Accessing dynamic memory in a struct pointer

Question

I'm writing some code to interact with dynamic memory that i' creating and i want to know why it's not working.

struct order{
    char *bookTitle;
    double price;
    char *category;
    double remain;
    int custID;
    char processed;
};
typedef struct order order;

struct orderNode{
    order *order;
    struct orderNode *next;
};
typedef struct orderNode orderNode;

Here is the code creatinng an array or orderNodes and trying to access them. The print statement is giving me a segmentation fault. Not sure why.

orderNode **processQueue = malloc(sizeof(orderNode)*numcats) ;
    //sort the orders into their respective categories
    //sortOrders(processQueue, unSortedQueue, numcats);
    processQueue[1]->order->category = "herro" ;
    puts("string set. \n");
    printf("%s\n",processQueue[1]->order->category);
    /* 
    gdb backtrace output.
    #0  strlen () at ../sysdeps/x86_64/strlen.S:106
    #1  0x00007ffff786794c in _IO_puts (str=0x1000000000a2074 <error: Cannot access memory at address 0x1000000000a2074>)
    at ioputs.c:36
    #2  0x000000000040192f in main (argc=4, argv=0x7fffffffdbd8) at main.c:444
    */

Thanks in advance!

you have to have memory defined for each pointer (a very good reason to consolidate your struct definitions into a single definition) so the problem is the pointer 'order' in the second struct is not pointing to anywhere in particular, so accessing an offset from the pointer is undefined behaviour and can/will result in a seg fault event. — user3629249
– user3629249, Commented Nov 25, 2014 at 20:19

Vlad from Moscow · Accepted Answer · 2014-11-25 20:33:30Z

I think that instead of

orderNode **processQueue = malloc(sizeof(orderNode)*numcats) ;

you mean

orderNode *processQueue = malloc(sizeof(orderNode)*numcats) ;

Also take into account that for each orderNode in the array you have to allocate data member order before trying to access its own data members.

And indices in C start from zero. It seems that in this statement

processQueue[1]->order->category = "herro" ;

you want to access processQueue[0] instead of processQueue[1]

The code could look like

orderNode *processQueue = malloc( sizeof( orderNode ) * numcats );

processQueue[0].order = malloc( sizeof( order ) );
processQueue[0].order->category = "herro";

puts( "string set." );
puts( processQueue[0].order->category );

JS1 · Accepted Answer · 2014-11-25 20:18:59Z

You never allocated any of your order:

orderNode *processQueue = calloc(numcats, sizeof(orderNode)) ;

for (i=0;i<numcats;i++)
    processQueue[i].order = calloc(1, sizeof(order));

Also, I used calloc for the first allocation to make sure all of your next pointers are NULL. I'm not sure what you plan on doing with those next pointers, though.

EDIT: also, as Vlad from Moscow pointed out, processQueue was of the wrong type (I fixed the type above).

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