How to allocate memory for char strings in a struct array?

Question

I am about to create a programm which stores data of students in a list. My question is, how to shall I allocate memory for each char string in my struct array. My code is down below. If there're some other mistakes, please correct me.

#include <stdio.h>
#include <stdlib.h>
#define DATA 10;
#define NAME 10;

typedef struct{
int id;
char *givenname;
char *familyname;

} students;

int main()
{
int answer;
int incr = 0; // Index for students in the list
int datalen = DATA;
int namelen = NAME;

students *studentlist;
studentlist = malloc(datalen * sizeof(students)); // Allocate memory for first ten students

if(NULL == studentlist){
    printf("Error: Couldn't allocate memory\n");
    exit(0);
}

for(incr = 0; incr < datalen; incr ++){
    printf("Add student to the list? Yes(1) No(2)\n");
    scanf("%d", &answer);

    if(answer != 1){
        break;
    }

    studentlist[incr]->givenname = malloc(namelen * sizeof(char)); // Allocate memory for each name
    studentlist[incr]->familyname = malloc(namelen * sizeof(char));

    printf("Insert ID: ");
    scanf("%d", &studentlist[incr].id);

    printf("Insert given name: \n");
    scanf("%s", studentlist[incr].givenname);

    printf("Insert family name: \n");
    scanf("%s", studentlist[incr].familyname);


}

free(studentlist);
free(studentlist.givename);
free(studentlist.familyname);


return 0;
}

The way you are allocating the memory looks ok. Do you have a specific problem? The way you free it is wrong ... — Fantastic Mr Fox
– Fantastic Mr Fox, Commented Dec 7, 2015 at 22:56
studentlist[incr]->givenname --> studentlist[incr].givenname — BLUEPIXY
– BLUEPIXY, Commented Dec 7, 2015 at 22:59
1. Don't forget space for a 0 terminator. 2. free studentlist.givename (and familyname) before studentlist or else you're invoking undefined behaviour (accessing freed memory). 3. Infact how can you free studentlist then access by .? If you freed it it ,must be a pointer and so access via ->. - Does this even compile? — John3136
– John3136, Commented Dec 7, 2015 at 23:00
free(studentlist.givename); --> do each element free(studentlist[incr].givename); and before free(studentlist); — BLUEPIXY
– BLUEPIXY, Commented Dec 7, 2015 at 23:01

Ratnadeep · Accepted Answer · 2015-12-07 23:13:20Z

1

Reference of some elements is wrong:

studentlist[incr]->givenname

It should be:

studentlist[incr].givenname

Allocation of strings seems fine.

Your freeing code needs change:

free(studentlist);
free(studentlist.givename);
free(studentlist.familyname);

You need to free studentlist.givename and studentlist.familyname in a loop and then free studentlist at the end:

for(incr = 0; incr < datalen; incr ++){
   free(studentlist[incr].givename);
   free(studentlist[incr].familyname);
}
free(studentlist);

answered Dec 7, 2015 at 23:13

Ratnadeep

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