---
name: "posix_memalign"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/posix_memalign"
---

# posix\_memalign(3)

allocate aligned memory

## NAME

posix\_memalign - allocate aligned memory

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <stdlib.h>
```

**int posix\_memalign(void \*\****memptr***, size\_t** *alignment***, size\_t** *size***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**posix\_memalign**():

```
    _POSIX_C_SOURCE >= 200112L
```

## DESCRIPTION

**posix\_memalign**() allocates *size* bytes and places the address of the allocated memory in *\*memptr*. The address of the allocated memory will be a multiple of *alignment*, which must be a power of two and a multiple of *sizeof(void \*)*. This address can later be successfully passed to [**free**(3)](/free/3.md). If *size* is 0, then the value placed in *\*memptr* is either NULL or a unique pointer value.

The memory is not zeroed.

[**aligned\_alloc**(3)](/aligned_alloc/3.md) is a standard function that provides the same functionality, and has a more ergonomic prototype. Use that instead.

## RETURN VALUE

**posix\_memalign**() returns zero on success, or one of the error values listed in the next section on failure. The value of *errno* is not set.

## ERRORS

- **EINVAL**

  The *alignment* argument was not a power of two, or was not a multiple of *sizeof(void \*)*.

- **ENOMEM**

  Out of memory.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **posix\_memalign**() | Thread safety | MT-Safe |

## STANDARDS

POSIX.1-2024.

## HISTORY

glibc 2.1.91. POSIX.1d, POSIX.1-2001.

## CAVEATS

On Linux (and other systems), **posix\_memalign**() does not modify *memptr* on failure. A requirement standardizing this behavior was added in POSIX.1-2008 TC2.

## SEE ALSO

[**aligned\_alloc**(3)](/aligned_alloc/3.md)