Segmentation fault when adding a node to a linked list

Question

typedef struct 
{
     uint32_t field_id;
     uint16_t length;
}entry_t;


struct node
{
        entry_t *sp_entry;
        struct node *next;
}*head;

I have a function called add() to add entry to the linked list.

void add( entry_t *entry )
{
        struct node *temp;
        temp=(struct node *)malloc(sizeof(struct node));
        temp->sp_entry = entry;
        if (head== NULL)
        {
                head=temp;
                head->next=NULL;
        }
        else
        {
                temp->next=head;
                head=temp;
        }
}

Please not that the "value" that is stored in the linked list node is itself a pointer to a structure.I am getting a segmentation fault at

temp->sp_entry = entry;

which is probably because I am not allocation memory for entry_t structure.What I want to know is that is this a usual use case? If yes how do I do it.Do I have to do

temp->sp_entry = malloc(sizeof (entry_t));

before making an assignment? Also is there a more elegant way to achieve this?

Additional information.

when I run gdb I get

p *temp
$3 = {sp_entry = 0x0, next = 0x3e64656269}

the sp_entry looks like it is a null pointer.This is printed after malloc in add() function.And also my code has been combined with "-g -O0 -Wall".There has been no warnings

are you ever initializing head to NULL? If not, your declaration of head after your struct definition leaves head uninitialized, and your head == NULL test will fail — pb2q
– pb2q, Commented Oct 2, 2012 at 1:51
Does your program compile without warnings, and do you have warnings enabled (e.g. gcc -Wall)? — John Kugelman
– John Kugelman, Commented Oct 2, 2012 at 2:01

John Kugelman · Accepted Answer · 2012-10-02 02:09:32Z

2

Your code looks okay. temp->sp_entry = entry; should not segfault since you allocated temp on the previous line.

Pointer errors can be insidious. The line that crashes isn't necessarily the line at fault. add() looks correct, so I suspect there's an error earlier in your program's execution that doesn't cause your program to crash immediately.

When I run gdb I get:
p *temp
$3 = {sp_entry = 0x0, next = 0x3e64656269}      
The sp_entry looks like it is a null pointer.

Not a problem. You haven't initialized temp->sp_entry or temp->next yet so their values are meaningless.

What's important is temp's value. It seems to be a valid pointer since gdb can print *temp. This is really where the segfault is occurring? I would have expected gdb to complain about temp being a pointer to an invalid memory location and refuse to print *temp.

edited Oct 2, 2012 at 2:09

answered Oct 2, 2012 at 1:56

John Kugelman

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1 Comment

liv2hak Over a year ago

when I run gdb I getp *temp $3 = {sp_entry = 0x0, next = 0x3e64656269}

Vivek · Accepted Answer · 2012-10-02 02:14:51Z

2

Not enough karma (?) to post a comment, so posting this as an answer...

If you are on a unixy system, run Valgrind (http://valgrind.org/) to see offending memory read/write(s)

answered Oct 2, 2012 at 2:14

Vivek

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Comments

Arun Taylor · Accepted Answer · 2012-10-02 04:04:25Z

1

The code seems fine. The only way it can give a segmentation fault is if malloc returned null or an invalid value. I would suggest checking the value returned by malloc for a null before attempting to use it.

answered Oct 2, 2012 at 4:04

Arun Taylor

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