shmctl(2) - NetBSD Manual Pages

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SHMCTL(2)                 NetBSD System Calls Manual                 SHMCTL(2)


NAME
shmctl -- shared memory control operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/shm.h> int shmctl(int shmid, int cmd, struct shmid_ds *buf);
DESCRIPTION
The shmctl() system call performs control operations on the shared memory segment specified by shmid. Each shared memory segment has a shmid_ds structure associated with it which contains the following members: 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 */ shmatt_t 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() */ The ipc_perm structure used inside the shmid_ds structure is defined in <sys/ipc.h> and contains the following members: uid_t cuid; /* creator user id */ gid_t cgid; /* creator group id */ uid_t uid; /* user id */ gid_t gid; /* group id */ mode_t mode; /* permission (lower 9 bits) */ 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 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 segment can do this. SHM_LOCK Lock the shared memory segment specified by shmid in memory. This operation can only be executed by the super-user. SHM_UNLOCK Unlock the shared memory segment specified by shmid. This operation can only be executed by the super-user. The read and write permissions on a shared memory identifier are deter- 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. [ENOMEM] The cmd is equal to SHM_LOCK and there is not enough physical memory. [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. An attempt was made to increase the value of shm_qbytes through IPC_SET but the caller is not the super-user. The cmd is equal to SHM_LOCK or SHM_UNLOCK and the caller is not the super-user.
SEE ALSO
ipcrm(1), ipcs(1), shmat(2), shmget(2)
STANDARDS
The shmctl system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5'').
HISTORY
Shared memory segments appeared in the first release of AT&T System V UNIX. NetBSD 9.99 November 25, 2006 NetBSD 9.99
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