Resolving the "dereferencing pointer to incomplete type" error
The “dereferencing pointer to incomplete type” error commonly occurs in C when one tries to dereference a type (usually a struct) that is:
- not declared at all.
- declared, but not defined.
The error in the first bullet point occurs because C cannot find the struct that is being dereferenced. The reason for this could be that the struct may not have been declared, or that the user made a typographical error in the struct’s name.
The following is an example of the error in the second bullet point:
#include<stdio.h>#include<stdlib.h>struct {int age;float weight;} person;int main(){// Declare a pointer to the struct and allocate memorystruct person *ptr;ptr = (struct person*)malloc(sizeof(struct person));// Fill in the struct with data.ptr->age = 10;ptr->weight = 55.5;}
Run the code above and note the two errors – both of them refer to the incomplete type struct person. Therefore, the fault lies in the declaration of the struct.
Lines 4-7 have defined a nameless struct type and a variable (person) of that type. Since the compiler has not associated a name with the struct, it throws an error whenever person is referenced.
The solution is to declare the struct with its name right after the struct keyword:
#include<stdio.h>#include<stdlib.h>struct person {int age;float weight;};int main(){struct person *ptr;ptr = (struct person*)malloc(sizeof(struct person));ptr->age = 10;ptr->weight = 55.5;printf("%s", "Successful.");}
