shmid_ds(4) [osf1 man page]
shmid_ds(4) Kernel Interfaces Manual shmid_ds(4) NAME
shmid_ds - Defines a shared memory region SYNOPSIS
#include <sys/shm.h> struct shmid_ds{ struct ipc_perm shm_perm; int shm_segsz; u_short shm_lpid; u_short shm_cpid; u_short shm_nattch; time_t shm_atime; time_t shm_dtime; time_t shm_ctime; }; DESCRIPTION
The shmid_ds structure defines a shared memory region associated with a shared memory region ID. There is one shared memory region per ID. Collectively, the shared memory regions are maintained in a shared memory table, with the shared memory region IDs identifying the entries in the table. The IPC permissions for the shared memory regions are implemented in a separate, but associated, ipc_perm structure. A shared memory region is created indirectly via the shmget() call. If shmget() is called with a non-existent shared memory region ID, the kernel allocates a new shmid_ds structure, initializes it, and returns the ID that is to be associated with the region. The kernel allocates actual memory of shm_segsz bytes only when a process attaches a region to its address space. Attached regions are maintained in a separate region table. The entries in the shared memory table point to the associated attached regions in the region ta- ble. The same shared memory region can be attached multiple times, by the same or different processes. Each attachment of the region cre- ates a new entry in the region table. After a process attaches a shared memory region, the region becomes part of the process's virtual address space. Processes access shared memory regions by using the same machine instructions used to access any virtual address. FIELDS
The ipc_perm structure that defines permissions for shared memory operations. See NOTES. The size of the shared memory region, in bytes. The process ID of the process that created the shared memory region ID. The process ID of the last process that performed a shmat() or shmdt() operation on the shared memory region. The number of processes that currently have this region attached. The time of the last shmat() operation. The time of the last shmdt() operation. The time of the last shmctl() operation that changed a member of the shmid_ds structure. NOTES
The shm_perm field identifies the associated ipc_perm structure that defines the permissions for operations on the shared memory region. The ipc_perm structure (from the sys/ipc.h header file) is shown here. struct ipc_perm { ushort uid; /* owner's user id */ ushort gid; /* owner's group id */ ushort cuid; /* creator's user id */ ushort cgid; /* creator's group id */ ushort mode; /* access modes */ ushort seq; /* slot usage sequence number */ key_t key; /* key */ }; The mode field is a nine-bit field that contains the permissions for shared memory operations. The first three bits identify owner permis- sions; the second three bits identify group permissions; and the last three bits identify other permissions. In each group, the first bit indicates read permission; the second bit indicates write permission; and the third bit is not used. RELATED INFORMATION
Functions: shmat(2), shmdt(2), shmctl(2), shmget(2) delim off shmid_ds(4)
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SHMCTL(2) BSD System Calls Manual SHMCTL(2) 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 */ 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 { 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) ), 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 inter- face, 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), shmdt(2), shmget(2), compat(5) BSD
August 17, 1995 BSD