c,strcpy , What does `strcpy(x+1, SEQX)` do?

## Question:

Tag: c,strcpy

I'm wondering what this syntax of `strcpy()` does in line 65 and 66:

`````` 24 #define SEQX "TTCATA"
25 #define SEQY "TGCTCGTA"
61   M = strlen(SEQX);
62   N = strlen(SEQY);
63   x = malloc(sizeof(char) * (M+2)); /* +2: leading blank, and trailing \0 */
64   y = malloc(sizeof(char) * (N+2));
65   strcpy(x+1, SEQX);            /* x_1..x_M now defined; x_0 undefined */
66   strcpy(y+1, SEQY);            /* y_1..y_N now defined; y_0 undefined */
``````

I know it's copying `SEQX` and `SEQY` into `x` and `y` but I don't understand what does the `+1` do? What's the formal name of this type of operation?

The `pointer + offset` notation is used as a convenient means to reference memory locations.

In your case, the `pointer` is provided by `malloc()` after allocating sufficient heap memory, and represents an array of `M + 2` elements of type `char`, thus the notation as used in your code represents the following address:

``````<x + 1 * sizeof(char)>
``````

And this also happens to be the same as:

``````&x[1]
``````

In other words, the address of `x[1]` (second element of `x`). After the `strcpy()` operation the memory would look like this:

``````[0] [1] [2] [3] [4] [5] [6] [7]
??? 'T' 'T' 'C' 'A' 'T' 'A' '\0'
^
x
``````

In other words:

``````strcmp(x + 1, SEQX) == 0
``````

Note that before practical use of `x` as a string, the first memory location should be defined, i.e.

``````x[0] = '='; // now the string holds "=TTCATA"
``````

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