Trouble with Malloc for Structs

Question

I keep constantly getting a segmentation error with a small portion of code and I have no idea why, I'm using malloc and cant see any reason with that, just as soono as it hits the portion of data, its crashes. The Data is:

listNode* node = (listNode*)malloc(sizeof(listNode));    <-This is the Line
strcpy(node->entry, string); 
node->next = NULL;

Using the struct definition:

typedef struct
{
    char* entry;
    struct listNode* next;
}listNode;

The data does get freed at one point, however it cannot reach said point. Thanks for the help!

You allocated memory for the struct itself. But you didn't for pointers that at in it. — Mysticial, Sep 14 '12 at 03:50

score 2 · Answer 1 · edited May 23 '17 at 10:34

Are you sure you're crashing on the malloc call? It's much more likely that you're crashing on the strcpy. In fact, the strcpy in this case is almost guaranteed to cause bad behavior, although to be precise the behavior is undefined.

The malloc should succeed as long as you haven't filled up your heap. However, because you called malloc and not calloc, you have no guarantees about the contents of node->entry. The C standard doesn't define what is stored in memory allocated with malloc, so node->entry is filled with garbage. By trying to copy to a bad pointer, your app is crashing for attempting to accessing an incorrect memory address.

Alexey Frunze is 100% correct on the fix - malloc space and change node->entry to point at the newly allocated memory, then copy string into node->entry. This avoids undefined behavior for using an uninitialized pointer, as I described in another SO answer.

score 1 · Accepted Answer · answered Sep 14 '12 at 03:51

1

You didn't allocate memory for the entry pointer. You're trying to copy your string to a bogus location specified by the uninitialized pointer entry. You might do this instead:

node->entry = malloc(strlen(string) + 1);
strcpy(node->entry, string);

answered Sep 14 '12 at 03:51

Alexey Frunze

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or `node->entry = strdup(string);` – John3136 Sep 14 '12 at 03:54
oops, Didn't even notice that hehe, I'm to use to it throwing segmentation faults for data I should be using haha, thanks! – Cail Demetri Sep 14 '12 at 03:54
@John3136 if it's available. It's not in the C standard from 1999. – Alexey Frunze Sep 14 '12 at 03:55

score 0 · Answer 3 · answered Sep 14 '12 at 04:45

You are not allocating allocate memory for the entry pointer. as while allocationg structure just 4 bytes allocated for *entry pointer you should do

listNode* node = (listNode*)malloc(sizeof(listNode)); // allocate memory for structure node->entry = malloc(strlen(string) + 1);// allocate memory for string strcpy(node->entry, string); //copying strings

now it will run fine

Trouble with Malloc for Structs

3 Answers3