Linked List Implementation Memory Allocation - c

Question

I've implemented a linked list using structs and pointers in C as follows:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/* these arrays are just used to give the parameters to 'insert',
* to create the 'people' array */
#define HOW_MANY 7
char * names[HOW_MANY] = {"Simon", "Suzie", "Alfred", "Chip", "John", "Tim", "Harriet"};
int ages[HOW_MANY] = {22, 24, 106, 6 ,18, 32, 24};
/* declare your struct for a person here */
struct p {
char *peopleName;
int age;
struct p* next;
} person;
struct p* insert_end (struct p *people,char *name, int age) {
/* put name and age into nthe enxt free place in the array parameter here */
/* modify nextfreeplace here */
struct p *nextPerson;
nextPerson = (struct p*)malloc(sizeof(struct p));
if (nextPerson == NULL) {
printf("Error performing malloc.\n");
return(NULL);
} else {
nextPerson -> peopleName = name;
nextPerson -> age = age;
if (people == NULL) {
nextPerson -> next = people;
return nextPerson;
} else {
struct p* temp = people;
while ((temp -> next) != NULL) {
temp = temp -> next;
}
temp -> next = nextPerson;
nextPerson -> next = NULL;
return people;
}
}
}
int main (int argc, char **argv) {
/* declare the people array here */
struct p *people = NULL;
for (int i = 0; i < HOW_MANY; i++) {
people= insert_end(people, names[i], ages[i]);
}
while (people != NULL) {
printf("Freeing\n");
free(people);
people = people -> next;
}
return 0;
}
This works fine, but what I don't understand is why it doesn't work when insert_end is declared as follows:
struct p* insert_end (struct p *people,char *name, int age) {
/* put name and age into nthe enxt free place in the array parameter here */
/* modify nextfreeplace here */
struct p *nextPerson;
nextPerson = (struct p*)malloc(sizeof(struct p));
if (nextPerson == NULL) {
printf("Error performing malloc.\n");
return(NULL);
} else {
nextPerson -> peopleName = name;
nextPerson -> age = age;
if (people == NULL) {
nextPerson -> next = people;
return nextPerson;
} else {
while ((people -> next) != NULL) {
people= people-> next;
}
people-> next = nextPerson;
nextPerson -> next = NULL;
return people;
}
}
}
As you can see, the memory is not being freed correctly when this implementation is used. Could anyone help me in understanding why this does not work?

Answer 1

The function is apparently supposed to return a pointer to the first entry in the list. The second implementation does not, instead returning a pointer to the next-to-last entry in the case where there's already at least one thing on the list.

Answer 2

Not the answer to your question, but even when you use the first version of insert_end which works fine, you have undefined behaviour in this piece of code :
while (people != NULL) {
printf("Freeing\n");
free(people);
people = people -> next; // << you are using people after it has been freed
}
On your platform the code may seem to work correctly, but this is only by chance.
This works in all cases on all platforms :
while (people != NULL) {
printf("Freeing\n");
struct p *temp = people ;
people = people -> next;
free(temp);
}