使用带有双指针的malloc时出现段错误

Question

I'm using something like this to allocate memory with a function (in C)

void myfunction(struct mystruct** ss) {
    // some code
    *ss = malloc( 1024 * sizeof (struct mystruct) );
    // some code
}    
int main()
{
   struct mystruct **x;
   *x = NULL;
   myfunction(x);
   return 0;
}

but I'm getting seg fault. What is wrong with this code?

Answer 1

After struct mystruct **x; , the variable x is uninitialized. It is illegal to read from it as your program does in *x = NULL; .

You may have wanted to write:

int main()
{
   struct mystruct *x;
   x = NULL;
   myfunction(&x);
   return 0;
}

But it is impossible to be sure, as your program does not do anything meaningful. Note that x = NULL; is unnecessary anyway: x will be initialized inside myfunction() .

Answer 2

you never make any storage for the underlying pointer, there is storage for the ** and the object but not the * ...

struct mystruct **x,*y;
x = &y;
myfunction(x);
return 0;