C struct into array of struct pointers compile error

Question

I've got a linked list and I'm trying to make a temporary array to help me address each node while I build the rest of the structure, then I intend to free the array but it seems I can't store the address of the struct into a pointer array.

Here's a boiled down version of where I'm having problems:

vertex *vertexIndex = NULL;
vertex *vertexHead = NULL;
vertexHead = malloc(sizeof(vertex));
vertexHead->vertexNum = 5;
vertexIndex = malloc(sizeof(vertex*));
vertexIndex[0] = vertexHead;            //<<<<<<<<<<<  Error on this line
printf("%u\n", (vertexHead[0])->vertexNum);

main.c:72:19: error: incompatible types when assigning to type ‘vertex’ from type ‘struct vertex *’

Any help would be greatly appreciated.

EDIT

Here are the structs

struct edgeStruct {
    unsigned int edgeTo;
struct edgeStruct *nextEdge;
};
typedef struct edgeStruct edge;

struct vertexStruct {
    unsigned int vertexNum;
    edge *edgeHead; 
    struct vertexStruct *nextVertex;
};
typedef struct vertexStruct vertex;

Can we see the declarations of vertexHead, and vertexIndex ? — Shao-Chuan Wang
– Shao-Chuan Wang, Commented Sep 14, 2012 at 18:27
Suspect the issue is with sizeof(vertex*)? Although you seem to be populate an array by generating a pointer? I'm trying to understand what you are trying to do, unsuccessfully?! — Andrew
– Andrew, Commented Sep 15, 2012 at 6:48

Todd Li · Accepted Answer · 2012-09-14 18:28:53Z

1

vertexIndex should be a pointer to a pointer, because you are using it as an array of pointers.

vertex **vertexIndex = NULL;

answered Sep 14, 2012 at 18:28

Todd Li

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P.P · Accepted Answer · 2012-09-14 18:29:07Z

1

vertexIndex is not an array of struct. It's just a struct pointer which is why you are getting the error.

If you want an array, declare an array: vertex

vertex *vertexHead[10]; //array of 10 pointers

Now, you'll be able to use it as you do now.

answered Sep 14, 2012 at 18:29

P.P

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2 Comments

Max Rahm Over a year ago

I'm trying to declare an array in such a way that I can use realloc on it later. Are "vertex *vertexIndex[10];" and "vertexIndex = malloc(sizeof(vertex*) * 10);" not equivalent?

P.P Over a year ago

@user1671981 First is allocated on stack and 2nd is allocated on heap. You can use either one depending on your need. Stack allocated variables are alive only in the scope they are declared.

Rohan · Accepted Answer · 2012-09-14 18:29:53Z

1

As the error message says, on that line you are trying to assign content of pointer to the pointer ie. assigning vertexHead which is vertex * to *vertexIndex (equivalent to vertexIndex[0] which are not compatible.

It will be better you post your code of definition of vertex so that people can suggest what should be done preferably.

answered Sep 14, 2012 at 18:29

Rohan

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