---
name: "shmdt"
section: "2"
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/shmat"
---

# shmdt(2)

attach or detach shared memory

Part of [shmat(2)](/shmat/2.md).

## NAME

**shmat**, **shmdt** – attach or detach shared memory

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<sys/types.h\>**

**#include \<sys/ipc.h\>**

**#include \<sys/shm.h\>**

*void* *\**\

**shmat**(*int shmid*, *const void \*addr*, *int flag*);

*int*\

**shmdt**(*const void \*addr*);

## DESCRIPTION

The **shmat**() system call attaches the shared memory segment identified by *shmid* to the calling process's address space. The address where the segment is attached is determined as follows:

- If *addr* is 0, the segment is attached at an address selected by the kernel.

- If *addr* is nonzero and *SHM\_RND* is not specified in *flag*, the segment is attached the specified address.

- If *addr* is specified and *SHM\_RND* is specified, *addr* is rounded down to the nearest multiple of SHMLBA.

If the *SHM\_REMAP* flag is specified and the passed *addr* is not NULL, any existing mappings in the virtual addresses range are cleared before the segment is attached. If the flag is not specified, *addr* is not NULL, and the virtual address range contains some pre-existing mappings, the **shmat**() call fails.

The **shmdt**() system call detaches the shared memory segment at the address specified by *addr* from the calling process's address space.

## RETURN VALUES

Upon success, **shmat**() returns the address where the segment is attached; otherwise, -1 is returned and *errno* is set to indicate the error.

The **shmdt()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

## ERRORS

The **shmat**() system call will fail if:

- \[EINVAL\]

  No shared memory segment was found corresponding to *shmid*.

- \[EINVAL\]

  The *addr* argument was not an acceptable address.

- \[ENOMEM\]

  The specified *addr* cannot be used for mapping, for instance due to the amount of available space being smaller than the segment size, or because pre-existing mappings are in the range and no *SHM\_REMAP* flag was provided.

- \[EMFILE\]

  Failed to attach the shared memory segment because the per-process *kern.ipc.shmseg* [sysctl(3)](/sysctl/3.md) limit was reached.

The **shmdt**() system call will fail if:

- \[EINVAL\]

  The *addr* argument does not point to a shared memory segment.

## SEE ALSO

[shmctl(2)](/shmctl/2.md), [shmget(2)](/shmget/2.md)