Pointer to Pointer with argv

Question

Based on my understanding of pointer to pointer to an array of characters,

% ./pointer one two

argv           
+----+          +----+
| .  |   --->   | .  |  ---> "./pointer\0"
+----+          +----+
                | .  |  ---> "one\0"
                +----+
                | .  |  ---> "two\0"
                +----+

From the code:

int main(int argc, char **argv) {
    printf("Value of argv[1]: %s", argv[1]);
}

My question is, Why is argv[1] acceptable? Why is it not something like (*argv)[1]?

My understanding steps:

Take argv, dereference it.
It should return the address of the array of pointers to characters.
Using pointer arithmetics to access elements of the array.

Dmitri · Accepted Answer · 2011-10-03 16:46:13Z

22

It's more convenient to think of [] as an operator for pointers rather than arrays; it's used with both, but since arrays decay to pointers array indexing still makes sense if it's looked at this way. So essentially it offsets, then dereferences, a pointer.

So with argv[1], what you've really got is *(argv + 1) expressed with more convenient syntax. This gives you the second char * in the block of memory pointed at by argv, since char * is the type argv points to, and [1] offsets argv by sizeof(char *) bytes then dereferences the result.

(*argv)[1] would dereference argv first with * to get the first pointer to char, then offset that by 1 * sizeof(char) bytes, then dereferences that to get a char. This gives the second character in the first string of the group of strings pointed at by argv, which is obviously not the same thing as argv[1].

So think of an indexed array variable as a pointer being operated on by an "offset then dereference a pointer" operator.

answered Oct 3, 2011 at 16:46

Dmitri

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1 Comment

Ben C. Over a year ago

Yes, I had the confusion of why we didn't need to derefrence argv, first but it turns out I forgotten that it's equal to *(argv + n), where n is the index subscript.

Jonathan Leffler · Accepted Answer · 2018-12-03 00:04:15Z

16

Because argv is a pointer to pointer to char, it follows that argv[1] is a pointer to char. The printf() format %s expects a pointer to char argument and prints the null-terminated array of characters that the argument points to. Since argv[1] is not a null pointer, there is no problem.

(*argv)[1] is also valid C, but (*argv) is equivalent to argv[0] and is a pointer to char, so (*argv)[1] is the second character of argv[0], which is / in your example.

edited Dec 3, 2018 at 0:04

answered Oct 3, 2011 at 5:27

Jonathan Leffler

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Comments

Ignacio Vazquez-Abrams · Accepted Answer · 2011-10-03 05:28:18Z

5

Indexing a pointer as an array implicitly dereferences it. p[0] is *p, p[1] is *(p + 1), etc.

answered Oct 3, 2011 at 5:28

Ignacio Vazquez-Abrams

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