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

# shmctl(3p)

XSI shared memory control operations

## PROLOG

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

## NAME

shmctl — XSI shared memory control operations

## SYNOPSIS

```
#include <sys/shm.h>
```

int shmctl(int *shmid*, int *cmd*, struct shmid\_ds \**buf*);

## DESCRIPTION

The *shmctl*() function operates on XSI shared memory (see the Base Definitions volume of POSIX.1‐2017, *Section 3.346*, *Shared Memory Object*). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in *Section 2.8*, *Realtime*.

The *shmctl*() function provides a variety of shared memory control operations as specified by *cmd*. The following values for *cmd* are available:

- IPC\_STAT

  Place the current value of each member of the **shmid\_ds** data structure associated with *shmid* into the structure pointed to by *buf*. The contents of the structure are defined in *\<sys/shm.h\>*.

- IPC\_SET

  Set the value of the following members of the **shmid\_ds** data structure associated with *shmid* to the corresponding value found in the structure pointed to by *buf*:

  ```
  shm_perm.uid
  shm_perm.gid
  shm_perm.mode    Low-order nine bits.
  ```

  Also, the *shm\_ctime* timestamp shall be set to the current time, as described in *Section 2.7.1*, *IPC General Description*.

  IPC\_SET can only be executed by a process that has an effective user ID equal to either that of a process with appropriate privileges or to the value of *shm\_perm.cuid* or *shm\_perm.uid* in the **shmid\_ds** data structure associated with *shmid*.

- IPC\_RMID

  Remove the shared memory identifier specified by *shmid* from the system and destroy the shared memory segment and **shmid\_ds** data structure associated with it. IPC\_RMID can only be executed by a process that has an effective user ID equal to either that of a process with appropriate privileges or to the value of *shm\_perm.cuid* or *shm\_perm.uid* in the **shmid\_ds** data structure associated with *shmid*.

## RETURN VALUE

Upon successful completion, *shmctl*() shall return 0; otherwise, it shall return -1 and set *errno* to indicate the error.

## ERRORS

The *shmctl*() function shall fail if:

- **EACCES**

  The argument *cmd* is equal to IPC\_STAT and the calling process does not have read permission; see *Section 2.7*, *XSI Interprocess Communication*.

- **EINVAL**

  The value of *shmid* is not a valid shared memory identifier, or the value of *cmd* is not a valid command.

- **EPERM**

  The argument *cmd* is equal to IPC\_RMID or IPC\_SET and the effective user ID of the calling process is not equal to that of a process with appropriate privileges and it is not equal to the value of *shm\_perm.cuid* or *shm\_perm.uid* in the data structure associated with *shmid*.\

The *shmctl*() function may fail if:

- **EOVERFLOW**

  The *cmd* argument is IPC\_STAT and the *gid* or *uid* value is too large to be stored in the structure pointed to by the *buf* argument.

*The following sections are informative.*

## EXAMPLES

None.

## APPLICATION USAGE

The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in *Section 2.7*, *XSI Interprocess Communication* can be easily modified to use the alternative interfaces.

## RATIONALE

None.

## FUTURE DIRECTIONS

None.

## SEE ALSO

*Section 2.7*, *XSI Interprocess Communication*, *Section 2.8*, *Realtime*, *shmat()*, *shmdt()*, *shmget()*, *shm\_open()*, *shm\_unlink()*

The Base Definitions volume of POSIX.1‐2017, *Section 3.346*, *Shared Memory Object*, ***\<sys\_shm.h\>***

## COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting\_bugs.html .