C - segmentation fault when loop condition is ptr != NULL

Question

I am getting segmentation fault when using ptr != NULL in shareRecord while loop but it seems to work when I use ptr->next != NULL.

So when I use ptr->next != NULL and uncomment the lines in shareRecord, the code works. But why doesn't it work using ptr != NULL only.

Kindly let me know what the issue is.

struct data *shareRecord(struct data *head, struct data *fileline){
    struct data *ptr;
    ptr = head;
    int flag = 0;
    while(ptr != NULL){
        if((strcmp(ptr->symbol, fileline->symbol) == 0) && (strcmp(ptr->type, fileline->type) == 0 )&& (ptr->time == fileline->time)){
            flag = 1;
            break;
        }
        ptr = ptr->next;
    }
    //if((strcmp(ptr->symbol, fileline->symbol) == 0) && (strcmp(ptr->type, fileline->type) == 0 )&& (ptr->time == fileline->time)){
            //flag = 1;
    //}
    if(ptr->next == NULL && flag == 0){
        return NULL;
    }   
    else
        return ptr;     
}
struct data *create_data(struct data *head, struct data *fileline){
    struct data *newdata, *ptr1, *ptr2;
    struct price *newprice, *priceptr1;
    newprice = (struct price *)malloc(sizeof(struct price *));
    newprice->amt = fileline->price->amt;
    newprice->next = NULL;
    if(head == NULL){

        newdata = (struct data *)malloc(sizeof(struct data));
        newdata->time = fileline->time;
        strcpy(newdata->symbol, fileline->symbol);
        strcpy(newdata->type, fileline->type);
        newdata->price = newprice;
        newdata->next = NULL;
        head = newdata;
    }
    else{
        ptr1 = head;
        ptr2 = head;
        if((ptr1 = shareRecord(head, fileline)) != NULL){
            priceptr1 = ptr1->price;
            while(priceptr1->next != NULL){
                priceptr1 = priceptr1->next;
            }
            priceptr1->next = newprice;
        }
        else{       
            while(ptr2->next != NULL){
                ptr2 = ptr2->next;
            }
            newdata = (struct data *)malloc(sizeof(struct data));
            newdata->time = fileline->time;
            strcpy(newdata->symbol, fileline->symbol);
            strcpy(newdata->type, fileline->type);
            newdata->price = newprice;
            newdata->next = NULL;
            ptr2->next = newdata;
        }
    }
    return head;
}

use a debugger. step through, check the values, you'll have your answer. — Sourav Ghosh, Sep 02 '18 at 14:06
Possible duplicate of [What is a debugger and how can it help me diagnose problems?](https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) — Max Vollmer, Sep 02 '18 at 14:06
Read [How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/) — Basile Starynkevitch, Sep 02 '18 at 14:58

score 1 · Answer 1 · answered Sep 02 '18 at 14:07

1

The loop will break when ptr != NULL will result in false. Which means when it breaks , ptr IS NULL . So the next line ,

if(ptr->next == NULL && flag == 0){

You're trying to use an attribute of NULL , which should result in an error .

answered Sep 02 '18 at 14:07

sagi

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C - segmentation fault when loop condition is ptr != NULL

1 Answers1