Using free() for char-pointer in struct

Question

typedef struct inventory 
{
   char *name; 
   int quantity; 
   double price; 
   struct inventory* next_inventory; 
   
} invent;

int main(void)
{
   invent *one=malloc(sizeof(invent)); 
   invent *two=malloc(sizeof(invent));
   invent *three=malloc(sizeof(invent));

   one->next_inventory=two;
   two->next_inventory=three;
   three->next_inventory=NULL;

   one->name=malloc(256);
   ...(repeat two->name, three->name)
   
   printf("name: ");
   scanf("%s", one->name);

   printf("qunatity: ");
   scanf("%d", &one->quantity);

   printf("price: ");
   scanf("%lf", &one->price);
   
   ...(repeat scanf() for two, three)

   while(one!=NULL)
   {
      printf("%s %d %.0f\n", one->name, one->quantity, one->price);
      printf("check\n");
      one=one->next_inventory;
   }

   free(one->name);
   free(two->name);
   free(three->name);
   free(one);
   free(two);
   free(three);

   return 0;

free(one->name); doesn't work. I checked another free(), using printf("check");, two->name, three->name, one ... its worked. Why only free(one->name) doesn't work? What can I do for solve this problem? Give me the advice.

free(one->name); doesn't work What do you mean by "doesn't work"? How exactly do you see that that statement "doesn't work"? — KamilCuk
– KamilCuk, Commented Jun 26, 2020 at 10:50
Well one is NULL at that point. I know it's NULL because if it wasn't NULL then the loop would've kept looping. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Jun 26, 2020 at 10:51
@user253751 Then what can I do for this problem? I should return the memory I used.(using malloc) — erehwyrevemai
– erehwyrevemai, Commented Jun 26, 2020 at 10:58
what if you have 100 elements in the inventory ? you go on doing one->next_inventory->next_inventory...100times...->name ? — farbiondriven
– farbiondriven, Commented Jun 26, 2020 at 11:52

farbiondriven · Accepted Answer · 2020-06-26 11:50:42Z

2

You are using your actual 'one' variable to do the iteration, in the end you are trying to free(one) where one points to NULL.

You should not lose track of the address of the memory you requested to allocate, to be able to release that memory later with free.

I would use a temporary pointer to iterate:

 invent* tmp = one;
 while(tmp!=NULL)
   {
      printf("%s %d %.0f\n", tmp->name, tmp->quantity, tmp->price);
      printf("check\n");
      tmp=tmp->next_inventory;
   }

// free all names and one, two, three.

In alternative:

you can define a function for printing:

void printAll(invent* one)
{
    while(one!=NULL)
   {
      printf("%s %d %.0f\n", one->name, one->quantity, one->price);
      one=one->next_inventory;
   }
}

then in the main, call

printAll(one)

This works as the pointer is passed by value in the function and you won't lose the original address.

edited Jun 26, 2020 at 11:50

answered Jun 26, 2020 at 10:54

farbiondriven

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3 Comments

erehwyrevemai Over a year ago

It's my homework, so it has conditions. Struct must have char *name, don't make struct without struct one, two, three, only using struct one for print. I appreciate your advice, But I can't using temporary pointer.

farbiondriven Over a year ago

you can't iterate using one, because how would you know after you are done with the printing where one was originally pointing to ?

erehwyrevemai Over a year ago

My professor said "There are only one, two, three pointer to point to struct. So I can't use temporary pointer. What should I do?

pifor · Accepted Answer · 2020-06-26 11:16:07Z

1

Here is a improved version of your program that includes also checking malloc return code:

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

typedef struct invent 
{
   char *name; 
   int quantity; 
   double price; 
   struct invent* next_inventory; 
   
} invent;

void enter(invent **s)
{
   printf("name: ");
   scanf("%s", (*s)->name);

   printf("quantity: ");
   scanf("%d", &(*s)->quantity);

   printf("price: ");
   scanf("%lf", &(*s)->price);
   
}

void *alloc(size_t size)
{
    void *p;
    
    p = malloc(size);
    if (p == NULL)
    {
        perror("malloc");
        exit(1);
    }
    return p;
}

int main(void)
{
   invent *start;

   invent *one;
   invent *two;
   invent  *three;

   one = alloc(sizeof(invent)); 
   two = alloc(sizeof(invent));
   three = alloc(sizeof(invent));

   start = one;

   one->next_inventory=two;
   two->next_inventory=three;
   three->next_inventory=NULL;

   one->name=alloc(256);
   two->name=alloc(256);
   three->name=alloc(256);


   enter(&one);
   enter(&two);
   enter(&three);

   while(one != NULL)
   {
      printf("%s %d %.0f\n", one->name, one->quantity, one->price);
      printf("check\n");
      one=one->next_inventory;
   }

   free(start->name);
   free(two->name);
   free(three->name);
   free(start);
   free(two);
   free(three);

   return 0;

}

answered Jun 26, 2020 at 11:16

pifor

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3 Comments

erehwyrevemai Over a year ago

I appreciate your advice, pifor. But my professor said "There are only one, two, three pointer to point to struct. What should I do without using start?

pifor Over a year ago

Please give all specifications that you have: do you really need to have a loop to print data ? Do you really need to free memory ? Is C structure mandatory ? Can we modify this structure ? etc. This homework looks a little bit too artificial ...

erehwyrevemai Over a year ago

I solved problem. Thank you pifor!!! When I think about it, I don't need to have a loop to print data.

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