CODEWITHSUNDEEP
ctypes
Pablo Francisco Pérez Hidalgo
Pablo Francisco Pérez Hidalgo

Reputation: 27702

What is the Opaque in this code?

These days almost everybody has heard about the HeartBleed exploit.

If you check the code which is the origin of the problem, you can find the following structure:

struct {
      HeartbeatMessageType type;
      uint16 payload_length;
      opaque payload[HeartbeatMessage.payload_length];
      opaque padding[padding_length]; 
   } HeartbeatMessage;

It has been a while since the last time I programmed actively on C but I still use it to program little programs or snippets.

The point is that I have never used opaque and I would like to know what does exactly opaque stand for.

Upvotes: 3

Views: 770

Answers (1)

Marco A.
Marco A.

Reputation: 43662

"Opaque" is often used in commercial and internal/proprietary APIs when you just need to "fill a gap" in a C/C++ declaration.

Since you don't know if that type is a pointer (in that case you would know its size) or an entire structure/class, you can't deduce anything and thus the code serves as a general idea of how the memory is laid out.

They hide information from you. It's a common technique especially used for security measures when there's sensible code like the one you posted above.

Upvotes: 3

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