Is there any difference between "x" and "&x[0]" in C?

Question

Going through some piece of software written in C, I encountered the following bit of code, which I can summarize as such:

void fill_array_with_some_data(int *_)
{
    // assign some arbitrary int values to _[0] and _[1]
    _[0] = 42;
    _[1] = 24;
}

int main(int argc, char **argv)
{
    int x[2] = {0};
    fill_array_with_some_data(&x[0]);
    
    // do something with the data which is now is x
    if (x[0] & 0x42)
        // do something
    
    return 0;
}

Seeing &x[0] instead of simply x seemed odd to me.

Is there any difference between both?

Is there any reason why one would want to use &x[0] rather than simply x?

Answer 1

In fill_array_with_some_data(&x[0]), x and &x[0] are equivalent.

After int x[2] = {0];, x designates an array. When an array is used in an expression, it is converted to a pointer to its first element except when:

• it is used as the operand of sizeof, as in sizeof x,
• it is used as the operand of unary &, as in &x, or
• it is a string literal used to initialize an array, as in char s[] = "abc";.

Thus sizeof x will give the size of the array, whereas sizeof &x[0] will give the size of a pointer to an element of x.

&x and &x[0] will both point to the same location in memory, but they have different types. x has type “pointer to array of 2 int, and &x[0] has type “pointer to int”. This means &x + 1 will point to after the end of the entire array x, but &x[0] + 1 will point to after the element x[0] (so it points to x[1]).

Generally, one would write &x[0] only when one wants to point specifically to the element. When filling the array with some data, programmers should write fill_array_with_some_data(x) to convey they expect the routine to operate on the entire array. This makes no difference to the compiler, but it helps humans understand what is being done. One should use &x[0] only to call out that element particularly. For example, if a routine foo operates on only one element, we might write foo(&x[0]) to indicate that to the reader.