check if pointer is null in a pointer array

Question

I'm trying to check if a pointer is NULL in an array of pointer. Program crash when running, debbugger point to the if() condition but I don't know what it wrong.

In main.c

  unsigned int** memory = malloc(sizeof(unsigned int*)*MEMORY_SIZE);

    /* malloc failed */
    if (!memory)
    {

       return EXIT_FAILURE;
    }

    Process myProcess = { 1, 2, -1};

    /* TEST THAT WORKS */
    /* memory[0] = &(myProcess.m_id); */
    /* printf("%u", *memory[0]); */


    AllocFirstFit(&myProcess, memory);

In another .c file

void AllocFirstFit(Process* process, unsigned int** memory)
{
    unsigned int itr_mry;
    /* Declaration of various other local variable here*/

    /* browsing the memory */
    for(itr_mry = 0; itr_mry < MEMORY_SIZE; ++itr_mry)
    {
        /* if memory unit is null */
        /* debugger point this line. This condition is never true for some reason */ 
        if(memory[itr_mry] == NULL)
        {

Bathsheba · Accepted Answer · 2015-12-16 10:02:11Z

3

You need to initialise the contents of the array memory to NULL yourself: the compiler will not do this for you on a malloc call. Currently the behaviour of your program is undefined since you're reading back an uninitialised pointer value.

The best thing to do is use calloc which will set pointers to the null pointer value.

edited Dec 16, 2015 at 10:02

answered Dec 16, 2015 at 9:55

Bathsheba

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1 Comment

Csi Over a year ago

Ah thank you, i didn't pay attention. I used calloc and the condition is working just find

Sourav Ghosh · Accepted Answer · 2015-12-16 11:53:32Z

2

In your code, while malloc()ing memory, you allocated some memory to the variable memory. You never initialized the contents of *memory or memory[i]. They may not be NULL, as you're expecting. They may very well contain garbage values.

So, basically, later,

 if(memory[itr_mry] == NULL)

is an attempt to use uninitialized memory which leads to undefined behavior.

Solution: You need to use calloc() to get a zero-initialized memory so that you can at least run the NULL check on *memory or memory[i].

edited Dec 16, 2015 at 11:53

answered Dec 16, 2015 at 9:54

Sourav Ghosh

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1 Comment

Csi Over a year ago

Ah thank you, i didn't pay attention. I used calloc and the condition is working just find.

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