Understanding pointer arithmetic

Question

I'm reading through K&R C. In t he back there's some sample fopen code. I don't particularly understand one part.

#define OPEN_MAX 20

FILE _iob[OPEN_MAX] = {
    { 0, NULL, NULL, _READ, 0 },             // _READ = 01
    { 0, NULL, NULL, _WRITE, 1 },            // _WRITE = 02
    { 0, NULL, NULL, _WRITE | _UNBUF, 2 }    // _UNBUF = 04
};

FILE *fopen(char *name, char *mode)
{
    FILE *fp;

    for (fp = _iob; fp < _iob + OPEN_MAX; fp++)
    /* ... */
}

The statement fp < _iob + OPEN_MAX is what's tripping me up.

fp: file pointer
_iob: file array
OPEN_MAX: integer

It seems to be adding a file array to an integer, then evaluating whether or not it's larger than a file pointer! How is the statement fp < _iob + OPEN_MAX possible?

It's possible due to pointer arithmetic being allowed in C. You can think of addresses (the values inside pointers) as integers that designate locations in memory. If two locations are adjacent, the difference between their addresses is a number. — StoryTeller - Unslander Monica, Mar 05 '17 at 21:40
When you use an array in an expression like that, it decays to a pointer to the first element. — Barmar, Mar 05 '17 at 21:40
That's very basic pointer arithmetics. It is explained in every C book. — too honest for this site, Mar 05 '17 at 21:42

score 2 · Answer 1 · edited May 23 '17 at 12:25

2

The expression _iob + OPEN_MAX causes _iob to be treated as a pointer to its first element within this expression (this is called array-to-pointer decaying).
The resulting FILE pointer is then offset by OPEN_MAX (the +).
The < then compares the two resulting FILE pointers (pointers are just integers under the hood).

edited May 23 '17 at 12:25

Community

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answered Mar 05 '17 at 21:42

Emil Laine

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score 1 · Accepted Answer · answered Mar 05 '17 at 21:45

In most contexts, when you use an array as an l-value, it decays to a pointer to the first element of the array. So

fp < _iob + OPEN_MAX

is equivalent to:

fp < &(_iob[0]) + OPEN_MAX

And when you perform arithmetic on a pointer to an array element, it's equivalent to array indexing. So this is the same as:

fp < &(_iob[OPEN_MAX])

Understanding pointer arithmetic

2 Answers2