CODEWITHSUNDEEP
cpointersstruct
Abdullah
Abdullah

Reputation: 541

Pointer syntax query

I can't seem to understand the difference between the following to pointer notations, can someone please guide me?

 typedef struct some_struct struct_name;
  struct_name this;
  char buf[50];
  this = *((some_struct *)(buf));

Now I tried to play around a bit and did the above thing like:

  struct some_struct * this;
     char buf[50];
     this=(struct some_struct *)buf;

As far as I am concerned I think both the implementations should generate the same result, Can someone guide me whether there is a difference between the two and if yes can some one point it out?

Thanks.

Upvotes: 0

Views: 81

Answers (3)

Jens Gustedt
Jens Gustedt

Reputation: 79003

Both approaches have their problems.

  • alignment: your buf might not be properly aligned for a variable of the structure type. If so this will produce undefined behavior (UB): in the best case it aborts your program, but it may make much worse things than that.
  • initialization: in the first cases you access uninitialized memory for reading. In the best case that gives you unspecific data, that is some random bytes. In the worst case, char is a signed integer type on your platform and you hit a trap representation for char => UB as above. (Your second case will encounter the same problem, once you try to access the object at the other end of the pointer.)

How to avoid all that:

  • Always initialize your variables. A simple = { 0 } should do in all cases.
  • never use char as a generic type for bytes but use unsigned char
  • never cast a byte buffer of arbitrary alignment to another data type. If needed, do it the other way round, cast a struct object to unsigned char.

Upvotes: 0

FatalError
FatalError

Reputation: 54631

In your first snippet, this is not a pointer, it's an instance of some_struct. The assignment you made did a shallow copy (i.e. memcpy()) of what's in buf as if it were an instance of some_struct as well.

In the second snippet, this is a pointer, and it's just pointed to the address of buf.

So, basically to sum up, first snippet this is not a pointer and the struct is copied into it. In the second, it's a pointer and assigned to the same memory as buf (i.e. not a copy).

Upvotes: 3

Jarred
Jarred

Reputation: 391

In the second one, "this" will point to the first memory location of "buf". In the first example, you will either get a compiler error (I don't think you can assign structs in C with =, I could be wrong though), or the contents of buf (up to sizeof(struct_name)) will be copied into this, which resides on the stack.

Upvotes: 0

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