration is not initialized explicitly,its value is indeterminate. If an object that has static storage duration is not initialized explicitly, then:

— if it has pointer type, it is initialized to a null pointer;

— if it has arithmetic type, it is initialized to (positive orunsigned) zero;

— if it is an aggregate, every member is initialized (recursively) according to these rules;

— if it is a union, the first named member is initialized (recursively) according to these rules.

3.17.2, §1:

indeterminate value: either an unspecified value or a trap representation

3.17.3, §1:

unspecified value: valid value of the relevant type where this International Standard imposes no requirements on which value is chosen in any instance. NOTE An unspecified value cannot be a trap representation.

automatic (local) variables are not guaranteed to be zero, can contain garbage.

global variables and static variables are guaranteed to be zero.

Variables declared (as int)at file scope are initialized to 0.

Example

int i;
int main()
{
   int x;
   printf("%d",i); // prints 0
   printf("%d",x); // prints some unspecified value
}

Is it guranteed that un-initialized variable will have 0 value in c at all?

Nopes! That doesn't hold for variables declared at function scope.

It depends on how the variable is allocated. Statically allocated variables are initialized to zero, whereas variables allocated on the stack or with malloc() are not.