Malloc function problem

[bouvett]

Hi Guys,

I have been using the malloc function in order to make use of a memory block.. The code is as shown below:

int * corner_image; 
corner_image = (int*) malloc (480*640);

When I try to access location 78282 i.e. corner_image[78282]=60; the compiler returns an error.

Now I only need to store 8 bit images, perhaps it is because type 'int' is 4 bytes wide? What type should I choose so that it becomes 1 byte wide only?

Thanks a lot. best regards,
bouvett

[quzah]

1. Don't cast malloc.
2. What compiler are you using? If you are using Turbo C, you can't have numbers as big as what you want.
3. You aren't checking to see if your malloc fails, at least not in this example.
4. If you want to store one byte at each array index, why aren't you using a char / unsigned char instead of an int?


Quzah.

[bouvett]

I changed the type to short int, the range doubled yet I cannot go up to (640*480) -1... any help is appreciated

[laserlight]

When I try to access location 78282 i.e. corner_image[78282]=60; the compiler returns an error.

You probably got a run time error instead of a compile error. Anyway, the point is that 480*640 is the number of bytes requested, so the number of elements is (480*640) / sizeof(*corner_image).

perhaps it is because type 'int' is 4 bytes wide?

If sizeof(int) is 4, then you only allocated space for 76800 elements, hence an index of 78282 is out of bounds.

Now I only need to store 8 bit images (...) What type should I choose so that it becomes 1 byte wide only?

char, perhaps unsigned char. CHAR_BIT is guaranteed to be at least 8, so this works for your purposes.

[bouvett]

oh sorry quzah i didn't see your post. changed to char and it worked. thanks

[quzah]

You probably got a run time error instead of a compile error. Anyway, the point is that 480*640 is the number of bytes requested, so the number of elements is (480*640) / sizeof(*corner_image).

If sizeof(int) is 4, then you only allocated space for 76800 elements, hence an index of 78282 is out of bounds.

char, perhaps unsigned char. CHAR_BIT is guaranteed to be at least 8, so this works for your purposes.

Since 480 * 640 is too big, you're going to need to divide by the pointer size first. Do something like:

size_t  howmany = 480 * (640 / sizeof(*corner_image));

Quzah.