Why are global variables always initialized to '0', but not local variables? [duplicate]

Question

Because that's the way it is, according to the C Standard. The reason for that is efficiency:

• static variables are initialized at compile-time, since their address is known and fixed. Initializing them to 0 does not incur a runtime cost.

• automatic variables can have different addresses for different calls and would have to be initialized at runtime each time the function is called, incurring a runtime cost that may not be needed. If you do need that initialization, then request it.

global and static variables are stored in the Data Segment (DS) when initialized and block start by symbol (BSS)` when uninitialized.

These variables have a fixed memory location, and memory is allocated at compile time.

Thus global and static variables have '0' as their default values.

Whereas auto variables are stored on the stack, and they do not have a fixed memory location.

Memory is allocated to auto variables at runtime, but not at compile time. Hence auto variables have their default value as garbage.

You've chosen simple variables, but consider:

void matrix_manipulation(void)
{
    int matrix1[100][100];
    int matrix2[100][100];
    int matrix3[100][100];

    /* code to read values for matrix1 from a file */
    /* code to read values for matrix2 from a file */
    /* code to multiply matrix1 by matrix2 storing the result in matrix3 */
    /* code to use matrix3 somehow */
}

If the system initialized the arrays to 0, the effort would be wasted; the initialization is overwritten by the rest of the function. C avoids hidden costs whenever possible.

Global variables are allocated and initialized before the main function starts, while local variables are generated on the stack as the instance of the program runs.