structurs

[michalb]

Hello,
I am tring to add new structure to slist.
I am not sure this is the right way to do it.
I have a broblem when i'm tring to put the fildes id and name (run time eror).

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

typedef struct student {
	char *name;
	int id;
	struct clist *courses;
} student;

typedef struct course {
	char *title;
	int number;
	struct slist *students;
} course;

typedef struct slist {
	student *info;
	struct slist *next;
} slist;

typedef struct clist {
	course *info;
	struct clist *next;
} clist;

//prototypes
slist* add_student(slist *students, char *name, int id);


//main function - only to check the program
void main(){
	char *name[256];
	int id;
	slist *students = NULL;

	printf("Enter your name\n");
		scanf("%s",&name);
		printf("Enter your id\n");
		scanf("%d",&id);
		add_student(students,name,id);
}

//Basic Operations
slist* add_student(slist *students, char *name, int id){

	slist *head = (slist*) malloc(sizeof(slist));
	
	strcpy(head->info->name, students->info->name);
	head->info->id = id;
	if (!head ) return NULL;
	head->next = students;

	return head;
	}

[nadroj]

In main, you declare "students" and assign it "NULL". Then, in "add_student", you try to access a field of it ("->info"), which will give you a segfault. You need to allocate memory for "students" before using it.

[quzah]

To expand upon that point:

slist* add_student(slist *students, char *name, int id){

	slist *head = (slist*) malloc(sizeof(slist));
	
	strcpy(head->info->name, students->info->name);
	head->info->id = id;
	if (!head ) return NULL;
	head->next = students;

	return head;
	}

Here again, you start fiddling with the memory, and afterwards you check to see if it's OK to do so. You should make sure it's valid before you start using it, not after.


Quzah.