Increasing Size of Struture W/ Realloc (ERROR On the second Increase)

Question

I am a begginer programmer trying to do a project for a class on college that requires the use of dynamic strutures. My problem: I use realloc to increase the size of my struture and it works the first time, but not on the second. the auxiliary pointer wich I use to recieve the new increased struture seems to get the value NULL on the second time.

Here's my code so far:

funtion where I try to increase structure:

TList *addToQueue (TList *plist, int *counter, char departure)
{
    TList *auxiliary;
    auxiliary = NULL;

    auxiliary = realloc(plist,(*counter+1)*sizeof(TList));

    if (auxiliary == NULL)
    {
        printf("Insufficient Memory");
        cleanstdin();
    }
    else
    {
        plist = auxiliary;
        plist[*counter].departure = departure;

        do{
        printf("\nPoint of Arrival: ");
        plist[*counter].arrival = checkMenuOption(OPTIONSLOC);
        }while(plist[*counter].arrival == '\0');

        printf("\nNumber of passengers:");
        plist[*counter].numberOfPassengers = readIntenger(MIN_SEATS, MAX_SEATS);

        (*counter) ++;

        printf("\nTicket Purchased successfully");
        cleanstdin();

    }
    return plist;
}

How I'ts declared above main:

TList *addToQueue (TList *plist, int *counter, char departure);

How I call in main:

char departure;

            departure = sellTicketsMenu ();

            switch (departure)
            {
            case 'A':
            {
                ListA = addToQueue (ListA, &counterA, departure);
                cleanstdin();
            }
            break;
            case 'B':
            {
                ListB = addToQueue (ListB, &counterB, departure);
                cleanstdin();
            }
            break;
            case 'C':
            {
                ListC = addToQueue (ListC, &counterC, departure);
                cleanstdin();
            }
            break;
            }

Other notes: readintenger - basic funtion to read intenger from keyboard cleanstdin - basic clear standarin function checkmenuoption - funtion to check the option from menus counters - counters to help with the index of the struture

If you need anything else to understand or to help me please do let me know. Thanks in advance.

What is clear standard input? why? And do you say that `realloc` is returning `NULL` becase the `"Insuf..."` message is printed or why? — Iharob Al Asimi, Jan 06 '15 at 14:09
Clear standard is just to avoid any errors when recieiving data from the user and to help create like ''pause'' moments, don't mind that to much, it's not that really important. — Unknown Char, Jan 06 '15 at 14:25
I say that it's returning NULL beacause I have an If condition positioned beneath the realloc to see if it did what it was supposded to do. My teachers said If it returns NULL it means that something ocurred and it wasn't able to increase the struture — Unknown Char, Jan 06 '15 at 14:26
It's just that I suspect you have a `fflush(stdin)` which has [undefined behavior](http://stackoverflow.com/a/11604639/1983495). — Iharob Al Asimi, Jan 06 '15 at 14:26
No. It's like fflush but my teacher uses a mac so we have to do universal codding so that it suits all os. — Unknown Char, Jan 06 '15 at 14:30
So you see `"Insufficient Memory"` in the output on the second call? — Iharob Al Asimi, Jan 06 '15 at 14:52
Please post your full `main` function, or see my answer, may be that's it. — Iharob Al Asimi, Jan 06 '15 at 14:57
this line: 'printf("Insufficient Memory");' should probably be preceded by: 'perror("realloc failed");' so the actual problem description and the failure event are recorded on stderr — user3629249, Jan 06 '15 at 18:52

score 1 · Answer 1 · answered Jan 06 '15 at 18:22

Better to post here rather than off-site

Off-site code (posted in deleted answer) calls reallloc(), via addToQueue(), with a pointer that has all ready been free'd.

 case 'C':
   ...
   free(ListA);
   switch (optionMTI) {
     case 'A': {
       ...
       ListA = addToQueue (ListA, &counterA, departure);

Problem is re-use of a free'd pointer.
If code is to re-use a pointer after free(), be sure to assign it to NULL.

            // Fixed code  
            free(ListA);
            ListA = NULL;  // add
            ...
            ListA = addToQueue (ListA, &counterA, departure);

Iharob Al Asimi · Answer 2 · 2015-01-06T15:13:04.747

0

The only possible explanation which cannot be confirmed because of the lack of source code is that the passed parameter counter is not initialized or list is not set to NULL before the first call.

You must initialize it to 0, a typical for loop would look like

int counter;
int i;
int numberOfStructs;
TList *list;

counter = 0; /* <--- this is the important part */
numberOfStructs = SOME_NUMBER_MAY_BE_FROM_INPUT;
list = NULL;
for (i = 0 ; i < numberOfStructs ; ++i)
    list = addToQueue (list, &counter, departure);

in your case you could just try

int counterA = 0;
int counterB = 0;
int counterC = 0;

at the counter* variables definition.

edited Jan 06 '15 at 15:13

answered Jan 06 '15 at 15:00

Iharob Al Asimi

52,653
6
59
97

counters are initiallized at 0 already, anymore thoughts? need more code? – Unknown Char Jan 06 '15 at 15:08
@UnknownChar yes more code please. I wouln't like to leave this answer as is or delete it. – Iharob Al Asimi Jan 06 '15 at 15:10

Increasing Size of Struture W/ Realloc (ERROR On the second Increase)

2 Answers2