Question

I was going to write a simple SDL app so I thought I should implement a simple linked list that I can extend later. Now after some struggling with it I decided to ask for help here.

I know the linked list, I read about it. I saw couple implementations, not so hard, easy to grasp. So I thought I should be able to implement one myself. I am guessing that I am severely overlooking something here. 🙁

First this code does not compile well, segmentation fault. Second I do not think that it is going to produce what I want if I can overcome the basic errors I am doing with the pointers unfortunately.

Can you please show me my mistakes in my code? Where am I doing wrong?

Here is a liveworkspace link for editing
http://liveworkspace.org/code/1qu1RN$1

Here is the error I get on Cygwin gcc

temp [v 0] -> 0.460241 0.000000 0.050356
temp [v 1] -> 0.072079 0.153807 0.864573
temp [v 2] -> 0.683917 0.709301 0.080479
temp [v 3] -> 0.295755 0.264795 0.894696
address of ptemp @ 1629101750

[v 0] -> linked vertex is  7731159828445146316135105405324534341657178000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000 578395405356271.125000 1310047834864691498231654014730274851713450000.000000
Segmentation fault (core dumped)

And here is the code

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

#define NEWline printf("\n")

typedef struct position {
    double x;
    double y;
    double z;
} position;

struct datalist {
    position v;
    struct datalist  *pnext;

};
typedef struct datalist datalist;

void initializeList (datalist *head);
void addList_II (datalist *head , double data[3] );

void initializeList (datalist *head){
    position pv= {0,0,0};
    head = (datalist *) malloc ( sizeof (head) );
        if (head == NULL){ printf("cannot allocate memory");  exit(1); }
    head->v=pv;
    head->pnext=NULL;
}

void addList_II ( datalist *head , double data[3] ) {

    datalist *adddata;
    adddata = (datalist *)malloc ( sizeof (datalist) );
        if (adddata == NULL) { printf("cannot allocate memory for insertion data"); exit(1); }
    adddata->v.x = data[0];
    adddata->v.y = data[1];
    adddata->v.z = data[2];

    adddata->pnext = head->pnext;
    head=adddata;
}

int main() {

    datalist *ptemp;
    datalist temp;
    int i,c; //counters
    datalist *head;

    initializeList(head);

    for ( i = 0; i < 4; i++ ) {
        double t[3];
        srand ( time ( NULL )*i+234 );
            temp.v.x = ((double ) rand()  )/ RAND_MAX;
        srand ( time ( NULL )*i*546 );
            temp.v.y = ((double ) rand()  )/ RAND_MAX;
        srand ( time ( NULL )*i*567+345 );
            temp.v.z = ((double ) rand()  )/ RAND_MAX;
        t[0] = temp.v.x; t[1] = temp.v.y; t[2] = temp.v.z;

        printf ( "temp [v %d] -> %F %F %F \n",i, temp.v.x, temp.v.y, temp.v.z );
        addList_II (head, t );
    }

    c=0;
    ptemp=head;
    while (ptemp!=NULL) {
        printf("[v %d] -> linked vertex is  %F %F %F \n", c, ptemp->v.x,ptemp->v.y, ptemp->v.z);
        ptemp=ptemp->pnext;

    }
}

Answer 1

In your addList_II you are setting the wrong pointer. You are taking your new node and pointing its next pointer to the head’s next pointer, which is null.

   void addList_II ( datalist *head , double data[3] ) {
        datalist *adddata;
        adddata = (datalist *)malloc ( sizeof (datalist) );
            if (adddata == NULL) { printf("cannot allocate memory for insertion data"); exit(1); }
        adddata->v.x = data[0];
        adddata->v.y = data[1];
        adddata->v.z = data[2];

        //    adddata->pnext = head->pnext; //not THIS
        adddata->pnext = head;        //DO THIS
        head=adddata;
    }

Additionally your pointers were going out of scope in your function calls. what you need to do is pass in ** variables. This means your passing in a pointer to a pointer, which points to your block of memory. You modify the original pointer by derefrencing your local *, and upon return you are good.

If you need more explanation, just ask, but heres the solution

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

#define NEWline printf("\n")

typedef struct position {
    double x;
    double y;
    double z;
} position;

struct datalist {
    position v;
    struct datalist  *pnext;

};
typedef struct datalist datalist;

void initializeList (datalist **head);
void addList_II (datalist **head , double data[3] );

void initializeList (datalist **head){
    position pv= {0,0,0};
    *head = (datalist *) malloc ( sizeof (datalist) );
        if (head == NULL){ printf("cannot allocate memory");  exit(1); }
    (*head)->v=pv;
    (*head)->pnext=NULL;
}

void addList_II ( datalist **head , double data[3] ) {
    datalist *adddata;
    adddata = (datalist *)malloc ( sizeof (datalist) );
        if (adddata == NULL) { printf("cannot allocate memory for insertion data"); exit(1); }
    adddata->v.x = data[0];
    adddata->v.y = data[1];
    adddata->v.z = data[2];

    adddata->pnext = *head;        //DO THIS
    *head=adddata;
}

int main() {

    datalist *ptemp;
    datalist temp;
    int i,c; //counters
    datalist *head;

    initializeList(&head);


    for ( i = 0; i < 3; i++ ) {
        double t[3];
        srand ( time ( NULL )*i+234 );
            temp.v.x = ((double ) rand()  )/ RAND_MAX;
        srand ( time ( NULL )*i*546 );
            temp.v.y = ((double ) rand()  )/ RAND_MAX;
        srand ( time ( NULL )*i*567+345 );
            temp.v.z = ((double ) rand()  )/ RAND_MAX;
        t[0] = temp.v.x; t[1] = temp.v.y; t[2] = temp.v.z;

        printf ( "temp [v %d] -> %F %F %F \n",i, temp.v.x, temp.v.y, temp.v.z );
        addList_II (&head, t );
    }

    ptemp=head;

    while (ptemp!=NULL) {
       NEWline; 
        printf("[v ] -> linked vertex is  %F %F %F \n", ptemp->v.x,ptemp->v.y, ptemp->v.z);
        ptemp=ptemp->pnext;

    }
}