SHMCTL(2) System Calls Manual SHMCTL(2)

shmctlshared memory control operations

#include <sys/shm.h>

int
shmctl(int shmid, int cmd, struct shmid_ds *buf);

The () 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 () is specified in cmd and is one of:

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

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

shmctl() will fail if:

[]
The command is IPC_STAT and the caller has no read permission for this shared memory segment.
[]
buf specifies an invalid address.
[]
shmid is not a valid shared memory segment identifier. cmd is not a valid command.
[]
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.

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

All of these include files are necessary.

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.

shmat(2), shmdt(2), shmget(2), compat(5)

August 17, 1995 macOS 14.4