---
name: "shmctl"
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/shmctl"
---

# shmctl(2)

shared memory control

## NAME

**shmctl** – shared memory control

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

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

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

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

*int*\

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

## DESCRIPTION

Performs the action specified by *cmd* on the shared memory segment identified by *shmid*:

- IPC\_STAT

  Fetch the segment's *struct shmid\_ds*, storing it in the memory pointed to by *buf*.

- IPC\_SET

  Changes the *shm\_perm.uid*, *shm\_perm.gid*, and *shm\_perm.mode* members of the segment's *struct shmid\_ds* to match those of the struct pointed to by *buf*. The calling process's effective uid must match either *shm\_perm.uid* or *shm\_perm.cuid*, or it must have superuser privileges.

- IPC\_RMID

  Removes the segment from the system. The removal will not take effect until all processes having attached the segment have exited. For the operation to succeed, the calling process's effective uid must match *shm\_perm.uid* or *shm\_perm.cuid*, or the process must have superuser privileges. If the *kern.ipc.shm\_allow\_removed* [sysctl(3)](/sysctl/3.md) variable is set to 0, once the IPC\_RMID operation has taken place, no further processes will be allowed to attach the segment.

The *shmid\_ds* structure is defined as follows:

```
struct shmid_ds {
    struct ipc_perm shm_perm;   /* operation permission structure */
    size_t          shm_segsz;  /* size of segment in bytes */
    pid_t           shm_lpid;   /* process ID of last shared memory op */
    pid_t           shm_cpid;   /* process ID of creator */
    int             shm_nattch; /* number of current attaches */
    time_t          shm_atime;  /* time of last shmat() */
    time_t          shm_dtime;  /* time of last shmdt() */
    time_t          shm_ctime;  /* time of last change by shmctl() */
};
```

## RETURN VALUES

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

- \[EINVAL\]

  Invalid operation, or no shared memory segment was found corresponding to *shmid*.

- \[EPERM\]

  The calling process's effective uid does not match the uid of the shared memory segment's owner or creator.

- \[EACCES\]

  Permission denied due to mismatch between operation and mode of shared memory segment.

## SEE ALSO

[shmat(2)](/shmat/2.md), [shmdt(2)](/shmdt/2.md), [shmget(2)](/shmget/2.md), [ftok(3)](/ftok/3.md)