dereferencing pointer to incomplete type

Question

I ve seen a lot of questions on this but I m going to ask the question differently without specific code   Is there a way of EASILY determining what is causing the type to be incomplete   In my case I m using someone elses code and I m completely sure I don t have the headers right  but  since computers do this stuff much faster and better than human eyeballs  is there a way to get the compiler to say   hey you think you have type X at line 34 but that s actually missing    The error itself only shows up when you assign  which isn t very helpful

User · Accepted Answer

I saw a question the other day where someone inadvertently used an incomplete type by specifying something like

struct a {
    int q; 
}; 
struct A *x; 
x->q = 3;

The compiler knew that struct A was a struct, despite A being totally undefined, by virtue of the struct keyword.

That was in C++, where such usage of struct is atypical (and, it turns out, can lead to foot-shooting). In C if you do

typedef struct a {
    ...
} a;

then you can use a as the typename and omit the struct later. This will lead the compiler to give you an undefined identifier error later, rather than incomplete type, if you mistype the name or forget a header.

User · Answer

Another possible reason is indirect reference  If a code references to a struct that not included in current c file  the compiler will complain      a- b- c   error if b not included in current c file

User · Answer

Outside of possible scenarios involving whole-program optimization  the code code generated for something like   struct foo  bar  struct foo  test struct foo  whatever  int blah      return blah   whatever  bar      will be totally unaffected by what members struct foo might contain   Because make utilities will generally recompile any compilation unit in which the complete definition of a structure appears  even when such changes couldn t actually affect the code generated for them  it s common to omit complete structure definitions from compilation units that don t actually need them  and such omission is generally not worthy of a warning   A compiler needs to have a complete structure or union definition to know how to handle declarations objects of the type with automatic or static duration  declarations of aggregates containing members of the type  or code which accesses members of the structure or union   If the compiler doesn t have the information needed to perform one of the above operations  it will have no choice but to squawk about it   Incidentally  there s one more situation where the Standard would allow a compiler to require a complete union definition to be visible but would not require a diagnostic  if two structures start with a Common Initial Sequence  and a union type containing both is visible when the compiler is processing code that uses a pointer of one of the structure types to inspects a member of that Common Initial Sequence  the compiler is required to recognize that such code might be accessing the corresponding member of a structure of the other type   I don t know what compilers if any comply with the Standard when the complete union type is visible but not when it isn t  gcc is prone to generate non-conforming code in either case unless the -fno-strict-aliasing flag is used  in which case it will generate conforming code in both cases  but if one wants to write code that uses the CIS rule in such a fashion as to guarantee correct behavior on conforming compilers  one may need to ensure that complete union type definition is visible  failure to do so may result in a compiler silently generating bogus code

User · Answer

I don t exactly understand what s the problem  Incomplete type is not the type that s  missing   Incompete type is a type that is declared but not defined  in case of struct types   To find the non-defining declaration is easy  As for the finding the missing definition    the compiler won t help you here  since that is what caused the error in the first place   A major reason for incomplete type errors in C are typos in type names  which prevent the compiler from matching one name to the other  like in matching the declaration to the definition   But again  the compiler cannot help you here  Compiler don t make guesses about typos

User · Answer

this error usually shows if the name of your struct is different from the initialization of your struct in the code  so normally  c will find the name of the struct you put and if the original struct is not found  this would usually appear  or if you point a pointer pointed into that pointer  the error will show up

User · Answer

A - Solution  Speaking for C language  I ve just found ampirically that following declaration code will be the solution   typedef struct ListNode       int data      ListNode   prev      ListNode   next    ListNode    So as a general rule  I give the same name both for both type definition and name of the struct   typedef struct X          code for additional types here     X  prev     reference to pointer     X  next     reference to pointer   X    B - Problemetic Samples  Where following declarations are considered both incomplete by the gcc compiler when executing following statement     removed- gt next- gt prev   removed- gt prev    And I get same error for the dereferencing code reported in the error output    gt gcc Main c LinkedList c -o Main exe -w LinkedList c  In function  removeFromList   LinkedList c 166 18  error  dereferencing pointer to incomplete type  struct ListNode       removed- gt next- gt prev   removed- gt prev    For both of the header file declarations listed below   typedef struct       int data      ListNode   prev      ListNode   next    ListNode    Plus this one   typedef struct ListNodeType       int data      ListNode   prev      ListNode   next    ListNode

User · Answer

What do you mean  the error only shows up when you assign  For example on GCC  with no assignment in sight   int main         struct blah  b   0       b     this is line 6     incompletetype c 6  error  dereferencing pointer to incomplete type   The error is at line 6  that s where I used an incomplete type as if it were a complete type  I was fine up until then   The mistake is that you should have included whatever header defines the type  But the compiler can t possibly guess what line that should have been included at  any line outside of a function would be fine  pretty much  Neither is it going to go trawling through every text file on your system  looking for a header that defines it  and suggest you should include that   Alternatively  good point  potatoswatter   the error is at the line where b was defined  when you meant to specify some type which actually exists  but actually specified blah  Finding the definition of the variable b shouldn t be too difficult in most cases  IDEs can usually do it for you  compiler warnings maybe can t be bothered  It s some pretty heinous code  though  if you can t find the definitions of the things you re using

[c] dereferencing pointer to incomplete type

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