Undefined structure type?

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Wed, 2016-01-13 21:14

#1

xenic

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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?

Thu, 2016-01-14 08:26

#2

salass00

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Re: Undefined structure type?

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.

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:

struct Notdef; // forward declaration
 
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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