Dynamically allocating array of structs

Question

I have a struct named clients, and i created this array of structs.

typedef struct auxiliarRegistre{
 char name[50];
 char CPF[20];
 char addr[100];
}clients;
clients PrimaryClients[100];

I'm calling a function to insert the data into this arrays, but i wanna to increase the amount of possible values until it reaches the limit. Is this the correct way?

int *pointer = (clients *) malloc(sizeof(clients));

• Some programmer dude: That malloc call allocates space for one client structure. You might also want to read about realloc.
• user529758: This cannot possibly be the correct way, since you are casting the return value of malloc(), which is wrong. Apart from that, are you looking for realloc()?
• Kninnug: Also watch out that you can't use realloc() to embiggen PrimaryClients as it is not allocated with malloc().
• Jens Gustedt: Why would pointer be of type int* that is a pointer to int and not a pointer to clients, clients*?
• PlayMa256: if i use clients *pointer = malloc(sizeof(clients), i will need to access PromaryClients using pointer->name?

Answer 1

Here's one example:

#include <stdlib.h>
typedef struct auxiliarRegistre { ... } clients;
int arrSize = SOME_START_SIZE;
clients *arr = malloc( arrSize * sizeof *arr );
/**
 * Do stuff with arr. When you need to extend the buffer, do the following:
 */
clients *tmp = realloc( clients, sizeof *arr * ( arrSize * 2));
if ( tmp )
{
 arr = tmp;
 arrSize *= 2;
}

Doubling the size of the buffer each time you need to extend it is a common strategy; this tends to minimize the number of calls to realloc. It can also lead to serious internal fragmentation; if you have 128 elements and you need to store just one more, you wind up allocating 256 elements overall. You can also extend by a fixed amount, such as

clients *tmp = realloc( clients, sizeof *arr * ( arrSize + extent ));
if ( tmp )
{
 arr = tmp;
 arrSize += extent;
}

Note that you don't want to assign the result of realloc directly to your buffer; if it returns NULL due to an error, you'll lose your reference to the memory you've already allocated, leading to a memory leak. Also, you don't want to update your array size until you know the call has succeeded.

• PlayMa256: interesting, to acess the fields on arr, i need to use arr->name, right?
• John Bode: @MatheusSilva: you'd use arr[i].name. The subscript operation implicitly dereferences arr; that is, if the type of arr is client *, then the type of arr[i] is client.
• PlayMa256: aparently i hade some errors, its shows me syntax error before cliente *tmp = realloc(client, sizeof(*arr * ( arrSize + 1 ))); and i dont know what is the error...
• John Bode: @MatheusSilva: what is the exact error message coming from the compiler? Note that the realloc call should be written as realloc(client, sizeof *arr * (arrSize + 1)); check your parentheses.