malloc struct pointer throwing segmentation fault (core dumped)

Question

I have these two structs

struct node{
    int val;
    struct node *up;
    struct node *down;
};

struct stack {
    struct node *bottom;
};

The typedef struct is defined in a header file

typedef struct stack stack_set;

I am trying to malloc the structs but I keep getting seg faults when I try to access the node struct. I have tried using malloc on the node struct to no avail.

    stack_set *set;
    set = malloc(sizeof(stack_set));
    set->bottom = NULL;
    set = malloc(sizeof(struct node));
    set->bottom->val = NULL;
    return set;

I hit a seg fault on the second last line of code.

How do I properly malloc my code so I don't keep throwing seg faults? I haven't been able to find an example that could aid in resolving mine.

Thanks

Ilya · Accepted Answer · 2014-08-19 15:45:40Z

3

It looks like misptint in fourth line (also you mix root and bottom). Try this:

stack_set *set;
set = malloc(sizeof(stack_set));
set->bottom = malloc(sizeof(struct node));
set->bottom->val = 0;     // it is better to initialize
set->bottom->up = NULL;   // all fields to avoid undefined
set->bottom->down = NULL; // behaviour in the future
return set;

Also it is better to check result of memory allocation functions:

stack_set *set;
set = malloc(sizeof(stack_set));
if (!set)
  return NULL;
set->bottom = malloc(sizeof(struct node));
if (!set->bottom) {
  free(set);
  return NULL;
}
set->bottom->val = 0;
set->bottom->up = NULL;
set->bottom->down = NULL;
return set;

edited Aug 19, 2014 at 15:45

answered Aug 19, 2014 at 15:29

Ilya

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1 Comment

connie xo Over a year ago

Wow, thankyou, it works now :D I'll definitely utilise this thorough checking method as well. Cheers!

Basile Starynkevitch · Accepted Answer · 2014-08-19 15:46:30Z

3

The standard malloc(3) function. is returning uninitialized memory. You need to initialize it. So after

    set = malloc(sizeof(struct node));

all of your set fields contain garbage values (or malloc failed and gave NULL). In particular you should not dereference set->root, this is undefined behavior, and might seems to work if unlucky.

I would recommend using callocto get zeroed memory:

   set = calloc(1, sizeof(struct stack));
   if (!set) {perror("calloc stack"); exit(EXIT_FAILURE); };
   set->bottom = calloc(1, sizeof(struct node));
   if (!set->bottom) {perror("calloc node"); exit(EXIT_FAILURE); };

If you can't use calloc clear the memory using memset.

Compile with all warnings and debug info (gcc -Wall -g) and use valgrind if available.

edited Aug 19, 2014 at 15:46

answered Aug 19, 2014 at 15:30

Basile Starynkevitch

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1 Comment

connie xo Over a year ago

Thankyou for the answer and the tips on using calloc. I will try use valgrind as well.

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