How does c assign values to an array of pointers when it is first defined?

Question

I have an array of int pointers.

int *ints[3];

What happens to memory when this array is defined? What would be the initial pointer values stored? When I try to dereference a value, I get a segmentation fault which tells me the values are arbitrary.

cnicutar · Accepted Answer · 2014-02-17 23:01:47Z

4

It depends on the storage of the array.

If it has static storage, i.e. declared outside of any function or declared with static in a function, then all the elements will be NULL
If it is declared in a function, without static, it will have automatic storage and the initial values will be indeterminate

The standard happens to be quite clear and helpful in this matter:

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static or thread storage duration is not initialized explicitly, then:

if it has pointer type, it is initialized to a null pointer;

if it has arithmetic type, it is initialized to (positive or unsigned) zero;

if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero
bits;

edited Feb 17, 2014 at 23:01

answered Feb 17, 2014 at 22:55

cnicutar

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2 Comments

This isn't my real name Over a year ago

I believe it should be noted that "indeterminate" is not the same as "random". The distinction may be less significant for pointers, because using a random pointer is just as much a problem as using an indeterminate pointer, but for number types on hardware that supports trap representations, "random" can imply "any valid value", whereas an indeterminate value may be a trap representation.

Jens Gustedt Over a year ago

It is as significant to pointers as for integer types. What traps are for integers are misaligned values for pointers. Even if by chance a pointer pointed into valid memory, it could still raise a signal when accessing the object if it isn't aligned for the base type.

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