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));

That malloc call allocates space for one client structure. You might also want to read about realloc. — Some programmer dude
– Some programmer dude, Commented Oct 28, 2013 at 13:31
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()? — user529758
– user529758, Commented Oct 28, 2013 at 13:31
Also watch out that you can't use realloc() to embiggen PrimaryClients as it is not allocated with malloc(). — Kninnug
– Kninnug, Commented Oct 28, 2013 at 13:40
Why would pointer be of type int* that is a pointer to int and not a pointer to clients, clients*? — Jens Gustedt
– Jens Gustedt, Commented Oct 28, 2013 at 13:55
if i use clients *pointer = malloc(sizeof(clients), i will need to access PromaryClients using pointer->name? — PlayMa256
– PlayMa256, Commented Oct 28, 2013 at 14:20

John Bode · Accepted Answer · 2013-10-28 14:28:33Z

2

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.

answered Oct 28, 2013 at 14:28

John Bode

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4 Comments

PlayMa256 Over a year ago

interesting, to acess the fields on arr, i need to use arr->name, right?

John Bode Over a year ago

@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 Over a year ago

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 Over a year ago

@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.

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