shmctl(2) [opendarwin man page]
SHMCTL(2) BSD System Calls Manual SHMCTL(2) NAME
shmctl -- shared memory control operations SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.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 */ int 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 { ushort cuid; /* creator user id */ ushort cgid; /* creator group id */ ushort uid; /* user id */ ushort gid; /* group id */ ushort mode; /* r/w permission (see chmod(2)) */ ushort seq; /* sequence # (to generate unique msg/sem/shm id) */ key_t key; /* user specified msg/sem/shm key */ }; 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) ), 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: [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. [EACCES] The command is IPC_STAT and the caller has no read permission for this shared memory segment. [EINVAL] shmid is not a valid shared memory segment identifier. cmd is not a valid command. [EFAULT] buf specifies an invalid address. SEE ALSO
shmat(2), shmdt(2), shmget(2) BSD
August 17, 1995 BSD
Check Out this Related Man Page
shmctl(2) System Calls shmctl(2) NAME
shmctl - shared memory control operations SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> int shmctl(int shmid, int cmd, struct shmid_ds *buf); DESCRIPTION
The shmctl() function provides a variety of shared memory control operations as specified by cmd. The permission required for a shared mem- ory control operation is given as {token}, where token is the type of permission needed. The types of permission are interpreted as fol- lows: 00400 READ by user 00200 WRITE by user 00040 READ by group 00020 WRITE by group 00004 READ by others 00002 WRITE by others See the Shared Memory Operation Permissions section of Intro(2) for more information. The following operations require the specified tokens: IPC_STAT Place the current value of each member of the data structure associated with shmid into the structure pointed to by buf. The contents of this structure are defined in Intro(2). {READ} IPC_SET Set the value of the following members of the data structure associated with shmid to the corresponding value found in the structure pointed to by buf: shm_perm.uid shm_perm.gid shm_perm.mode /* access permission bits only */ This command can be executed only by a process that has appropriate privileges or an effective user ID equal to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid. IPC_RMID Remove the shared memory identifier specified by shmid from the system. The segment referenced by the identifier will be destroyed when all processes with the segment attached have either detached the segment or exited. If the segment is not attached to any process when IPC_RMID is invoked, it will be destroyed immediately. This command can be executed only by a process that has appropriate privileges or an effective user ID equal to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid. SHM_LOCK Lock the shared memory segment specified by shmid in memory. This command can be executed only by a process that has appro- priate privileges. SHM_UNLOCK Unlock the shared memory segment specified by shmid. This command can be executed only by a process that has appropriate privileges. A shared memory segment must be explicitly removed using IPC_RMID before the system can deallocate it and the resources it uses. RETURN VALUES
Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The shmctl() function will fail if: EACCES The cmd argument is equal to IPC_STAT and {READ} operation permission is denied to the calling process. EFAULT The buf argument points to an illegal address. EINVAL The shmid argument is not a valid shared memory identifier; or the cmd argument is not a valid command or is IPC_SET and shm_perm.uid or shm_perm.gid is not valid. ENOMEM The cmd argument is equal to SHM_LOCK and there is not enough memory, or the operation would exceed a limit or resource con- trol on locked memory. EOVERFLOW The cmd argument is IPC_STAT and uid or gid is too large to be stored in the structure pointed to by buf. EPERM The cmd argument is equal to IPC_RMID or IPC_SET, the effective user ID of the calling process is not equal to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid, and {PRIV_IPC_OWNER} is not asserted in the effec- tive set of the calling process. The cmd argument is equal to SHM_LOCK or SHM_UNLOCK and {PRIV_PROC_LOCK_MEMORY} is not asserted in the effective set of the calling process. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ SEE ALSO
ipcs(1), Intro(2), shmget(2), shmop(2), attributes(5), privileges(5), standards(5) SunOS 5.11 10 Apr 2007 shmctl(2)