SHMCTL(2) | System Calls Manual | SHMCTL(2) |
shmctl
— shared
memory control operations
#include
<sys/shm.h>
int
shmctl
(int shmid,
int cmd, struct shmid_ds
*buf);
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
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
IPC_SET
IPC_RMID
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:
EACCES
]EFAULT
]EINVAL
]EPERM
]#include
<sys/types.h>
#include
<sys/ipc.h>
#include
<sys/shm.h>
All of these include files are necessary.
The
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.
August 17, 1995 | macOS 14.6 |