Copyright (c) 1995 Frank van der Linden
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.Dd November 25, 2006 .Dt SHMCTL 2 .Os .Sh NAME .Nm shmctl .Nd shared memory control operations .Sh LIBRARY .Lb libc .Sh SYNOPSIS n sys/shm.h .Ft int .Fn shmctl "int shmid" "int cmd" "struct shmid_ds *buf" .Sh DESCRIPTION The .Fn shmctl system call performs control operations on the shared memory segment specified by .Fa shmid .
p Each shared memory segment has a .Sy shmid_ds structure associated with it which contains the following members: d -literal 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() */ .Ed
p The .Sy ipc_perm structure used inside the .Sy shmid_ds structure is defined in n sys/ipc.h and contains the following members: d -literal 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) */ .Ed
p The operation to be performed by .Fn shmctl is specified in .Fa cmd and is one of: l -tag -width IPC_RMIDX t Dv IPC_STAT Gather information about the shared memory segment and place it in the structure pointed to by .Fa buf . t Dv IPC_SET Set the value of the .Va shm_perm.uid , .Va shm_perm.gid and .Va shm_perm.mode fields in the structure associated with .Fa shmid . The values are taken from the corresponding fields in the structure pointed to by .Fa buf . This operation can only be executed by the super-user, or a process that has an effective user id equal to either .Va shm_perm.cuid or .Va shm_perm.uid in the data structure associated with the shared memory segment. t Dv IPC_RMID Remove the shared memory segment specified by .Fa shmid and destroy the data associated with it. Only the super-user or a process with an effective uid equal to the .Va shm_perm.cuid or .Va shm_perm.uid values in the data structure associated with the segment can do this. t Dv SHM_LOCK Lock the shared memory segment specified by .Fa shmid in memory. This operation can only be executed by the super-user. t Dv SHM_UNLOCK Unlock the shared memory segment specified by .Fa shmid . This operation can only be executed by the super-user. .El
p The read and write permissions on a shared memory identifier are determined by the .Va shm_perm.mode field in the same way as is done with files (see .Xr chmod 2 ) , but the effective uid can match either the .Va shm_perm.cuid field or the .Va shm_perm.uid field, and the effective gid can match either .Va shm_perm.cgid or .Va shm_perm.gid . .Sh RETURN VALUES Upon successful completion, a value of 0 is returned. Otherwise, -1 is returned and the global variable .Va errno is set to indicate the error. .Sh ERRORS .Fn shmctl will fail if: l -tag -width Er t Bq Er EACCES The command is .Dv IPC_STAT and the caller has no read permission for this shared memory segment. t Bq Er EFAULT .Fa buf specifies an invalid address. t Bq Er EINVAL .Fa shmid is not a valid shared memory segment identifier.
p .Va cmd is not a valid command. t Bq Er ENOMEM The .Fa cmd is equal to .Dv SHM_LOCK and there is not enough physical memory. t Bq Er EPERM .Fa cmd is equal to .Dv IPC_SET or .Dv IPC_RMID and the caller is not the super-user, nor does the effective uid match either the .Va shm_perm.uid or .Va shm_perm.cuid fields of the data structure associated with the shared memory segment.
p An attempt was made to increase the value of .Va shm_qbytes through .Dv IPC_SET but the caller is not the super-user.
p The .Fa cmd is equal to .Dv SHM_LOCK or .Dv SHM_UNLOCK and the caller is not the super-user. .El .Sh SEE ALSO .Xr ipcrm 1 , .Xr ipcs 1 , .Xr shmat 2 , .Xr shmget 2 .Sh STANDARDS The .Nm system call conforms to .St -xsh5 . .Sh HISTORY Shared memory segments appeared in the first release of .At V .
Indexes created Sun Sep 21 14:09:52 GMT 2025