Ask about how to determine whether the pointer has been allocated memory Thank you!

黑与白的礼赞

E.g:

struct LNode
{
    int data;
    LNode * next;
}
void main ()
{
    LNode * p;
    // So far, p has not been allocated memory,
    // I want to judge here, how can I know that p does not point to the object instance
    if (...)
    {
        // other operations
    }
}

Note: I have tried both p! = '\0' and p! = NULL, to no avail.

hzxszmh

LNode * p = 0; give 0 when declared

黑与白的礼赞

If I release it with free (), it seems to be useless:

void main ()
{
LNode * p = 0;
p = (LNode *) malloc (sizeof (LNode));
free (p);
if (p == NULL)
{
printf ("P is NULL\n");
}
}

druboy

free (p);
p = NULL;

孤单浪子

void main ()
{
    LNode * p;
    p = (LNode *) malloc (sizeof (LNode));

    if (NULL == P)
    {
        printf ("alloc memory failed.\n");
    }

    if (...)
    {
        // other operations
    }
    free (p);
    p = NULL;
}

孤单浪子

Of course, pointers are best initialized when they are declared:
void main ()
{
    LNode * p = NULL;

Because if you do not initialize, some compilers will automatically initialize it, such as VC will initialize the pointer to 0xcccccccc

黑与白的礼赞

I don't mean this. I don't want to determine whether the malloc function is allocated successfully. I want to determine whether a pointer is allocated to an object instance. This requires considering the following situations, such as a function:

int AppendElem (LNode * header)
{
    if (...) // determine here whether the header is a null pointer
    {
        // if it is a null pointer
    }
}

Another example:

int Function (LNode * p)
{
    if (...) // determine if an object instance exists
    {
        // if an object instance exists
    }
}

Before the above two function calls, the p pointer may or may not have been malloc

hzxszmh

LNode * p = 0; give 0 when declared
p = (LNode *) malloc (sizeof (LNode));
if (0 == p) {
Allocation failed
}
Generally, assigning a value of 0 when the pointer is not used indicates an invalid pointer.

黑与白的礼赞

Tohzxszmh:

    Still don't understand what I mean, if so:

    LNode * p;
    AppendElem (p);

    p is neither assigned 0 nor malloc, and I now want a function entry judgment that can consider exceptions.

暗夜刺炎

Is sizeof (p) OK?