Segmentation fault when different OS

Question

I have a C project, and I'm quite new with C and the linux environment.

I'm working on a linux distribution with the following system info

Linux bt 3.2.6 #1 SMP Fri Feb 17 10:34:20 EST 2012 x86_64 GNU/Linux

After compiling with gcc, on the said OS, I got the waited results.

Before submitting my project to the professor, I thought of trying to compile and execute the program on another linux distribution with the following system information

Linux feistyFawn 2.6.20-15-generic #2 SMP Sun Apr 15 07:36:31 UTC 2007 i686 GNU/Linux

And I get segmentation fault under this one. To illustrate the output console. Here are the images.

Works

Fails

I'm not sure what to do now.

---

Code

A call to this function causes segmentation fault on another OS.

fileInEvenements(listEvents, 'A', time, queueId);

What it does is file in a event called A into a queue structure, listEvents.

And its definition

void fileInEvenements(eventStructure *listEvents, char eventType, int time_exec, int id_queue)
{
    Event *newEvent = malloc(sizeof(*newEvent));
    if (newEvent == NULL || listEvents == NULL){
        exit(EXIT_FAILURE);
    }

    newEvent->type = eventType;
    newEvent->execution_time = time_exec;
    newEvent->id = id_queue;
    if (listEvents->firstEvent != NULL)
    {
        // the list contains at least one event, go to the end of list 
        Event *evCurrent =  listEvents->firstEvent;
        while (evCurrent->next != NULL)
        {
            evCurrent = evCurrent->next;
        }
        evCurrent->next = newEvent;
    }
    else // the list contains no event, new event becomes first event
    {
        listEvents->firstEvent = newEvent;
    }

}

Answer 1

When you are creating a new entry in your linked list, you are attempting to append it to the list by starting at the head and iterating the list until you find a NULL in evCurrent->next. When you do find a NULL you stop iterating the list and, at that point, assign newEvent as the next entry in the list via evCurrent->next = newEvent; — and, of course, if there are no entries in the linked list, you make the new entry the head of the list via listEvents->firstEvent = newEvent;

However, at no time do you initialize the value of newEvent->next. Note that malloc() does not initialize the block of memory it returns. It simply allocates a block and returns it to you. See the docs here http://www.cplusplus.com/reference/cstdlib/malloc/

The key bit is this...

The content of the newly allocated block of memory is not initialized, remaining with indeterminate values.

Therefore, newEvent->next is, for all practical purposes, a random value. Thus, your code is off into random memory walks because you are counting on it being NULL to terminate the linked list.

I might suggest you try calloc() instead.

Evenement *newEvent = calloc( 1, sizeof(*nvEvent) );

Otherwise, ensure that the value of the next element is initialized to NULL when you create it.

Answer 2

Add a line:

newEvent->next = NULL;

Using an unitialized field is an error.

Alternatively you can use on the following:

newEvent = calloc(sizeof(*newEvent), 1); // instead of malloc

Or

memset(newEvent, 0, sizeof(*newEvent)); // set all fields to 0