C, How to acess a structure from another structure?

Question

I am getting an error:

error: request for member ‘clsf_ptr’ in something not a structure or union

From this C Program:

#include<stdio.h>
#define M 3

struct class_stud
{
    char name[16];
    char subject[16];
    int roll_num;
}

struct node
{
    int n;
    struct branch *p[M];
    struct class_stud cls[M-1];
}*root=NULL;

main()
{
    int clsf_cnt;
    struct class_stud clsf, *clsf_ptr;
    clsf_ptr = &clsf;
    clsf_cnt = 0;
    root = malloc(sizeof (struct node));
    printf("enter source address \n");
    scanf("%s",&root.clsf_ptr[clsf_cnt]->name);
    printf("%s",root.clsf_ptr[clsf_cnt]->name);

    printf("enter key\n");
    scanf("%d",&root.clsf_ptr[clsf_cnt]->key);
    printf("%d",root.clsf_ptr[clsf_cnt] -> key);
    clsf_cnt++;
}

What does the error mean?

Answer 1

There are a number of problems in this code. Let's walk through it line by line:

#include<stdio.h>     
#define M 3      

struct class_stud     
{     
  char name[16];     
  char subject[16];     
  int roll_num;     
}      

First problem; you need a closing ; after the struct definition above. Also, it's interesting that you don't define a preprocessor macro for the lengths of the name and subject arrays, but that's a minor issue at this point.

struct node     
{     
  int n;     
  struct branch *p[M];     
  struct class_stud cls[M-1];     
}*root=NULL;        

Where is the definition for struct branch? What is p supposed to represent in the structure definition? Why do you have one fewer elements in cls than p?

main()

As of C99, you cannot get away with an implicit type for main; it must be explicitly typed to return int. And, since you're not messing with any command-line arguments, it's best to explicitly set the parameter list to void, like so:

int main(void)
{     
  int clsf_cnt;     
  struct class_stud clsf, *clsf_ptr;     
  clsf_ptr = &clsf;     

What purpose does the above operation serve?

  clsf_cnt = 0;     
  root = malloc(sizeof (struct node)); 

That's fine, but a preferred way of writing this would be

  root = malloc(sizeof *root);

That way you're sure to allocate the right amount of memory based on root's type.

  printf("enter source address \n");     
  scanf("%s",&root.clsf_ptr[clsf_cnt]->name);     

Ack. Multiple problems in this one line.

First of all, clsf_ptr is not a member of struct node. You likely meant to access cls. Secondly, since root is a pointer to struct, you need to use the -> operator to access its members (or explicitly dereference root and then use .). Third, cls[clsf_cnt] is not a pointer type, so you would use the . operator to access name. Finally, since name is an array type, it will implicitly be converted to a pointer type before being passed to scanf, so the & isn't necessary.

In order to avoid buffer overflow, you should either put a maximum field width in the %s conversion specifier, or use fgets() instead of scanf().

Sticking with scanf(), that line should be written as

  scanf("%15s", root->cls[clsf_cnt].name);

I'm using 15 instead of 16 in the conversion specifier so that we leave one character for the 0 terminator.

Using fgets(), that line would be written

  fgets(root->cls[clsf_cnt].name, sizeof root->cls[clsf_cnt].name, stdin);
  printf("%s",root.clsf_ptr[clsf_cnt]->name);     

Again, you're trying to access something that isn't a member of root, and you have your access operators backwards:

  printf ("%s", root->cls[clsf_cnt].name);

  printf("enter key\n");     
  scanf("%d",&root.clsf_ptr[clsf_cnt]->key);     
  printf("%d",root.clsf_ptr[clsf_cnt] -> key);

struct class_stud has no member named key; perhaps you meant roll_num?

  scanf("%d", &root->cls[clsf_cnt].roll_num);
  printf("%d", root->cls[clsf_cnt].roll_num);    
  clsf_cnt++;     
} 

It looks like you've posted something that isn't complete; make sure you're showing us the same code you're trying to build.

Answer 2

The following code should work for you. I'm assuming key should be a member in class_stud (and by your formatting options in scanf, a signed integer):

#include<stdio.h>
#define M 3

typedef struct
{
    char name[16];
    char subject[16];
    int roll_num;
    int key;
} class_stud;

typedef struct
{
    int n;
    struct branch *p[M];
    struct class_stud cls[M-1];
} node;

int main(void)
{
    int clsf_cnt = 0;
    node *root;

    root = malloc(sizeof(node));
    if (root == NULL) {
       /* handle error */
    }

    printf("enter source address \n");
    scanf("%s",&root->cls[clsf_cnt]->name);
    printf("%s",root->cls[clsf_cnt]->name);

    printf("enter key\n");
    scanf("%d",&root->cls[clsf_cnt]->key);
    printf("%d",root->cls[clsf_cnt]->key);
    clsf_cnt++;
}

You had the following problems:

• Incorrectly accessing class_stud. It's a part of root, so you want root -> class_stud[member]->value.
• key was omitted
• Both were changed to typedefined structures as shown in Lundin's answer, however you need not make them types (just using struct is fine).
• We just declare root to be a pointer to node, rather than declaring struct node, and then a pointer and then making the assignment.
• We aren't creating a structure as a pointer, we're creating a pointer to a structure
• Main should return the type int, if you don't want arguments, tell the compiler that you expect argc to be void.

Stuff I didn't do:

• Initialize the allocated structure with memset() or via calloc()
• Handle malloc() failing
• Compile my example (I don't know what struct branch is)

Answer 3

Here is some general source code cleanup that may be helpful:

#include <stdio.h>
#define M 3

typedef struct
{
  char  name [16];
  char  subject [16];
  int   roll_num;

} Class_stud;


typedef struct
{
  int         n;
  Branch*     p[M];
  Class_stud  cls[M-1];

} Node;



int main()
{
  Node*        root;
  int          clsf_cnt;
  Class_stud   clsf;
  Class_stud*  clsf_ptr;

  clsf_ptr = &clsf;
  clsf_cnt = 0;

  root = malloc (sizeof(Node));

  if(root == NULL)
  {
    /* error handling here */
  }
}

Answer 4

Any instance of root.clsf_ptr you have in your code should be replaced with root->clsf_ptr, which indicates that root is a pointer to a struct.

You also need to make sure the member clsf_ptr is located in the node struct.

Answer 5

I think malloc syntax is wrong. once check that. Memory is not allocated.Thats why it shows an error.