Cannot access index of array pointer C

Question

I am trying to set up an adjacent list array in C:

mygraph->table = (Node *)malloc(MaxSize * sizeof(Node));
check(mygraph->table, error);
int i;
for(i = 1; i <= MaxSize; i++)
{
    mygraph->table[i].name = NULL;
    mygraph->table[i].outlist = NULL;
    mygraph->table[i].outdegree = 0;
}
...

When I run this code, it works fine. But when I try to access an index in the table I get a segmentation fault:

mygraph->table[n].name = name;

I checked the n index, and it is correct. MaxSize is 10, but I get a segmentation fault even when n = 1.

EDIT:

typedef struct linkedlist { // linked list of ints (for use in Node)
  int index;
  struct linkedlist *next;
} List;

typedef struct { // a Node of a Graph
  char *name;
  List *outlist; // adjacency list
  int outdegree; // length of outlist
  //double pagerank_score; //not needed for this exercise
} Node;

typedef struct {
  // your code goes here
  int MaxSize;      /*Maximum number of vertices that the graph can constain*/
  Node *table;     /*Adjacency lists' array*/
} Graph;

did you allocate memory for name?

t0mm13b
– t0mm13b

2016-04-17 11:42:39 +00:00
Commented Apr 17, 2016 at 11:42 — t0mm13b
– t0mm13b, Commented Apr 17, 2016 at 11:42

Sergey Kalinichenko · Accepted Answer · 2016-04-17 11:36:10Z

2

I get a segmentation fault even when n = 1.

This is because your code has undefined behavior. It writes past the end of the array:

for(i = 1; i <= MaxSize; i++) // should be i=0 ; i<MaxSize

When n is 10 the code does not crash on your system - perhaps because malloc adds enough padding at the end of the block to accommodate an extra element past the end of the array, but the error is still there.

You can find such hidden errors using a memory profiler, e.g. valgrind.

The fix is to use correct indexes in the initialization:

for(int i = 0 ; i != MaxSize ; i++) {
}

answered Apr 17, 2016 at 11:36

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

Alessandro Over a year ago

It crashes for every number, even 10

Alessandro Over a year ago

I tried your fix but it still crashes... I'll post the declaration of Node and Graph

Sergey Kalinichenko Over a year ago

@Alessandro It's probably crashes in some other place now, because this is the error in the code that you have posted.

Alessandro Over a year ago

There's not much I do really, just initialise and try to change one index.

Sergey Kalinichenko Over a year ago

@Alessandro You may want to post the "try to change one index" code, too.

|

t0mm13b · Accepted Answer · 2016-04-17 11:45:11Z

1

This is the key:

mygraph->table[n].name = name;

There is no mention of allocating memory for the name variable.

Also, assigning name variable on the right hand side of expression, it would be better to indicate, by using strncpy or strdup to state your intention.

The onus is on you to ensure you do indeed, free the memory occupied by the name member also.

answered Apr 17, 2016 at 11:45

t0mm13b

34.7k8 gold badges85 silver badges114 bronze badges

3 Comments

Ashalynd Over a year ago

If the name variable on the right was allocated properly, the assignment should be valid, but indeed, if the original pointer gets deallocated somewhere else later, then there is a problem.

t0mm13b Over a year ago

Indeed, it is valid, but to the discerning eye, what is name, where did it come from? Cue looking around elsewhere, it would be easier if it had the intention of indicating that it was strcpy or strdup , which clues in more in 'a-ha, its a pointer variable'. :)

Alessandro Over a year ago

I tried using strcpy(mygraph->table[n].name, name); but it still fails

Elazar · Accepted Answer · 2017-05-29 17:37:00Z

1

Your loop need to run from 0 to MaxSize-1, like this:

for(i = 0; i < MaxSize; i++)
{
    mygraph->table[i].name = NULL;
    mygraph->table[i].outlist = NULL;
    mygraph->table[i].outdegree = 0;
}

so the first element will be at mygraph->table[0] and the last element will be at
mygraph->table[MaxSize-1].

In pointer arithmetic, the first element will start at mygraph->table and the last element will start at:

((mygraph->table) + MaxSize-1)

and would end at:

((mygraph->table) + MaxSize)

table[MaxSize] is equivalent *((mygraph->table) + MaxSize) and this is out of the array boundaries.

edited May 29, 2017 at 17:37

Elazar

22k4 gold badges51 silver badges68 bronze badges

answered Apr 17, 2016 at 11:49

Nuf

313 bronze badges

Collectives™ on Stack Overflow

Cannot access index of array pointer C

3 Answers 3

8 Comments

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

8 Comments

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related