reallocarray(3bsd) 3bsd reallocarray(3bsd)

# [NAME](#NAME)

`reallocarray`, `recallocarray`, `freezero` — memory allocation and deallocation

# [LIBRARY](#LIBRARY)

library “libbsd”

# [SYNOPSIS](#SYNOPSIS)

`#include <stdlib.h>` (See [libbsd(7)](/man/os/libbsd/libbsd.7) for include usage.)  
`void *`  
`reallocarray`(`void *ptr`, `size_t nmemb`, `size_t size`);

`void *`  
`recallocarray`(`void *ptr`, `size_t oldnmemb`, `size_t nmemb`, `size_t size`);

`void`  
`freezero`(`void *ptr`, `size_t size`);

# [DESCRIPTION](#DESCRIPTION)

Designed for safe allocation of arrays, the [`reallocarray`](#reallocarray)() function is similar to `realloc`() except it operates on `nmemb` members of size `size` and checks for integer overflow in the calculation `nmemb` * `size`.

Used for the allocation of memory holding sensitive data, the [`recallocarray`](#recallocarray)() function guarantees that memory becoming unallocated is explicitly [*discarded*](#discarded), meaning cached free objects are cleared with [explicit\_bzero(3)](/man/os/libbsd/explicit_bzero.3).

The [`recallocarray`](#recallocarray~2)() function is similar to `reallocarray`() except it ensures newly allocated memory is cleared similar to `calloc`(). If `ptr` is `NULL`, `oldnmemb` is ignored and the call is equivalent to `calloc`(). If `ptr` is not `NULL`, `oldnmemb` must be a value such that `oldnmemb` * `size` is the size of the earlier allocation that returned `ptr`, otherwise the behavior is undefined. The `freezero`() function is similar to the [`free`](#free)() function except it ensures memory is explicitly discarded. If `ptr` is `NULL`, no action occurs. If `ptr` is not `NULL`, the `size` argument must be equal to or smaller than the size of the earlier allocation that returned `ptr`. `freezero`() guarantees the memory range starting at `ptr` with length `size` is discarded while deallocating the whole object originally allocated.

# [RETURN VALUES](#RETURN_VALUES)

The `reallocarray`() and `recallocarray`() functions return a pointer to the allocated space if successful; otherwise, a null pointer is returned and `errno` is set to `ENOMEM`.

If multiplying `nmemb` and `size` results in integer overflow, `reallocarray`() and `recallocarray`() return `NULL` and set `errno` to `ENOMEM`.

If `ptr` is not `NULL` and multiplying `oldnmemb` and `size` results in integer overflow `recallocarray`() returns `NULL` and sets `errno` to `EINVAL`.

# [IDIOMS](#IDIOMS)

Consider [`calloc`](#calloc)() or the extensions `reallocarray`() and `recallocarray`() when there is multiplication in the `size` argument of `malloc`() or `realloc`(). For example, avoid this common idiom as it may lead to integer overflow:

```
if ((p = malloc(num * size)) == NULL)
	err(1, NULL);
```

A drop-in replacement is [`reallocarray`](#reallocarray~2)():

```
if ((p = reallocarray(NULL, num, size)) == NULL)
	err(1, NULL);
```

Alternatively, [`calloc`](#calloc~2)() may be used at the cost of initialization overhead.

When using [`realloc`](#realloc)(), be careful to avoid the following idiom:

```
size += 50;
if ((p = realloc(p, size)) == NULL)
	return (NULL);
```

Do not adjust the variable describing how much memory has been allocated until the allocation has been successful. This can cause aberrant program behavior if the incorrect size value is used. In most cases, the above sample will also result in a leak of memory. As stated earlier, a return value of `NULL` indicates that the old object still remains allocated. Better code looks like this:

```
newsize = size + 50;
if ((newp = realloc(p, newsize)) == NULL) {
	free(p);
	p = NULL;
	size = 0;
	return (NULL);
}
p = newp;
size = newsize;
```

As with [`malloc`](#malloc)(), it is important to ensure the new size value will not overflow; i.e. avoid allocations like the following:

```
if ((newp = realloc(p, num * size)) == NULL) {
	...
```

Instead, use [`reallocarray`](#reallocarray~3)():

```
if ((newp = reallocarray(p, num, size)) == NULL) {
	...
```

Calling [`realloc`](#realloc~2)() with a `NULL` `ptr` is equivalent to calling `malloc`(). Instead of this idiom:

```
if (p == NULL)
	newp = malloc(newsize);
else
	newp = realloc(p, newsize);
```

Use the following:

```
newp = realloc(p, newsize);
```

The [`recallocarray`](#recallocarray~3)() function should be used for resizing objects containing sensitive data like keys. To avoid leaking information, it guarantees memory is cleared before placing it on the internal free list. Deallocation of such an object should be done by calling [`freezero`](#freezero)().

# [SEE ALSO](#SEE_ALSO)

[malloc(3)](/man/os/libbsd/malloc.3), [calloc(3)](/man/os/libbsd/calloc.3), [alloca(3)](/man/os/libbsd/alloca.3)

# [HISTORY](#HISTORY)

The `reallocarray`() function appeared in OpenBSD 5.6, DragonFly 5.5 and glibc 2.26.

The `recallocarray`() function appeared in OpenBSD 6.1 and DragonFly 5.5. The `freezero`() function appeared in OpenBSD 6.2 and DragonFly 5.5.

September 14, 2019 Linux 4.19.0-gvisor

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Document: reallocarray(3bsd) from libbsd 0.12.2-r9 (Chainguard OS)
Source: https://gitlab.freedesktop.org/libbsd/libbsd