如何在指向结构时正确使用malloc（）和realloc（）？

Question

This is my code:

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

typedef struct{
    char name;
    char surname;
    int month;
    int day;
} person;

person *ptr;
int action, number_of_friends=0, a, b, day, month;
char decision;

int main(void)
{
    ptr=(person*)malloc(sizeof(person));
    while(1)
    {
        printf("Please enter the data about the person number %d\n", number_of_friends+1);
        person entered_person;
        printf("Initial of the name: "); scanf(" %c", &entered_person.name);
        printf("Initial of the surname: "); scanf(" %c", &entered_person.surname);
        printf("The day of birth: "); scanf("%d", &entered_person.day);
        printf("And the month: "); scanf("%d", &entered_person.month);
        *(ptr+number_of_friends) = entered_person;
        printf("Do you want to add more friends? (y/n) ");
        scanf(" %c", &decision);
        if (decision=='n' || decision=='N') {
            break;
        }
        number_of_friends++;
        ptr=realloc(ptr, number_of_friends);
    }
    number_of_friends++;
    person get_person;
    for (a=0; a<number_of_friends; a++)
    {
        get_person = *(ptr+a);
        printf("Person number %d\n", a+1);
        printf("Initial of the name: %c\n", get_person.name);
        printf("Initial of the surname: %c\n", get_person.surname);
        printf("The day of birth: %d\n", get_person.day);
        printf("And the month: %d\n\n", get_person.month);
    }
}

The problem is that it won't display the list of entered people correctly if the number of people is greater than... something like 5.

I believe this is connected with malloc() and realloc() (writing out-of-bounds?) but as a beginner I don't know how to solve that problem.

Answer 1

Your realloc() size is wrong, you want number_of_friends multiplied by the size of a person struct (I'm guessing):

ptr=realloc(ptr, number_of_friends*sizeof(person));

Edit: Also this is going to crash after the first loop:

number_of_friends++;
ptr=realloc(ptr, number_of_friends);

Since number_of_friends starts at 0

Answer 2

number_of_friends++;
ptr=realloc(ptr, number_of_friends);

should be

person *tmp = realloc( ptr, sizeof *ptr * (number_of_friends + 1) );
if ( tmp )
{
  ptr = tmp;
  number_of_friends++;
}

Remember that realloc , like malloc , takes a number storage units (bytes) as an argument, not number of elements of a particular type. Also, realloc will return NULL on failure, so you want to preserve your original pointer until you're sure that the realloc operation succeeded. Similarly, you don't want to update number_of_friends until after the realloc call has completed.