Function can not allocate memory for a pointer in C

Question

I have a struct which is called "visitor" and I want to write function which allocates the memory for my pointer to this struct ...

this Codes works:

visitor *visitorPtr = NULL;
int visitorCounter = 0;
visitorPtr = realloc(visitorPtr, ((visitorCounter++) * sizeof(visitor)));

but now I wanted to put this functionality to a function

void getMoreSpace(void *ptr, unsigned int counter)
{
    counter++;
    ptr = realloc(ptr, counter * sizeof(visitor));

    if (ptr == NULL)
    {
        printf("\n Error allocating memory! \n");
        return EXIT_FAILURE;
    }
}

//Call in MAIN:
getMoreSpace(visitorPtr, visitorCounter);

Unfortunately it seems not to work because I cant create data for this pointer and get this Error when I make that for that struct: Exception thrown at 0x0FDAE559 (ucrtbased.dll) in My-C-Project.exe: 0xC0000005: Access violation writing location 0x00000000.

You understand the concepts: call-by-value and call-by-reference? — Sourav Ghosh, Apr 03 '19 at 12:13
You modify a _local_ variable, not the caller's variable. So the caller cannot see the change. Lookup call-by-value and call-by-reference as Sourav Gosh suggests. — Paul Ogilvie, Apr 03 '19 at 12:18
call-by-reference means for me if after the function the var is still in the changed state — BlindRob, Apr 03 '19 at 12:20
*call-by-reference means for me if after the function the var is still in the changed state* And how does C pass variables... — Andrew Henle, Apr 03 '19 at 12:20
generally always by value and if you use a pointer its by reference! :-) — BlindRob, Apr 03 '19 at 12:26

kiran Biradar · Accepted Answer · 2019-04-03T12:20:46.470

1

visitorPtr is passed as value to getMoreSpace function. Thus any changes done to ptr within the function will not update the vistorPtr in main.

What you need is call the function by reference as below.

void getMoreSpace(void **ptr, unsigned int counter)
{
    counter++;
    *ptr = realloc(*ptr, counter * sizeof(visitor));

    if (*ptr == NULL)
    {
        printf("\n Error allocating memory! \n");
    }
}

And from main you call as below.

getMoreSpace(&visitorPtr, visitorCounter);
if (visitorPtr)
   ...do stuff....

edited Apr 03 '19 at 12:20

answered Apr 03 '19 at 12:16

kiran Biradar

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That looks good to me. – Jabberwocky Apr 03 '19 at 13:05

Function can not allocate memory for a pointer in C

1 Answers1