Struct with pointer in other struct (segmentation fault)

Question

So, I have this 2 structs and the one global variable

struct Tasks{

    int tid;                      
    int difficulty;               
    struct Tasks *next;           
};

struct Head_GL{

    int tasks_count[3];           
    struct Tasks *head;           
};

struct Head_GL *tasks_head;

and I have to create a linked list with acceding order by difficulty. How is it possible to make comparision and read the difficulty. I did this tasks_head->head->difficulty and gives me segmentation fault

tasks_head->head=(struct Tasks*)malloc(sizeof(struct Tasks)); like this? — Ηροδοτος Αποστολου
– Ηροδοτος Αποστολου, Commented Nov 9, 2020 at 14:17
so before i allocate tasks_head->head i have to allocate tasks_head tasks_head=(struct Head_GL*)malloc(sizeof(struct Head_GL)); — Ηροδοτος Αποστολου
– Ηροδοτος Αποστολου, Commented Nov 9, 2020 at 14:21
You can drop the casts, they are useless: (struct Tasks*)malloc(sizeof(struct Tasks)) -> malloc(sizeof(struct Tasks)) — Jabberwocky
– Jabberwocky, Commented Nov 9, 2020 at 14:25

kiran Biradar · Accepted Answer · 2020-11-09 14:21:40Z

1

You need to create each item:

tasks_head = calloc(1, sizeof(struct Head_GL));
tasks_head->head = calloc(1, sizeof(struct Tasks));

You can then populate and use them. You also need to remember to free these later.

edited Nov 9, 2020 at 14:21

kiran Biradar

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answered Nov 9, 2020 at 14:18

Danny Staple

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2 Comments

Danny Staple Over a year ago

The structs (does he need typedefs around that too...)

MikeCAT Over a year ago

They should be struct Head_GL and struct Tasks.

Ηροδοτος Αποστολου · Accepted Answer · 2020-11-09 20:13:30Z

I allocate the memory but i have a problem to printf the results. Segmantation fault in printf in main (tasks_head->head->tid). Any help?

struct Tasks *new=(struct Tasks*)malloc(sizeof(struct Tasks));


tasks_head=(struct Head_GL*)malloc(sizeof(struct Head_GL));
tasks_head->head=(struct Tasks*)malloc(sizeof(struct Tasks));

tasks_head->tasks_count[0]=0;
tasks_head->tasks_count[1]=0;
tasks_head->tasks_count[2]=0;
tasks_head->head->difficulty=0;
tasks_head->head->tid=0;
tasks_head->head->next=NULL;

if(new==NULL)
    return 0;
new->tid = tid;
new->difficulty = difficulty;
new->next = NULL;

if(difficulty==1)
    tasks_head->tasks_count[0]++;
else if(difficulty==2)
    tasks_head->tasks_count[1]++;
else
    tasks_head->tasks_count[2]++;


if(tasks_head==NULL){
    tasks_head->head = new;
    return 1;
}
if( tasks_head->head->difficulty > difficulty){
    new->next = tasks_head->head;
    tasks_head->head= new;
    return 1;
}
else{
    prev = tasks_head->head;
    temp = tasks_head->head->next;
    while(temp != NULL && temp->difficulty < difficulty){
        prev = temp;
        temp = temp->next;
    }
    if(temp==NULL){
        prev->next = new;
        return 1;
    }
    else{
        new->next = temp;
        prev->next = new;
        return 1;
    }
}

}

int main(){

printf("hello1\n");

if(1==insert_task(1,1))
    printf("alo");

if(1==insert_task(4,1))
    printf("alo");

if(1==insert_task(3,2))
    printf("alo\n");


printf("%d\n",num);

for(int i=0; i<num; i++){
    printf("%d\n",tasks_head->head->tid);
    tasks_head->head=tasks_head->head->next;
}

return 0;

}

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