pointer to array

Question

I'm wondering, can you make a pointer to a group of variables in an array? like this

array[20]{'a','b','c',...}
pointer = array[6 through 10];

so then you could say...

*pointer[0] == array[6];

and

*pointer[5] == array[10];

and the length of *pointer

5 == sizeof(*pointer) \ sizeof(type);

OK

Let me explain what I'm trying to accomplish, maybe that will clear me up a bit. I want to read a full file into a buffer, but I want to do it piece by piece. passing a small array into read() and then looping it into the larger array defeats the purpose. I was hoping that I could directly 'point' to an area in the buffer I want to fill, and pass that to the read() function.

I DO NOT want to use streams or anything that buffers behind my back

that would be counterproductive as I'm trying read the whole file into memory at once. As fast as possible.

I need speed!!!

Answer 1

Despite what everyone has said, this is possible, with one small caveat - unless you have a C99 compiler, you have to know the size of the "slice" that you want at compile time.

To declare and use the pointer correctly, you also have to know that the subscript operator [] binds before the * dereference operator, so parantheses are needed.

Behold:

int array[20] = {'a','b','c','d','e','f','g','h','i','j','k','l'};
int (*pointer)[10 - 6 + 1] = (int (*)[10 - 6 + 1])&array[6];    /* = array[6 through 10] */

printf("(*pointer)[0] = %c\n", (*pointer)[0]);
printf("(*pointer)[4] = %c\n", (*pointer)[4]);
printf("sizeof *pointer / sizeof **pointer = %lu\n", (unsigned long)(sizeof *pointer / sizeof **pointer));

Addendum:

For the answer to your actual problem that you've put in, that's a lot easier. Just use a pointer that is set to an offset from the buffer array, eg:

unsigned char buffer[102400];
unsigned char *ptr;

/* ... */
ptr = buffer + 500;
read(fd, ptr, 1024); /* Try and read up to 1024 bytes at position buffer[500] */