Question

This is just a rough code,so there are no free(s) in there yet.
I am just trying to figure out where its messing up my linked list.

The purpose of the following function is to accept something like :

add 1 2

or

add 1 "some quote" maybe more stuff

and make a linked list with elements..
in first case:

[add]->[1]->[2]

in second case:

[add]->[1]->["some quote" maybe more stuff]

I know that it is actually doing the steps, because the ‘count/total’ is right in the output. However when I try to iterate through the linked list, it only prints the first element.

typedef struct command{
    char* args;
    struct command *next;
}command;
typedef struct commands_list{
    command *head;  /*Start of the queue*/
    int total;  /*Total commands passed*/
}commands_list;

commands_list* process_command(char *command){
    char curr_char;                 /*Keeps track of current character*/
    int start_pos;
    int i;
    int len;    

    /*Length of user input*/
    int quote=0;
    int empty =1;
    commands_list *commands;
    struct command *conductor;

    len = strlen(command);              /*Calculate length*/    
    /*Initialize the List*/
    commands=malloc(sizeof(commands_list));         /*Allocate memory for the linked list*/
    commands->head = malloc(sizeof(struct command));
    conductor = commands->head;

    for(i=0,start_pos=0;i<strlen(command);i++){
        curr_char = command[i];
        if (empty==0){
            conductor = malloc(sizeof(struct command));
        }
        if (curr_char == ' '){      /*If there was a space found copy the stuff before the space*/
            if ( i>0 && command[i-1]==' ') {
                start_pos++;
                continue;
            }
            conductor->args = malloc(i-start_pos+1*(sizeof(char))); /*Allocate memory for the word to be copied*/
            strncpy(conductor->args,command+start_pos,i-start_pos); /*Copy the word/command to the memory allocated*/
            conductor->args[i-start_pos+1]='\0';            /*Add null terminator at end*/
            commands->total++;              /*Increase total # of commands*/
            conductor=conductor->next;          /*Conductor points to the first element now*/
            start_pos =i+1;
            if (empty==1){
                empty=0;
            }
        }
        else if (curr_char == '\"'){        /*If a quote was found, copy the rest of the string and exit loop*/
            conductor->args = malloc(len-i+1*(sizeof(char)));
            strncpy(conductor->args,command+i,len-i);
            conductor->args[len-i+1]='\0';
            conductor->next=NULL;
            commands->total++;
            quote=1;
            //empty_queue = 0;
            conductor = conductor->next;
            if (empty==1){
                empty=0;
            }
            break;
        }
    }
    if (quote==0){                  /*If there was no quote in the string, get the last element*/
        if (empty==0){
            conductor = malloc(sizeof(struct command));
        }
        conductor->args = malloc(len-start_pos+1*(sizeof (char)));
        strncpy(conductor->args,command+start_pos,len-start_pos);
        conductor->args[len-start_pos+1]='\0';
        conductor->next=NULL;
        commands->total++;
    }   /*Finish find quote*/
    printf("%d commands found\n",commands->total);
    //free(conductor);
    return commands;    
}

And a temporary method that I am using to print the linked list:

int print_list(commands_list **headNode){
    commands_list *top = *headNode;
    struct command *temp = top->head;           /*Temporary variable for command*/
    while(temp!=NULL){
        printf("I was here to print: [%s]\n",temp->args);
        temp = temp->next;
    }
    printf("It was all null\n");

    free(temp);
}

Thanks

Answer 1

There are many problems in the code that make it hard to debug.

The main issue is how you add a struct command * to the end of the list. The line

        conductor=conductor->next;

will just assign conductor to NULL (or whatever malloc did). You never assign conductor->next to anything.

When you malloc a new struct command *, you need to update the old conductor->next to the newly allocated element. So instead of:

        conductor = malloc(sizeof(struct command));

You need something like:

        struct command *tmp = malloc(sizeof(struct command));
        conductor->next = tmp;
        conductor = tmp;

Also, it might help your debug if you added the line
conductor->args = “not allocated yet #1“;
after the last line above. This is gross, and should not appear in production code, but will help you debug your issues.