C，malloc数组中的分段错误

Question

I have problem with my school project, the server which is testing this build returns this error:

./__runscript: line 2: 489 Segmentation fault ./a.out < __input

Compile is OK, but this error is displayed just when it starts running. But if I run it on Windows 7 with visual studio 10, everything is fine. I think the mistake might be in the insert() function.

POSTUP *insert(POSTUP *first)
{
    POSTUP* current,*pom;
    current=(POSTUP*)malloc(sizeof(POSTUP));
    pom=(POSTUP*)malloc(sizeof(POSTUP));
    int i,count_of_elements;

    scanf("%d", &count_of_elements);

    if(first==NULL)
    {
        first=current;
        first->array= (int*)malloc(count_of_elements*sizeof(int));
        for(i=0; i<count_of_elements; i++)
        {
            scanf("%d",&first->array[i]);
        }
        first->elements=count_of_elements;
        first->next=NULL;
        return first;
    }
    else
    {
        pom->array= (int*)malloc(count_of_elements*sizeof(int));
        for(i=0; i<count_of_elements; i++)
        {
            scanf("%d",&pom->array[i]);
        }
        pom->next=NULL;
        pom->elements=count_of_elements;
        current=first;
        while(1)
        {
            if(current->next==NULL)
            {
                current->next=pom;
                return first;
            }   
        }
    }
    return 0;
}
int main(void)
{
    int count, i;
    POSTUP* first,*current;
    first= (POSTUP*)malloc(sizeof(POSTUP));
    first=NULL;

    scanf("%d",&count);
    for(i=0; i<count; i++)
    {
        first=insert(first);

    }
}    

Answer 1

This code in main() :

POSTUP *first, *current;
first = (POSTUP*)malloc(sizeof(POSTUP));
first = NULL;

scanf("%d", &count);
for (i = 0; i < count; i++)
{
    first = insert(first);
}

is clearly problematic. You allocate space and store the only pointer to it in first ; you then overwrite the pointer, leaking memory. You should officially check that the allocation was not null. But your insert() code is equipped to handle a null pointer as input, so the memory allocation in main is redundant.

In insert() , you allocate memory without checking for success; this is a recipe for trouble, though more usually later than sooner.

Your code is never checking the result of scanf() ; that means you could be getting rubbish into your system if there's a format error in the data.

Likely cause of crash: In the 'if (first == NULL) block in insert()`, you have:

    for(i=0; i<count_of_elements; i++)
    {
        scanf("%d", &prvy->array[i]);
    }

There's no declaration of prvy in sight in the code you show; it should almost certainly be referring to &first->array[i] anyway. This could easily be good for a crash. Otherwise, you've carefully stashed the data in a different set of memory than you thought, so the array for first is uninitialized gibberish.

The same code does not use pom , so you've leaked memory in this part of the code.

In the main else clause in insert() , you overwrite current (which contained a memory allocation) with first , thus leaking memory there too.

Don't allocate memory until you're about to use it.