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 The return class; statement returns a pointer that's no longer valid by the time the caller receives it.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 .