---
name: "alloca"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/alloca"
---

# alloca(3)

memory allocator

## NAME

**alloca** – memory allocator

## SYNOPSIS

**#include \<stdlib.h\>**

*void* *\**\

**alloca**(*size\_t size*);

## DESCRIPTION

The **alloca**() function or macro allocates *size* bytes of space in the stack frame of the caller. This temporary space is automatically freed on return.

## RETURN VALUES

**alloca**() returns a pointer to the beginning of the allocated space.

## SEE ALSO

[brk(2)](/brk/2.md), [calloc(3)](/calloc/3.md), [getpagesize(3)](/getpagesize/3.md), [malloc(3)](/malloc/3.md), [realloc(3)](/realloc/3.md)

## HISTORY

**alloca**() appeared in AT&T UNIX v32.

## BUGS

**alloca**() is machine and compiler dependent; its use is discouraged.

**alloca**() is slightly unsafe because it cannot ensure that the pointer returned points to a valid and usable block of memory. The allocation made may exceed the bounds of the stack, or even go further into other objects in memory, and **alloca**() cannot determine such an error. Avoid **alloca**() with large unbounded allocations.

The use of C99 variable-length arrays and **alloca**() in the same function will cause the lifetime of **alloca**(*Ns*, *'s*) storage to be limited to the block containing the **alloca**(). For example, in the following snippet, *p*'s lifetime does not extend outside of the block, whereas it would've if *vla* hadn't been defined or had been defined as a fixed-length array:

```
char *p;
{
	const int n = 100;
	int vla[n];
	p = alloca(32);
	strcpy(p, "Hello, world!");
	printf("Inside: %s\n", p); /* Valid. */
}
printf("Outside: %s\n", p); /* Undefined. */
```