CODEWITHSUNDEEP
c++memory-management
Mark
Mark

Reputation: 8688

Minor memory allocation question C++

int main() {


  char **k;
  char *s ="abc"; 
  char *b ="def";

  *k = s;


}

//Why does this produce segmentation fault? Shouldn't everything be store on the stack without any problems?

Upvotes: 2

Views: 135

Answers (3)

hookenz
hookenz

Reputation: 38967

Ok, I hope I don't get beaten up with with any slight error... here is my attempt to explain it as fully as I can.

With an ordinary char* it will point to a char.

with a char** it points to a pointer that points to a char. That *k value is on the heap and not the stack.

i.e. like this:

stack (1)    heap (2)   heap or ... (3)
+-----+    +-----+     +----+
|char*| -> |char*| ->  |char|
+-----+    +-----+     +----+

Now char*'s are not really strings but they are treated as blocks of contiguous printable characters in memory that are terminated by a null or zero byte. So the string would be stored and be referenced at (3)

So to fix your code you'll want to allocate space for a char* (not a char).

i.e. put

k = (char**)malloc(sizeof(char*));

before the line 

*k = s;

Not that it's a good code, But it shouldn't crash.

Upvotes: 2

sarnold
sarnold

Reputation: 104100

Alexander is correct, you're dereferencing k with *k = s;. Your initialization of char *s="abc"; may look the same, but it is syntactic sugar for the longer: char *s; s="abc";

Upvotes: 1

Alexander Gessler
Alexander Gessler

Reputation: 46657

k has no defined value yet, so dereferencing it (*k) causes undefined behaviour. If you add an initialization, i.e. k = &b;, *k = s; will work afterwards.

Upvotes: 4

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