9.82 shmctl ¶

struct ipc_perms {
   uint32_t __key;
   uint32_t uid;
   uint32_t gid;
   uint32_t cuid;
   uint32_t cgid;
   uint32_t mode;
   uint32_t __seq;
   uint64_t reserved[2];
}

struct shmid_ds {
   struct ipc_perms shm_perm;
   uint64_t shm_segsz;
   uint64_t shm_atime;
   uint64_t shm_dtime;
   uint64_t shm_ctime;
   uint32_t shm_cpid;
   uint32_t shm_lpid;
   uint64_t shm_nattch;
}

#define IPC_RMID 0
#define IPC_SET  1
#define IPC_STAT 2

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

This syscall performs the control operation specified by cmd on the shared memory segment whose identifier is given in shmid.

cmd can be one of:

IPC_RMID

The passed shmid is set to be deleted when the attached count of the segment goes to 0. Otherwise, it will linger forever. buf is ignored.

IPC_SET

Information passed on the structure pointed to by buf is used to modify the permission fields of the passed segment, namely shm_perm.uid, shm_perm.gid, and shm_perm.mode.

IPC_STAT

Get the associated information of the passed segment id and write it to the structure pointed to by buf.

The syscall returns 0 on success or -1 on failure, with the following errno:

EINVAL

size is less than needed and/or fd is not a TTY.

EBADF

fd is not opened to anything.

EFAULT

buffer points to invalid memory.