链接列表程序在malloc上崩溃

Question

I'm fairly new to C and coding in general so please bear with me. I've been trying to implement a linked list recently and this is the code i came up with

typedef struct something{
    int data;
    struct something *next;
} thing ;

int main ()
{
thing *head, *current;

head=malloc(sizeof(thing));
puts("head=malloc(sizeof(thing));");

if (head != NULL)
    puts("malloc success");

head=NULL;

current=head;
puts("current=head;");
if (current == NULL)
    puts("current is NULL");


puts("while");
while (current!=NULL)
{
    current = current->next;
}
puts("end while");


current->next=malloc(sizeof(thing));
puts("current->next=malloc(sizeof(thing));");

//free at end of program
}

While the compiler shows 0 errors, when i run the program it only runs until the final malloc part before crashing. It doesnt run the final puts so i will assume it's something to do with the way i'm trying to use malloc. I'll gladly appreaciate for someone to tell me what im doing wrong.

Answer 1

The problem is that your while loop goes to far. You want to stop when current points to the last element of the list, so you can add to it. But you're going one step further, and stopping when current == NULL . It's then too late to assign to current->next .

First, you need to initialize head->next to NULL.

head = malloc(sizeof(thing));
head->next = NULL;

Get rid of the line:

head = NULL;

as this is overwriting the result of malloc() .

Then your while loop needs to test current->next , not current itself:

while (current->next != NULL) {
    current = current->next;
}

And when you add the new node, you have to set its next pointer to NULL as well:

current->next = malloc(sizeof(thing));
current->next->next = NULL;

These should fix your problem.

Answer 2

You allocate head and then immediately after few checks point its pointer to NULL

// Allocation here

head=malloc(sizeof(thing));
puts("head=malloc(sizeof(thing));");

// Not a null 
if (head != NULL)
    puts("malloc success");

// Point to NULL again ???
head=NULL;

Then your current points to head viz NULL again that makes current NULL

current=head;
puts("current=head;");
if (current == NULL)
    puts("current is NULL");

and then you dereference current and try to malloc

puts("while");
while (current!=NULL)
{
    current = current->next;
}
puts("end while");


current->next=malloc(sizeof(thing)); //current is NULL here NULL->next is invalid
puts("current->next=malloc(sizeof(thing));");