how to make structs point to different strings?

Question

I have a struct and in that a struct i have a character pointer but and i am creating different instances of this struct but when i am changing the pointer in one struct the other is also changing.

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

typedef struct human{
    int age;
    char name[100];
} Human;

int main(){
    FILE *s = fopen("h.txt","r");
    if(s==NULL){
        printf("file not available");
    }

    for(int i=0 ;i<5;i++){
        Human h;

        fscanf(s,"%d",&h.age);
        fscanf(s,"%s",h.name);

        insertintolinkedlist(h);
        // this method is going to insert the human into the linked list
    }

    return 0; 
}

what is happening that all humans in the linked list have different ages but same name!

h.name doesn't have any memory allocated to it. Based on the sample code provided, I'd expect the second fscanf to crash. — , Mar 05 '16 at 07:02

Support Ukraine · Accepted Answer · 2016-03-05T08:21:11.353

1

You need to allocate memory to hold the name.

char* name is just a pointer - it has no memory for saving the name.

You change it to

char name[100];

Remember to check that the names you put into Human.name isn't longer than 100 characters.

To use a linked list you can do something like:

typedef struct human{
    int age;
    char name[100];
    struct human* next;
} Human;

int main()
{
    Human* head = NULL;
    Human* tail = NULL;

    for(.....)
    {
        Human* h = malloc(sizeof(Human));
        if (head == NULL) head = h;
        if (tail != NULL)
        {
            tail->next = h;
        }
        tail = h;
        h->next = NULL;
        h->age = ....;
        strncpy(h->age, "..name..", 100);
    }

    // ..... other code

    // Remember to free all allocated memory
}

edited Mar 05 '16 at 08:21

answered Mar 05 '16 at 07:44

Support Ukraine

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thank you you are right but even if i do this the pointer still point to the same memory location – Sara Hamad Mar 05 '16 at 07:52
i am always doing a human and then adding it to the linked list but this human is going to have diiffernernt age but same name – Sara Hamad Mar 05 '16 at 07:53
@SaraHamad - you don't have any linked list! – Support Ukraine Mar 05 '16 at 07:54
imagine this code is in a loop and i am always making humans and adding to linked list these humans have different ages but when i change the name it change for all humans thanks a lot – Sara Hamad Mar 05 '16 at 07:54
here i only showed a part of my code for simplicity but what i really am doing is making the struct and then calling a function that is adding it to the linked list – Sara Hamad Mar 05 '16 at 07:55
How do you make new humans? `malloc` ? You should probably show that code as well. – Support Ukraine Mar 05 '16 at 07:58
i edited the code , please note that i am new in c and thank you for your hellp much appreciated – Sara Hamad Mar 05 '16 at 08:06
You don't get a new variable in each loop using `Human h;` inside the loop. You must allocate it dynamic using malloc. – Support Ukraine Mar 05 '16 at 08:11
the name is read from i file how can i use strcpy in this case? – Sara Hamad Mar 05 '16 at 08:13
I'm not using `fscanf` much but I think you can do `fscanf(s, "%s", h->name);` instead of a strcpy. However, it is not good because you can't make sure the name is shorter than 100 characters. However, take a look at this: http://stackoverflow.com/questions/12306591/ensure-scanf-only-reads-so-many-characters-in-a-string Maybe you can do `fscanf(s, "%100s", h->name);` – Support Ukraine Mar 05 '16 at 08:20

how to make structs point to different strings?

1 Answers1