CODEWITHSUNDEEP
cpointersinitializationdereference
Terry Li
Terry Li

Reputation: 17268

How to check if allocated memory is initialized or not

I am exposed to the pointer int* i, of which I only know its memory is allocated but am not sure it has been initialized to some integer or not.

If I try to deference it, what would happen? In other words, how should I check if it is initialized or not? If it is not, I'd like to assign an integer value to that address; otherwise I do nothing.

Upvotes: 3

Views: 7794

Answers (5)

Agnius Vasiliauskas
Agnius Vasiliauskas

Reputation: 11267

You can also re-define malloc in such as way that it will allocate memory block and initialize it to zero bytes at once by using such macro:

#define malloc(p, size) do {if (p != NULL) {p=malloc(size); memset(p, 0, size);}} while(0);

Or if you prefer malloc can be re-defined in terms of calloc:

#define malloc(p, elCount , elSize) do {if (p != NULL) {p=calloc(elCount,elSize);}} while(0);

By forcing such construct you will be sure that any programmer which uses malloc() - also initializes memory properly, so that no pointer shows to garbage data.

Upvotes: -1

Keith Thompson
Keith Thompson

Reputation: 263177

i is typically a name for an integer (usually int) object. Calling a pointer i is confusing.

So let's assume you have

int *p;

You say "its memory is allocated". Do you mean memory for the pointer object itself (which is allocated when you declare p), or do you mean memory for the int object it points to?

In either case, you can't tell whether it's been initialized or not.

If p has static storage duration, then its initial value (in the absence of an explicit initializer) is a null pointer; it doesn't point to anything. Otherwise, its initial value is garbage, and you cannot safely do anything with it other than assigning some valid value to it.

If, on the other hand, you mean that p has been initialized to point to some int object, but that object may or may not have been initialized, then you have a similar problem. If you have:

int *p;
p = malloc(sizeof *p);
if (p == NULL) {
    /* malloc() failed! */
    exit(EXIT_FAILURE);
}

then p points to an allocated int object, but that object's value is garbage. On most systems, you can safely access the value (*p) -- but there's no point in doing so. But it's possible that int has trap representations; if so, just accessing the value of *p has undefined behavior, and could cause your program to crash. And an optimizing compiler can do unexpected things in the presence of undefined objects; it can assume that it's been initialized to some value, and not bother to actually fetch the stored value.

There is no special value, marker, or flag for an uninitialized int object. It is entirely up to you, the programmer, to ensure that any object has a valid value before you attempt to access it. (Some languages keep track of these things for you; C is not one of those languages. Of course, a lot of the implementations of those languages are written (very carefully) in C.)

So you're asking the wrong question. If you don't know whether an object has been initialized, the solution is to find out (logically, when you write the code, not during program execution) whether it's been initialized. The best way do to this is usually to ensure that it is initialized.

If you're trying to do something like the following pseudo-code:

 if (/* *p has been initialized */) {
     do something with *p;
 }
 else {
     do something else;
 }

then you're doing it wrong. Instead:

/* Ensure p points to a valid int object */
*p = some_value;
/* Now you *know* *p has been initialized */
do something with *p;

Upvotes: 0

dbeer
dbeer

Reputation: 7203

Good programming practice includes assigning NULL to pointers that are uninitialized. If you're handling a pointer from somewhere else, it is common to check to make sure it isn't NULL. In your case:

if (i != NULL)
  *i = the_int_you_assign;

However, if there is no specified way to handle it and you can't count on good programming practices, then you can't really defend yourself.

Upvotes: 0

NoMoreZealots
NoMoreZealots

Reputation: 5320

I am exposed to the pointer int* i, of which I only know its memory is allocated but am not sure it has been initialized to some integer or not.

If i was allocated as a pointer and not initialized you will get a segment violation. If i was allocated and initialized to some integer X... i.e.

int X = SOMEVALUE;

    int* i;
    i = &X; // It would seem silly to malloc a single int...

Then it is initialized IF and only if X was initialized. If you know some expected bounds for the value it is pointing to then it would be recommended to perform a bounds check before using...

If I try to deference it, what would happen? In other words, how should I check if it is initialized or not? If it is not, I'd like to assign an integer value to that address; otherwise I do nothing.

My typical practice is initialize pointers to 0 or NULL until I have a real value to feed them. Prior to calling an *X*alloc function of somesort... Then if it anywhere I would try to use it... I do a...

if (myPtr == NULL)
{
 printf("Run failure handling code...\r\n");
 return FUNC_FAILED_CONST;
}

But I would have to more info to know your situation....

Upvotes: 0

user405725
user405725

Reputation:

Define "initialized". There will always be some value, and there is no way you can tell whether that value is a garbage or an integer, because any 32 bits of garbage will yield some value. You can dereference it though, no problem.

Upvotes: 5

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