Memory management errors in the C language. Why are these errors?

Question

I am trying to find two errors related to memory allocation in this code snippet. It was said in lecture that the first error was returning class, and the second was not freeing memory. I don't understand why the first is an error. The second one I am not sure of, but have a good idea of why.

For the first error, class is an array of students and the function getClassInfo returns a pointer to this array, seems fine to me.

I think I understand why memory should be free'd though. On each iteration of the for loop thisStudent is allocated memory by malloc and then class[i] is assigned to the value of thisStudent. On each iteration thisStudent is reassigned a new block of memory, but the last one is never free'd.

Any clarification as to why the first error is an error would be greatly appreciated! Thank you - Chris

#include <stdio.h>
#define CLASS_SIZE 500

typedef struct student {
    int studentNr;
    char grade;
} student;

student * getClassInfo(void){
    int i;
    student class[CLASS_SIZE];

for(i = 0; i < CLASS_SIZE; i++){
    student * thisStudent = (student*) malloc(sizeof(student));
        if(thisStudent == NULL) return(NULL);
        scanf("%d %c", &(thisStudent->studentNr),&(thisStudent->grade));

        class[i] = *thisStudent;
        }
    return class;
}

I do get an warning saying its return the address of a local variable, when compiling, but how is that a error related to memory allocation?

Answer 1

class is a local variable within the getClassInfo() function, which means that it ceases to exist when that function returns. The return class; statement returns a pointer that's no longer valid by the time the caller receives it.

You should declare class as a student* and allocate it dynamically with malloc(), just like you do for the students themselves. That allocates it on the heap, so it continues to exist until you explicitly free() it. You'll want to do malloc(sizeof(student) * CLASS_SIZE) to allocate contiguous space for all the student structures.

You're right that not freeing the student structures allocated in the loop is a bug (specifically, a memory leak). You don't actually need to dynamically allocate students at all; you can just store the student number and grade into class[i].studentNr and class[i].grade.