未初始化的指针变量有什么用？

Question

I was asked this question in an interview.

If there is a pointer that is declared but not initialized like this: int *ptr;

Is it assigned a default value? Or will it be a null pointer? Also, what happens if we try to use this pointer like:

if(ptr) { //block of code }

Will the if condition pass? Also, will this work on debug/release build?

I tried to run this program at home and found that if condition passes, and if I try to print the value of the ptr like this:

printf("%x %d", ptr, *ptr);

it prints some random value, but it doesn't crash the program. What is the explanation behind this?

Answer 1

If it's a global variable (or static ), it will be implicitly initialized to NULL . Otherwise, its value is undefined. Therefore, the behaviour of that if statement is undefined (ie you can make no assumptions about what it might do). The behaviour of your second example is also undefined (and it could result in a segmentation fault).

Uninitialized pointers are considered dangerous; imagine the consequences of this code:

int *ptr;

...

*ptr = 42;  // Dereference pointer and trash somewhere random in memory

But note that this might not crash; it may corrupt things silently.

So the convention is to always explicitly initialize it to point at something useful. If that's not possible (eg you don't yet have something to point to), the next best thing you can do is initialise it to NULL . Dereferencing a NULL pointer will raise a segmentation fault on most platforms, which will at least help you track down the bug.

Answer 2

Uninitialized pointers are good for security vulnerabilities and possibly contributing to seeding random number generators.

Your if condition will only skip the block of code if the pointer had been initialized to NULL (0).

Because the initialization value is undefined by default it is always considered good practice to initialize to NULL .

Your printf statement just illustrates the fact that it defaulted to a value other than NULL and in this case it was a valid area of memory that could be dereferenced.

Answer 3

What does an un-initialized pointer variable good for?

Efficiency. If C automatically initialized pointers to NULL , it would waste a whole CPU instruction (or possibly more if the variable had to be stored in main memory instead of a register), and that was unacceptable to the designers of C. Would you believe programmers these days wanting to sacrifice valuable nanoseconds of runtime just to save a few hours of debugging?