segmentation error when accessing a linked list or doing a malloc() on linux

Question

I'm having problems with a segmentation fault on a Linux system. I'm using code from Aho and Ullman's "Foundations of Computer Science" C edition. This is the code

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct que_element
{
    long mem_address;
    long mem_data;

    int msg;
} Qq;

typedef struct CELL *LIST;
struct CELL
{

    Qq el;

    int element;
    LIST next;
};


Qq l1element;

typedef struct
{
    LIST front;
    LIST rear;
} QUEUE;

main()
{
    QUEUE *l1c2l1d; /*L1 Controller to L1 Data */

    l1c2l1d->front = malloc(sizeof *l1c2l1d);
}

If you want to represent true/false, use typedef char BOOLEAN. ;) int usually has 4 bytes on 32-bit, while char has always 1 byte. — Gandaro
– Gandaro, Commented Feb 17, 2012 at 21:11
@Gandaro But an int is usually faster to access than a char. — cnicutar
– cnicutar, Commented Feb 17, 2012 at 22:33

cnicutar · Accepted Answer · 2012-02-19 07:06:27Z

2

Is there an initializing step I'm missing?

Yes, l1c2l1d is uninitialized in your code. Dereferencing it means dereferencing NULL (since l1c2l1d is global). Try this:

l1c2l1d = malloc(sizeof *l1c2l1d);
l1c2l1d->front...

EDIT In light of last edit

You have this:

main()
{
    QUEUE *l1c2l1d; /*L1 Controller to L1 Data */

    l1c2l1d->front = malloc(sizeof *l1c2l1d);
}

In this case l1c2l1d is uninitialized, it points to garbage. Try this (copy-paste it this time):

main()
{
    QUEUE *l1c2l1d; /*L1 Controller to L1 Data */

    l1c2l1d = malloc(sizeof *l1c2l1d);
    l1c2l1d->front = malloc(*l1c2l1d->front);
}

edited Feb 19, 2012 at 7:06

answered Feb 17, 2012 at 21:02

cnicutar

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9 Comments

Mark Over a year ago

I just tried l1c2l1d = malloc(sizeof *l1c2l1d) , but it also causes the segmentation error.

Mark Over a year ago

I just tried making defining l1c2l1d in main(), still segmentation error.

cnicutar Over a year ago

@Mark Something else must be causing it (like your enqueue function).

Mark Over a year ago

I removed all of the function definitions (enque, deque, etc.). I just have main() and the QUEUE and link list definitions. It still dies on that first line (the malloc).

Mark Over a year ago

I think it's time to punt. I'll just use an array of structures as my linked list. Not as fancy but I won't have to worry about malloc(). Thank you for your help.

|

Heath Hunnicutt · Accepted Answer · 2012-02-19 07:39:55Z

0

You seem to be going for something along the lines of:

main()
{
    QUEUE l1c2l1d = { 0, 0 }; /*L1 Controller to L1 Data */

    l1c2l1d->front = l1c2l1d->rear = malloc(sizeof CELL);
}

answered Feb 19, 2012 at 7:39

Heath Hunnicutt

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