---
name: "shmctl"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/shmctl"
---

# shmctl(2)

shared memory control operations

## NAME

**shmctl** – shared memory control operations

## SYNOPSIS

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

*int*\

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

## DESCRIPTION

The **shmctl**() system call performs some control operations on the shared memory area specified by *shmid*. Each shared memory segment has a data structure associated with it, parts of which may be altered by **shmctl**() and parts of which determine the actions of **shmctl**(). This structure is defined as follows in ⟨*sys/shm.h*⟩:

```
struct shmid_ds {
    struct ipc_perm  shm_perm;     /* operation permissions */
    size_t           shm_segsz;    /* size of segment in bytes */
    pid_t            shm_lpid;     /* pid of last shm op */
    pid_t            shm_cpid;     /* pid of creator */
    short            shm_nattch;   /* # of current attaches */
    time_t           shm_atime;    /* last shmat() time*/
    time_t           shm_dtime;    /* last shmdt() time */
    time_t           shm_ctime;    /* last change by shmctl() */
    void            *shm_internal; /* sysv stupidity */
};
```

The ipc\_perm structure used inside the shmid\_ds structure is defined in ⟨*sys/ipc.h*⟩ and looks like this:

```
struct ipc_perm {
  uid_t           uid;   /* Owner's user ID */
  gid_t           gid;   /* Owner's group ID */
  uid_t           cuid;  /* Creator's user ID */
  gid_t           cgid;  /* Creator's group ID */
  mode_t          mode;  /* r/w permission (see chmod(2)) */
  unsigned short  _seq;  /* Reserved for internal use */
  key_t           _key;  /* Reserved for internal use */
};
```

The operation to be performed by **shmctl**() is specified in *cmd* and is one of:

- IPC\_STAT

  Gather information about the shared memory segment and place it in the structure pointed to by *buf*.

- IPC\_SET

  Set the value of the *shm\_perm.uid*, *shm\_perm.gid* and *shm\_perm.mode* fields in the structure associated with *shmid*. The values are taken from the corresponding fields in the structure pointed to by *buf*. This operation can only be executed by the super-user, or a process that has an effective user id equal to either  *shm\_perm.cuid* or *shm\_perm.uid* in the data structure associated with the shared memory segment.

- IPC\_RMID

  Remove the shared memory segment specified by *shmid* and destroy the data associated with it. Only the super-user or a process with an effective uid equal to the  *shm\_perm.cuid* or *shm\_perm.uid* values in the data structure associated with the queue can do this.

The read and write permissions on a shared memory identifier are determined by the *shm\_perm.mode* field in the same way as is done with files (see [chmod(2)](/chmod/2.md) ), but the effective uid can match either the *shm\_perm.cuid* field or the *shm\_perm.uid* field, and the effective gid can match either *shm\_perm.cgid* or *shm\_perm.gid*.

## RETURN VALUES

Upon successful completion, a value of 0 is returned. Otherwise, -1 is returned and the global variable *errno* is set to indicate the error.

## ERRORS

**shmctl**() will fail if:

- \[EACCES\]

  The command is IPC\_STAT and the caller has no read permission for this shared memory segment.

- \[EFAULT\]

  *buf* specifies an invalid address.

- \[EINVAL\]

  *shmid* is not a valid shared memory segment identifier. *cmd* is not a valid command.

- \[EPERM\]

  *cmd* is equal to IPC\_SET or IPC\_RMID and the caller is not the super-user,nor does the effective uid match either the *shm\_perm.uid* or *shm\_perm.cuid* fields of the data structure associated with the shared memory segment. An attempt is made to increase the value of *shm\_qbytes* through IPC\_SET but the caller is not the super-user.

## LEGACY SYNOPSIS

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

All of these include files are necessary.

## LEGACY DESCRIPTION

The ipc\_perm structure used inside the shmid\_ds structure, as defined in ⟨*sys/ipc.h*⟩, looks like this:

```
struct ipc_perm {
    __uint16_t cuid;  /* Creator's user id */
    __uint16_t cgid;  /* Creator's group id */
    __uint16_t uid;   /* Owner's user id */
    __uint16_t gid;   /* Owner's group id */
    mode_t     mode;  /* r/w permission (see chmod(2)) */
    __uint16_t seq;   /* Reserved for internal use */
    key_t      key;   /* Reserved for internal use */
};
```

This structure is maintained for binary backward compatibility with previous versions of the interface. New code should not use this interface, because ID values may be truncated.

Specifically, LEGACY mode limits the allowable uid/gid ranges to 0-32767. If the user has a UID that is out of this range (e.g., "nobody"), software using the LEGACY API will not behave as expected.

## SEE ALSO

[shmat(2)](/shmat/2.md), [shmdt(2)](/shmdt/2.md), [shmget(2)](/shmget/2.md), [compat(5)](/compat/5.md)