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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 exe-cuted executed cuted 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 deter-mined determined mined 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 seg-ment. segment. ment. 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 pre-vious previous vious 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"), soft-ware software ware using the LEGACY API will not behave as expected. SEE ALSO shmat(2), shmdt(2), shmget(2), compat(5) BSD August 17, 1995 BSD |