CODEWITHSUNDEEP
cmemory-management
Vikram
Vikram

Reputation: 2069

Unable to understand Memory allocation of structure

Consider following C code -

typedef struct node
{
    int data;
}node;

int main()
{
    node *temp;
    temp->data=100;
    printf("%d",temp->data);
    return 0;
}

It gives Segmentation fault on line containing temp->data=100; because (I think) I haven't allocate memory for it. So, there is nothing such as temp->data.
But, when I try with -

int main()
{
    node *temp,*n;
    n=(node*)malloc(sizeof(node));
    n->data=100;
    temp->data=n->data;
    printf("%d",temp->data);
    retrun 0;
}

It gives proper output 100. I haven't allocate memory where temp will point. But still I am copying n->data to temp->data. How ??

Upvotes: 1

Views: 346

Answers (5)

Prashant Anuragi
Prashant Anuragi

Reputation: 390

Your first code is also right just "temp" is pointing to a garbage value so you have to initialized that.

temp=(node *)malloc(sizeof(node));

Upvotes: 1

Pablo
Pablo

Reputation: 13580

That you haven't got a segfault in temp->data=n->data; is just coincidence. temp is not initialized and hence it points into the digital nirvana.

When you've called malloc you've allocated sizeof node bytes (in reality it may be even a little more) and then you have full memory access to the block pointed to by n.

In general you should call malloc in this way:

node *n;
n = malloc(sizeof *n);
  1. without a cast
  2. with sizeof *n instead of sizeof <datatype>. If you change the datatype (for example you have a typo and instead of typedef .... node you've writen typedef ... nhode. Then you only have to change the declarations of the variables and the rest of the code doesn't have to be changed at all.

Upvotes: 1

s.s
s.s

Reputation: 138

The temp pointer takes some garbage value as its not initialised. By chance temp has a garbage value which may happen to be legal address for your program so it runs. If garbage value has illegal adress it will generate segment fault.

Upvotes: 2

Adam Liss
Adam Liss

Reputation: 48300

You're lucky.

Local variables are not initialized automatically, so when the program begins, both temp and n contain whatever values happen to be on the stack. Next, memory is allocated and n is set to point to it. The value 100 is stored in the data member of the structure.

But temp is still uninitialized, so the value 100 is copied into an unspecified area of memory. Depending on where that memory happens to be, the program may segfault, or it may simply corrupt memory that it doesn't own.

Upvotes: 1

Mark Stevens
Mark Stevens

Reputation: 2366

Your right on the first part, it segfaults because node* temp isn't pointing anywhere (well, somewhere, but not to allocated memory).

I don't know why the second one "works". I suspect it only appears to work, in that it's not crashing. But since temp was never initialized, who know where it's sticking that '100'. Maybe just hanging around to crash later. Either way, writing to unitialized memory isn't a good idea ;-0

Upvotes: 0

Related Questions