Undefined structure type?

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xenic's picture
Last seen: 11 months 1 day ago
Joined: 2011-05-07 04:52
Undefined structure type?
While debugging a program I noticed a structure declaration that contains an undefined type but GCC isn't flagging it with a warning or error. Why would that be the case? Here is an example using an undefined structure type of 'notdef': struct Notdef *mystruct; Notdef isn't defined anywhere but the declaration isn't flagged. On the other hand if I declare a simple pointer the same way it gets flagged as an error by GCC. Notdef *myptr; gets flagged as an error. Can anyone explain why the structure declaration doesn't get flagged as an error?
salass00's picture
Last seen: 1 month 3 weeks ago
Joined: 2011-02-03 11:27
Re: Undefined structure type?
  1. struct Notdef *mystruct;
This can be seen as a forward declaration of the structure Notdef. As it's a pointer the compiler doesn't need to know the size or contents of the structure yet.
  1. Notdef *myptr;
In this case the compiler can't know if this is a struct, a class, an enum, a union, a typedef or whatever else it could be since you haven't specified it so it will produce an error. OTOH this code will work:
  1. struct Notdef; // forward declaration
  3. Notdef *myptr;
Leaving out struct keyword as in the last two code fragments is only allowed in C++ code and not in plain C. In C the only way to get rid of 'struct' is to use a typedef.
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