Thread: realloc

How can we allocate more memory for an array?
I have a ar[10] and I want to grow it to ar[100] in runtime.

As I guessed, tnx.
But I thought it maybe possible, because arrays are pointers in theory.

I read the faq and used a pointer instead of an array it worked fine. But I read something there that really confused me:

Originally Posted by FAQ

It is useful to realize that a reference like x[3] generates different code depending on whether x is an array or a pointer. Given the declarations above, when the compiler sees the expression a[3], it emits code to start at the location ``a'', move three past it, and fetch the character there. When it sees the expression p[3], it emits code to start at the location ``p'', fetch the pointer value there, add three to the pointer, and finally fetch the character pointed to. In other words, a[3] is three places past (the start of) the object named a, while p[3] is three places past the object pointed to by p. In the example above, both a[3] and p[3] happen to be the character 'l', but the compiler gets there differently. (The essential difference is that the values of an array like a and a pointer like p are computed differently whenever they appear in expressions, whether or not they are being subscripted, as explained further in question 6.3.) See also question 1.32.

If so why this code prints out "y b", as I expected ??

Code:

int main()
{
	char* cp="xyz";
	char ca[]="abcd";
	 printf( "%s, %s\n", cp+1, ca+1 ); ;

	return 0;
}

I thought the only difference is that compiler knows that array points to a static size block of memory but ptr can points to anywhere.

>> it emits code to start at the location ``a'', move three past it <<from FAQ
Three past of it? Why? When we can notice?
Why there should be difference between ptr and array?

Code:

char arr[]="abcd";

Array is a sequence of bytes, the first one is addressed in arr, but compiler keeps in mind that this pointer points to an array so if you put a bigger number in subscription it will go to a illegal place (ex. arr[7]).
Look at this code

Code:

int main()
{
	char ca[]="abcd";
	char* cp=ca;
	
	printf( "%s, %s\n", cp+1, ca+1 );           

	return 0;
}

OUTPUT IS THE SAME!!!
It means ca acts exactly like a pointer, so what is the difference?

Originally Posted by siavoshkc

When we can notice?

When the output generated from the code is examined. Some pseudo-assembly might be something like this.

Code:

   load value,[array+3]

Code:

   load a,pointer
   load value,[a+3]

Originally Posted by siavoshkc

Why there should be difference between ptr and array?

Because an array is an array and a pointer is a pointer. Why should there be a difference between a car and a truck if they go can get you to the same place? Because they do different things.

Try to keep reading that FAQ until it makes sense.

Originally Posted by Salem

Apparently, you didn't run my code to discover the difference in the value of sizeof()

I knew they differ. But I think the difference is because compiler knows where the ca points to(an array) and looks at the definition of the array then returns a value for size.

What about two dimentional arrays?

Code:

int ar[x][z];

ar[] is pointer, ar[][] is value?