comparison of the pointers in C

I have some simple question, when I do something like that:

int* ptr1
int* ptr2

if(ptr1 == ptr2)...

Wha开发者_如何学Ct do I actually compare:

1. addresses where ptr1 and ptr2 saved
2. addresses where content of pointers saved

If there is 1, how can I check 2?

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That compares the pointer values, which are addresses.

So ptr1==ptr2 tests whether the two pointers point to the same address -- your #2.

You could express #1 -- comparing the addresses of the pointers themselves -- with &ptr1 == &ptr2, but here you know that that will be false.

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You are comparing wether ptr1 points to the same address as ptr2 does (and reciprocally). That means, your option 2.

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You compare the actual pointers.

I am not sure from your phrasing what you actually want to do, but it is probably one of:

if (*ptr1 == *ptr2) ...

or

if (&ptr1 == &ptr2) ...

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To compare what the pointers actually point towards, use:

if(*ptr1 == *ptr2)...

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When you declare

  int *ptr1;

you're defining a variable (that has an address) that points to an integer. You can get the address of that variable by taking the address (&) of that variable, that is,

&ptr1

fundamentally, ptr1 will contain the address of a memory location (where an integer may be stored, if it point to a location where space has been allocated for such a thing).

Comparing the values of the ptr1 and ptr2 variables will tell you if they point at the same variable; comparing the dereferenced values of the ptr1 and ptr2 variables will tell you if the values they point at are the same.

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2) the values of the pointers and therefore the addresses they point to.

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To compare the values pointed to by the pointers:

1. check if any of the pointers are NULL
2. Figure out what both pointers being NULL means in context
3. compare with if (*ptr1 == *ptr2)

To compare the addresses where the pointers are stored use:

if ( &ptr1 == &ptr2 ) since these are the addresses of the pointers.

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Any time you use have something in the form "a==b" you are comparing the contents of variable a to the contents of variable b. In the case that a and b are pointers, their contents are the addresses of some data so a==b will return true if they point to the same location and false otherwise.

To compare the addresses of two variables you need to do &a==&b.