Segmentation Fault occurring when accessing the same memory in one function instead of another

Question

I am implementing a LinkedList with a bunch of ListNodes that are structs which contain some basic information about people. I have two files, one that is main.c, and another that is main.h. Whenever I attempt to print the list by passing the root to a function, I get an error.

Here is main.c:

#include "main.h"

/* Edward Nusinovich

    This C file is going to have a LinkedList containing information about people.
    The user can interact with it and manipulate the list.

*/

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

    if(!checkIfValidArguments(argc).value){return -1;}

    ListNode *root = askForDetails(1,argv);
    ListNode *current = root;
    current->next = NULL;

    ListNode *temp;

    int index = 2;

    while(index<argc){
        temp = askForDetails(index,argv);
        current->next = temp;
        current = current->next;
        index++;
    }

    userLoop(root);

    return 0;
}

In my main.h file, I have no problem printing attributes of the root parameter in the function "userLoop", but as soon as I pass ListNode *root to "print" or "stuff" I get a seg fault when trying to print its values.

Here is main.h:

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

#define true 1
#define false 0

typedef struct bool{
    int value:1;
} boolean;

//this is going to store info about a person
typedef struct people{

    char *name;
    char *hairColor;
    char *eyeColor;
    char *age;
    struct people *next;    

} ListNode;

//using a 1 bit bitfield we have constructed, stores whether or not the user has entered a proper number of inputs
boolean checkIfValidArguments(int numArgs){

    boolean validArguments;

    if(numArgs>1){validArguments.value=true;}
    else{validArguments.value=false; printf("We need some names of people.\n");}

    return validArguments;
}

//this will construct a new Person and return him to 
ListNode *askForDetails(int personIndex, char **argv){

    ListNode toReturn;

    char *name = argv[personIndex];
    printf("Please enter the hair color, eye color, and age of %s.\n",name);

    char *inBuf=malloc(100);
    char nextchar=getchar();
    int index = 0;

    if(nextchar!='\n'){inBuf[index]=nextchar; index++;}

    while((nextchar=getchar())!='\n'){
        inBuf[index] = nextchar;
        index ++;
    }

    toReturn.name = name;
    toReturn.hairColor = strtok(inBuf," ");
    toReturn.eyeColor = strtok(NULL," ");
    toReturn.age = strtok(NULL,"\n");

    ListNode *newNode = malloc(sizeof(ListNode)); 
    newNode = &toReturn;    
    return newNode;
}

char *getInput(char *message){

    printf("%s",message);
    char *inBuf = malloc(40);
    char nextchar=getchar();
    int index = 0;
    if(nextchar!='\n'){inBuf[index]=nextchar; index++;}

    while((nextchar=getchar())!='\n'){
        inBuf[index] = nextchar;
        index ++;
    }

    return inBuf;

}

void addToEnd(ListNode *root){

    ListNode *current = root;

    char *inBuf = getInput("\nEnter the name of a person who you want to add: \n"); 

    ListNode *toAdd = malloc(sizeof(ListNode));
    toAdd = askForDetails(0,&inBuf);
    toAdd->name = inBuf;
    toAdd->next = NULL;

    while((current->next) != NULL){
        current = current->next;
    }

    current->next = toAdd;
}


void print(ListNode *root){

    printf("The name is %s.\n",root->name);
/*
    ListNode *current = root;

    do{
        printf("\n%s's hair is %s, their eyes are %s and they are %s years old.\n",current->name,current->hairColor,current->eyeColor,current->age);
        current = current->next;
    }while(current!=NULL);
*/  

}

void remEnd(ListNode *root){

    ListNode *current = root;

    if(current == NULL){ return; }





}

void addAfter(ListNode *root){

    ListNode *current = root;

    char *name = getInput("\nWho do you want to add after?\n");
    int comparison = 0; 

    while(current!=NULL&&(comparison = strcmp(name,current->name))!=0){
        current = current -> next;
    }

    if(current==NULL){printf("\nIndividual not found.\n"); return;}

    else{
        char *newPerson = getInput("What's the name of the person you wish to add? ");

        ListNode *toAdd = askForDetails(0,&newPerson);
        ListNode *next = current->next;
        current -> next = toAdd;
        toAdd->next = next;
        return;
    }

}

void stuff(ListNode *root){
    printf("name is %s.\n",root->name);
    printf("Root lives at %u.\n",root);
}

void userLoop(ListNode *root){

    char input = ' ';

    printf("Root lives at %u.\n",root);

    while(true){
        printf("name is %s.\n",root->name);     

        if(input!='\n'){
            printf("\nWhat would you like to do with your list:\n");
            printf("A) Add an element at the end of the list\n");
            printf("B) Remove an element from the end of the list\n");
            printf("C) Add an element after an element on your list\n");
            printf("D) Print your list\n");
            printf("E) Quit this program\n\n");
        }

        input = getchar();

        switch(input){
            case 'A': addToEnd(root); break;            
            case 'B': remEnd(root); break;
            case 'C': addAfter(root); break;
            case 'D': stuff(root); break;
            case 'E': return;
        }
    }

}

When printing the address in memory of root in both functions, I get the same value, so I'm not sure why I am unable to access the values in root in one function instead of the other.

Thank you so much for any help.

Answer 1

You don't seem to quite grasp how the memory model works in C. Your offending piece of code is probably:

ListNode *newNode = malloc(sizeof(ListNode)); 
newNode = &toReturn;    
return newNode;

This returns a pointer to the local variable toReturn, not the malloc'd memory address. You need to copy the data from toReturn into your malloc'd memory space. However, even worse you don't malloc space for each of your strings, so each of your nodes point to the same input buffer. This should work, since you malloc with for every node, however if you are going to start deleting nodes managing your memory is going to be a little awkward. You also don't make sure there is enough room in your buffer.

I suggest looking at some online resources (or ideally a book) to refresh on how C memory works.

Edit: I did not learn C online, but a quick google search turned up this, which at a quick glance seems to be a decent resource.

I would recommend looking into book list. I used C Programming A Modern Approach.

Answer 2

I'm going to assume you're on a 64 bit system so that pointers are 8 bytes & integers are 4 bytes. Use %p for pointers, not %u