Using a struct in a header file "unknown type" error

c, header-files, kdevelop, struct

Solution

In C one has two possibilities to declare structure:

struct STRUCT_NAME {} ;

or

typedef struct {} STRUCT_ALIAS;

If you use first method (give struct a name) - you must define variable by marking it explicitly being `a struct`:

struct STRUCT_NAME myStruct;

However if you use second method (give struct an alias) then you can omit `struct` identifier - compiler can deduce type of variable given only it's `alias` :

STRUCT_ALIAS myStruct;

Bonus points: You can declare struct with both it's name and alias:

typedef struct STRUCT_TAG {} STRUCT_TAG;
// here STRUCT_NAME == STRUCT_ALIAS

Then in variable definition you can use either first or second method. Why both of two worlds is good ? Struct alias lets you to make struct variable definitions shorter - which is a good thing sometimes. But struct name let's you to make `forward declarations`. Which is indispensable tool in some cases - consider you have circular references between structs:

struct A {
  struct B * b;
}
struct B {
  struct A * a;
}

Besides that this architecture may be flawed - this circular definition will compile when structs are declared in the first way (with names) AND struct pointers are referenced explicitly by marking them as `struct`.

Problem

I am using Kdevelop in Kubuntu. I have declared a structure in my datasetup.h file: ``` #ifndef A_H #define A_H struct georeg_val { int p; double h; double hfov; double vfov; }; #endif ``` Now when I use it in my main.c file ``` int main() { georeg_val gval; read_data(gval); //this is in a .cpp file } ``` I get the following error: georeg_chain.c:7:3: error: unknown type name 'georeg_val' (This is in the `georeg_val gval;` line) I would appreciate if anyone could help me resolve this error.

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