C - compiler error: dereferencing pointer to incomplete type

Question

I've seen many questions here about dereferencing pointers to incomplete types but every single one of them is related to not using typedef or to having the structs declared in the .c, not in the header file. I've been trying to fix this for many hours and can't seem to find a way.

stable.h (cannot be changed):

typedef struct stable_s *SymbolTable;

typedef union {
    int i;
    char *str;
    void *p;
} EntryData;

SymbolTable stable_create();

stable.c:

SymbolTable stable_create() {
    SymbolTable ht = malloc(sizeof (SymbolTable));
    ht->data = malloc(primes[0] * sizeof(Node));
    for (int h = 0; h < primes[0]; h++) ht->data[h] = NULL;
    ht->n = 0;
    ht->prIndex = 0;
    return ht;
}

aux.h:

#include "stable.h"

typedef struct {
    EntryData *data;
    char *str;
    void *nxt;
} Node;


typedef struct {
    Node **data;
    int n;
    int prIndex;
} stable_s;

typedef struct {
    char **str;
    int *val;
    int index;
    int maxLen;
} answer;

freq.c:

answer *final;
static void init(SymbolTable table){
    final = malloc(sizeof(answer));
    final->val = malloc(table->n * sizeof(int));
}

int main(int argc, char *argv[]) {
    SymbolTable st = stable_create();
    init(st);
}

compiler error (using flags -Wall -std=c99 -pedantic -O2 -Wextra):

freq.c:13:30: error: dereferencing pointer to incomplete type ‘struct stable_s’
 final->val = malloc(table->n * sizeof(int));

Please make a [mcve]. The code you show is incomplete. The code I guess is behind it cannot plausibly provoke the described error. — Yunnosch, May 13 '18 at 20:18
One thing to always observe though. Always make sure that information you need is available. I.e. include any header declaring anythign you need. Your stable.h does not match that rule. Your .c happens to include in an order which should avoid the problem (see first comment), but that is not good practice. — Yunnosch, May 13 '18 at 20:20
The `stable_create` function leaks memory, and dereferences an uninitialized pointer. (`T *p = malloc(...); return *p;`), you copy the object allocated and never free it. You should not use the first malloc (and use `T p;`). This looks weird because pointer typedefs are bad. — M.M, May 13 '18 at 20:59
@M.M i have a stable_destroy() to free the memory, I didn't include it because it wasn't relevant to the error ;) — Bruna, May 13 '18 at 21:22

score 1 · Accepted Answer · answered May 13 '18 at 20:24

This code

 typedef struct stable_s *SymbolTable;

defines the type SymbolTable as a pointer to struct stable_s.

This code

typedef struct {
    Node **data;
    int n;
    int prIndex;
} stable_s;

defines a structure of type stable_s. Note that stable_s is not struct stable_s.

A simple

struct stable_s {
    Node **data;
    int n;
    int prIndex;
};

without the typedef will solve your problem.

See C : typedef struct name {...}; VS typedef struct{...} name;

score 0 · Answer 2 · answered May 13 '18 at 20:29

As Andrew points out, declaring a "struct stable_s { ... }" will make things compile.

However, you don't say whether this is a class assignment or real-world. If real-world, it's probably an extremely bad idea to declare the struct yourself. You are being given an opaque type to use to reference a library; you are not supposed to know or access the stuff inside. The library is relying on various semantics that you might mess up, and as software versions change, the contents of the struct can (and almost certainly will) change, so your code will break in the future.

It's an assignment, I was given the API in stable.h and I'm supposed to implement in (along with a test client). — Bruna, May 13 '18 at 20:34

C - compiler error: dereferencing pointer to incomplete type

2 Answers2