Implementing a Linked List C

Question

I am doing an assignment, and I thought i should use a linked list to store some data. The problem is that the list doesn't retain all nodes.

When adding finishes and I try to see the nodes, it only shows the last node that was added to list.

I will write the relevant part below, I hope someone could point out what the problem is.( I suspect that it must be something related to malloc. the address gets destroyed after the functions finishes its work, but am not sure.

Also I should point out, that I tested, printing data while they were being added, and it did show that they were being added properly to the list).

/**
 * Adds command name and it's hash onto the linked list
 * returns 1, if successful
 * returns 0, if failed
 */

int addToList(struct CMDList *head, char *pathCommand[], char *hash){

int result = 0;

/** If head was pointing to NULL, list empty, add at the beginning */
if(head->path == NULL){

    head->path = pathCommand[0];
    head->command = pathCommand[1];
    head->hash = hash;
    head->next = NULL;
    result = 1;

}else{

    struct CMDList *current = head;

    /** Find tail of the list */
    while(current->next != NULL){

        current = current->next;
    }
    current->next = (struct CMDList *)malloc(sizeof(struct CMDList));
    if(current->next != NULL){

        current->path = pathCommand[0];
        current->command = pathCommand[1];
        current->hash = hash;
        current->next = NULL;
        result = 1;

    }

}

return result;

}

MAIN Program:

int main(int argc, char *argv[]){

/** CODE DELETED */

/** initialize list for storing cmds from config file */
/** cmdList is the head node that i use to traverse the list */
cmdList = (struct CMDList *)malloc(sizeof(struct CMDList));
if(cmdList != NULL){

    cmdList->path = NULL;
    cmdList->command = NULL;
    cmdList->hash = NULL;
    cmdList->next = NULL;
}else{

    printError("Silent Exit: couldn't initialize list to store commands of config file");
    exit(1);
}


/** CODE DELETED **/

/** add new data to the list */
if(!addToList(cmdList,arrayCommand,sha)){

        printError("Silent Exit: couldn't add to list");
        exit(1);
}

}

Answer 1

In this part of your code:

if(current->next != NULL){

    current->path = pathCommand[0];
    current->command = pathCommand[1];
    current->hash = hash;
    current->next = NULL;
    result = 1;

}

You have to use current->next->... instead of current->..., because your new element is in current->next, not in current (in fact, you checked that current->next != NULL).

Because of this error, the first element added is fine, but when you try to add the second, you just allocate its space but then you overwrite the first.

Answer 2

In this part of the code you want to set the variables for the current->next

  if(current->next != NULL){

    current->path = pathCommand[0];
    current->command = pathCommand[1];
    current->hash = hash;
    current->next = NULL;
    result = 1;

}

Another option is, when doing the malloc you set a temp pointer to your new struct, give the values you need and then set current->next = temp;

Also you can skip the while step. You can have a pointer named tail to point at the end of your list.

And do something like

temp=malloc(...)
temp->path = ...
...
tail->next = temp;
tail = temp;