How to properly use realloc in c? [duplicate] - c

Question

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How can I create a dynamically sized array of structs?
(10 answers)
Closed 4 years ago.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#define LENGHT 20
typedef struct {
unsigned id;
char name[LENGHT];
char genre[LENGHT];
char nazionality [LENGHT];
int year_carrier_started;
bool check;
}Artist;
void print(Artist *arr, int *i);
void addOne(Artist *arr, int *i);
int main(void) {
int index = 5;
Artist *array = malloc(sizeof(int)*index);
for (int i=0;i<index;i++)
{
printf("Insert a number:\n");
scanf("%d",&array[i].id);
}
do
{
addOne(array,&index);
print(array,&index);
}while(1);
system("pause");
free(array);
return EXIT_SUCCESS;
}
void addOne(Artist *arr, int *i)
{
realloc(arr,sizeof(Artist)*(*i)+1);
printf("Insert another one:\n");
scanf("%d",&arr[*i].id);
*i = *i +1;
print(arr,i);
}
void print(Artist *arr, int *i)
{
for (int j=0;j<*i;j++)
{
printf("Number: %d position %d\n",arr[j].id,j);
}
}
Hi guys, I got this struct from a piece of my program, what I need to is to realloc everytime the user wants to add a new Artist. After adding like 10/15 more users, it crashes for no reason (at least for me there is not). What am I doing wrong? (Dat *i is passed by reference, it counts how many artists are there already).

Answer 1

How to properly use realloc in c(?)
After adding like 10/15 more users, it crashes for no reason
#Christian Gibbons well advises: use the return value of realloc(). The former value of array may not be valid.
Multiply the right values
// sizeof(Artist)*(*i)+1
sizeof(Artist)*((*i)+1)
Use the return value of realloc()
// realloc(array,sizeof(Artist)*(*i)+1);
void *new_ptr = realloc(array,....
Check realloc() results
// realloc(array,sizeof(Artist)*(*i)+1);
void *new_ptr = realloc(array,....
if (new_ptr == NULL) OutOfMemory();
else array = new_ptr;
Consider sizing by the object than the type
// realloc(array,sizeof(Artist)*(*i)+1);
void *new_ptr = realloc(array, sizeof *array *((*i)+1));
All together
Artist *array = malloc(sizeof *array);
if (array == NULL) Handle_OutOfMemory();
...
void *new_ptr = realloc(array, sizeof *array * ((*i)+ 1));
if (new_ptr == NULL) {
// `array` still has *i elements assigned to it.
Handle_OutOfMemory();
} else {
array = new_ptr;
(*i)++;
}

Answer 2

realloc will attempt to resize your allocated memory in place if it can, but if there's not enough room to expand to the new size where it is, it will have to move the data to a new location. Realloc returns a pointer to the location for this reason. You should also test to make sure that it did not err. So something like this:
Artist *arr = malloc(sizeof(*arr) * i);
Artist *temp = realloc(arr, sizeof(Artist)*((*i)+1) // Borrowing this from Yunnosch
if(temp) {
arr = temp;
}
else {
free(arr);
// more error handling
}