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!

Yann Ramin · Accepted Answer · 2012-02-21 23:48:47Z

6

 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*.

answered Feb 21, 2012 at 23:48

Yann Ramin

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Comments

paulsm4 · Accepted Answer · 2012-02-21 23:55:11Z

-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;
}

edited Feb 21, 2012 at 23:55

answered Feb 21, 2012 at 23:50

paulsm4

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4 Comments

Karoly Horvath Over a year ago

hi is just doing the initalization... perfectly fine. there is no dereferencing.

Karoly Horvath Over a year ago

that update is even worse.. append(NULL).. that's just doesn't make any sense.

paulsm4 Over a year ago

@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 Over a year ago

you should take a rest, then re-read this thing, because you're talking nonsense. he allocated the memory in the previous line.

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