seg fault for linked list in another linked list--help!!

This is a discussion on seg fault for linked list in another linked list--help!! within the C Programming forums, part of the General Programming Boards category; hi i have a linked list in another linked list Code: /* the branch linked list */ struct nodebranch{ unsigned ...

  1. #1
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    Unhappy seg fault for linked list in another linked list--help!!

    hi
    i have a linked list in another linked list
    Code:
    /* the branch linked list */
    struct nodebranch{
    	unsigned int offset;
    	unsigned char data[130];
    	struct nodebranch *next;
    };
    
    struct node{
    	unsigned int id;
    	struct nodebranch *branch;
    	struct node *next;
    };
    I have a function insertToList(), in the function i have
    struct node *current;
    to point to the current node position of the list. in the function, i will call insertToBranch()

    static void insertToBranch(struct node **bufferNode, L2_IDU l2idu);

    to insert data into the branch linked list from the list node current. i call the function use:

    insertToBranch(&(current->branch), l2idu);

    current->branch is the node linked to branch(another linked list), the data will be inserted to that linked list.
    l2idu is a data structure and is not important for this question.

    in the insertToBranch() function, i have
    void insertToBranch(struct node **bufferNode, L2_IDU l2idu){

    struct nodebranch *currentbranch;
    ....
    which i use to point to the current position of the *bufferNode.
    but while i do that

    currentbranch = *bufferNode;

    i got a seg fault!??
    i know if i pass a linked list reference, thats the rite way to do, but not sure about pass a linked list which is in another linked list.
    anyone has any idea?
    ps. im quit sure the seg fault is caused by pointing currentbranch to *bufferNode.

    cheers
    dave

  2. #2
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    i think i should post my code to make it easier(or harder???) for u guys to understand
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    
    #include "bool.h"
    #include "layer2.h"
    #include "layer3.h"
    
    /* The Layer 3 PDU length is the MTU of Layer 3. */
    /* (even though we can't really put all that into one packet) */
    #define L3_PDU_MAXLENGTH	L3_MTU
    
    /* the fill in for other headers which need to be set to zero*/
    #define ZERO 0x00;
    
    /* a global varible for identification */
    static int identificator = 0;
    
    
    /* the branch linked list */
    struct nodebranch{
    	unsigned int offset;
    	unsigned char data[130];
    	struct nodebranch *next;
    };
    
    struct node{
    	unsigned int id;
    	struct nodebranch *branch;
    	struct node *next;
    };
    
    /* the linked list we use to store different msgs before send them*/
    static void insertToList(struct node **msgBuffer, L2_IDU l2idu);
    static void insertToBranch(struct nodebranch **bufferNode, L2_IDU l2idu);
    static int countLength(struct nodebranch **branch);
    
    /* the function to check if all the packets have been stored, if is
     * sort and send the msg and return a 1, else return 0
     */
    void StoreAndSendMessage(struct nodebranch * branch, int branchLength, int lastPacketDataLength);
    
    void checkAndSend(struct nodebranch **branch, L2_IDU l2idu); 
    
    //void insert( NodePtr *sPtr, char value);
    //void delete( NodePtr *sPtr, char value);
    
    /* the packet is a Layer 3 protocol data unit */
    typedef unsigned char L3_PDU[L3_PDU_MAXLENGTH];
    
    int L3_connected=FALSE;
    
    /* the function that Layer 2 calls */
    static void L3_receiveL2_IDU(L2_IDU l2idu);
    
    /* the pointer to a function, to call when giving a DU to Layer 4 */
    static void (* receiveL3_IDU)(L3_IDU l3idu);
    
    
    /**
     * Place your function header and description here.
     *  - describe how you implemented the reassembly
     *  - describe any data structures you used
     *  - etc.
     */
    static void L3_receiveL2_IDU(L2_IDU l2idu)
    {
    	/* the linked list of linked list which store all the packets
    	 * before sending them
    	 */
    	printf("show something before the seg fault!!\n");
    	struct node *msgBuffer = NULL;
    	
    	insertToList(&msgBuffer, l2idu);
    }
    
    void insertToList(struct node **Buffer, L2_IDU l2idu){
    
    	int id;							/* identification */		
    	/* check if the linked list already have node with the same
    	 * identificaton
    	 */
    	struct node *currentList;			/*currentListList pointer*/
    	struct node *new;				/*use to add new node*/
    	struct node *preList;				/*point to one before msgBuffer*/
    	
    	/*allocate memory to new*/
    	new = malloc(sizeof(struct node));
    	
    	/* get id */
    	id = ((l2idu.data[4]<<8) | l2idu.data[5]);
    	
    	/* if there s space, insert packet*/
    	if(new != NULL){
    		
    		preList = NULL;
    		currentList = *Buffer;
    		
    		/* loop to the last node in the list or the node which got
    		 * the same id no. with the packet*/
    		while(currentList != NULL && currentList->id != id){
    			preList = currentList;
    			currentList = currentList->next;
    		}
    		/*case 1, there s a node in the list have same id */
    		if(currentList != NULL && preList!=NULL){
    			
    			/* insert the packet into the node */
    			insertToBranch(&(currentList->branch), l2idu);
    			printf("out of insertToBranch function01");
    
    		}
    		/* otherwise, the list comes to the end, and we ll add the packet
    		 * on the end of the list*/
    		else{	
    			new->id = id;
    			insertToBranch(&(currentList->branch), l2idu);
    			printf("out of insertToBranch function02");
    			new->next = NULL; /*node doesnt link to other node*/
    			
    			/* case 3, empty list */
    			if (preList == NULL){
    				new->next = *Buffer;
    				*Buffer = new;
    				/* point currentList to new */
    				currentList = new;
    			}
    			/* case 2, no id in the list, add to the end of list */
    			else {
    				preList->next = new;
    				new->next = currentList;
    				/* point currentList to new */
    				currentList = new;
    			}
    		}
    	} 
    	else {
    		printf("%c not inserted. No memory available.\n", id );
    	}
    	
    	/* after insert the packet, we need to check if the branch which 
    	 * just insert the packet has a MF set to true. if its true, means
    	 * there s chance the whole msg has been recieved. if its so, store 
    	 * and send the msg, and clear the branch free the memory. 
    	 */
    	 checkAndSend(&currentList->branch, l2idu);
    	 	
    	 if(currentList->branch == NULL){
    	 	preList->next = currentList->next;
    	 	free(currentList);
    	 }	
    }
    
    		
    
    /*insert the packet into the node with same id, along
     * with data, a offset and a counter will be saved as well
     * for later use. after the packet has been saved onto branch,
     * check if all the packets has been recieved, by check if 
     * the count equl to the offset/16. sort and send them and 
     * make branch to NULL and free the memory.
     * the packet with MF = 0 (the last packet) will be inserted
     * on the end of the list while others will be inserted in the
     * front. 
     */
    void insertToBranch(struct nodebranch **bufferNode, L2_IDU l2idu){
    	
    	printf("in the insertToBranch function\n");
    	int offset;    					 /* the offset of incoming packet l2idu*/
    	int i;							 /* counter i */
    	int length;						 /* totle length of the packet */
    	int lastPacketDataLength = 0;		 /* the data length of the last packet(where MP=0)*/
    	struct nodebranch * newNode; 	 /* newNode to add into branch */
    	struct nodebranch * pre;    	 /* a linked list point one node before current*/
    	struct nodebranch * current;	  
    	
    	/* current point to bufferNode */
    	current = *bufferNode;
    		
    	/* get the offset, while the MF is 1, need to change that bit 
    	 * into 0 1st. 
    	 */
    	 
    	printf("the current is pointed to *bufferNode\n");
    	if( (l2idu.data[6] ^ (1<<5)) > l2idu.data[6]){
    		offset = ((l2idu.data[6]<<8) | l2idu.data[7]);
    		printf("the offset is : %d \n", offset);
    	}
    	else {
    		offset = ( ( (l2idu.data[6] ^ (1<<5)) <<8) | l2idu.data[7]);
    		printf("the offset is : %d \n", offset);
    	}
    	
    	/* get totle length of the packet */
    	length = ((l2idu.data[2]<<8) | l2idu.data[3]);
    	
    	/* asign memory to the new node */
    	newNode = malloc(sizeof(struct nodebranch));
    	
    	/*if the MF = 0, the bufferNode->MF will be set to TRUE 
    	 */
    	if((l2idu.data[6] ^ (1<<5)) > l2idu.data[6]) {
    		
    		/* the data length of packet which have MF=0 */
    		lastPacketDataLength = length - 20;
    	}
    	/* for all the packets, will be inserted into a sorted list */
    	/* loop to get the sorted position of the packet */
    	while(current != NULL && offset > current->offset){
    		pre = current;
    		current = current->next;
    	}
    		
    	/* put packet into a new node */	
    	newNode->offset = offset;
    	for(i=0; i<(length-20); i++){
    		newNode->data[i] = l2idu.data[20+i];
    	}
    		
    	/* for empty list */
    	if(pre == NULL){
    		newNode->next = *bufferNode;
    		*bufferNode = newNode;
    	}
    	/* if list s not empty, insert new node */
    	else{
    		pre->next = newNode;
    		newNode->next = current;
    	}
    }

  3. #3
    CSharpener vart's Avatar
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    Code:
    if(currentList != NULL && preList!=NULL)
    {
    }
    else
    {	
       new->id = id;
       insertToBranch(&(currentList->branch), ...
    When currentList is NULL (true in your case) you're still trying to access its members - so exception occures
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  4. #4
    Registered User ssharish2005's Avatar
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    this is the part of the code which might give u a problem.

    Code:
    new->id = id;
    			insertToBranch(&(currentList->branch), l2idu);
    			printf("out of insertToBranch function02");
    			new->next = NULL; /*node doesnt link to other node*/
    			
    			/* case 3, empty list */
    			if (preList == NULL){
    				new->next = *Buffer;
    				*Buffer = new;
    				/* point currentList to new */
    				currentList = new;
    			}
    			/* case 2, no id in the list, add to the end of list */
    			else {
    				preList->next = new;
    				new->next = currentList;
    				/* point currentList to new */
    				currentList = new;
    u are trying to reference the branch when currentlist is NULL. There is no sign of currentList reference a node a the first run. Assign the new node to the cuurrentlist and then then try referencing the branch from the currentlist.

    ssharish2005

  5. #5
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    but my code is :
    Code:
    if(currnetList != NULL .....){
    
    }
    so when currentList is not NULL, do insert....... isn't it?
    i used '!='

  6. #6
    CSharpener vart's Avatar
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    You have this call also in the else statement I just skipped the if block of yours
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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