pointer malloc fail

Question

I tried to define a node struct which includes a node* next. I write an append(node* n) function to add a new node next to the previous one, but every time I run the code, it gives me seg fault. My code is following:

#include<stdlib.h>
#include<stdio.h>
typedef struct _log_t {
 struct _log_t* next;
}log_t;
void initi(log_t* l) {
 l = (log_t*)malloc(sizeof(log_t));
 l -> next = NULL;
}
void append(log_t* l){
 l->next = (log_t*)malloc(sizeof(log_t)); 
 l->next->next = NULL;
 l = l->next;
}

Thanks in advance for any help!

Answer 1

 l = l->next;

That line is not doing what you think it is - in fact, its doing nothing.

Perhaps you want to pass log_t* as a log_t**, or return the new log_t*.

Answer 2

You're dereference a pointer you never initialized. That's why it's crashing :)

// OK (but you should check for malloc() failing, too!)
void initi(log_t* l) {
 l = (log_t*)malloc(sizeof(log_t));
 l -> next = NULL;
}
void append(log_t* l){
 // OK...
 l->next = (log_t*)malloc(sizeof(log_t)); 
 // BAD!!!! l->next: allocated. l->next->next: *NOT* initialized!
 l->next->next = NULL;
 l = l->next;
}

Here's what I think you possibly mean:

log_t * append (log_t* l) {
 // Initialize the head
 if (l == NULL) {
 l = (log_t *)malloc(sizeof (log_t));
 l->next = NULL;
 }
 // Initialize a sub-node
 else {
 l->next = (log_t *)malloc(sizeof (log_t));
 l->next->next = NULL;
 }
 // Always return the head
 return l;
}

• Karoly Horvath: hi is just doing the initalization... perfectly fine. there is no dereferencing.
• Karoly Horvath: that update is even worse.. append(NULL).. that's just doesn't make any sense.
• paulsm4: @yi_H: the point is you can't dereference "l->next->next" until you've both 1) allocated "l", and 2) initialized "l->next". It's perfectly appropriate to initialize l->next either to NULL, or to a newly allocated value. It's NOT OK to use it BEFORE you've initialized it.
• Karoly Horvath: you should take a rest, then re-read this thing, because you're talking nonsense. he allocated the memory in the previous line.