C Adding value to array? `array + value`

Arrays are a continuous block of memory accessed by an address (the location of element 0) and an offset.

wavehead+4 evaluates to the address of the 5th element in the array. The method it's passed into (write32le) will see the 5th element of the array as the 1st element.

// If we take an array
int myArr[] = { 1, 2, 3, 4, 5, 6 }
// And pass it into a method as
myMethod(myArr+2);
// the method will see (assuming it knows the length of the array which it won't in c)
{ 3, 4, 5, 6 }

The purpose of write32le() is to take a word that you wish to re-write as its first argument, and an index into to an array of unsigned int in the form of of a pointer. wavhead[index] is that kind of pointer.

If you wanted to replace the first 4 bytes of wavhead with the word 0xDEADBEEF, you would do this:

int main()
{
    int in=0xDEADBEEF;
    printf("Before write32le:\n ");
    for(int i=0; i<=3; i++)
    {
        printf("0x%hhx ", wavhead[i]);
    }
    printf("\n");

    write32le(in, wavhead);
    printf("After write32le:\n ");
    for(int i=0; i<=3; i++)
    {
        printf("0x%hhx ", wavhead[i]);
    }
    printf("\n");

    return 0;
}

Output:

Before write32le:
 0x52 0x49 0x46 0x46
After write32le:
 0xef 0xbe 0xad 0xde

In majority of contexts (with only a few exceptions) an object of type "array" is implicitly converted to a value of type "pointer". The pointer points to the first element of the array. E.g. in such contexts plain wavhead is implicitly interpreted as &wavhead[0].

In your case wavhead is used as an operand of binary + operator. Binary + is not an exception. So, wavhead decays to &wavhead[0] and your call is interpreted as

write32le(outsizetotal, &wavhead[0] + 4);

which means that what you really have here is "pointer + value", not "array + value".

The second argument results in a pointer to wavhead[4]. You could also rewrite it as

write32le(outsizetotal, &wavhead[4]);

in this case, the name of the array wavhead[] is equal to a pointer of "unsigned char *", adding 4 to this pointer is just moving the pointer by 4 steps and the step is sizeof (unsigned char), which is 1. So wavhead+4 = the address of pointer wavhead + 4, and it points to wavhead[4] after the addition.