.\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993 .\" Portions Copyright 1993 Giorgio Ciucci .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" Modified 1993-07-28, Rik Faith .\" Modified 1993-11-28, Giorgio Ciucci .\" Modified 1997-01-31, Eric S. Raymond .\" Modified 2001-02-18, Andries Brouwer .\" Modified 2002-01-05, 2004-05-27, 2004-06-17, .\" Michael Kerrisk .\" Modified 2004-10-11, aeb .\" Modified, Nov 2004, Michael Kerrisk .\" Language and formatting clean-ups .\" Updated shmid_ds structure definitions .\" Added information on SHM_DEST and SHM_LOCKED flags .\" Noted that CAP_IPC_LOCK is not required for SHM_UNLOCK .\" since kernel 2.6.9 .\" Modified, 2004-11-25, mtk, notes on 2.6.9 RLIMIT_MEMLOCK changes .\" .TH SHMCTL 2 2004-11-10 "Linux 2.6.9" "Linux Programmer's Manual" .SH NAME shmctl \- shared memory control .SH SYNOPSIS .ad l .B #include .sp .B #include .sp .BI "int shmctl(int " shmid ", int " cmd ", struct shmid_ds *" buf ); .ad b .SH DESCRIPTION \fBshmctl()\fP allows the caller to obtain information about a shared memory segment, set the owner, group, and permissions of a segment, or destroy a segment. .PP The .I buf argument is a pointer to a \fIshmid_ds\fP structure, defined in as follows: .PP .in +4n .nf struct shmid_ds { struct ipc_perm shm_perm; /* Ownership and permissions */ size_t shm_segsz; /* Size of segment (bytes) */ time_t shm_atime; /* Last attach time */ time_t shm_dtime; /* Last detach time */ time_t shm_ctime; /* Last change time */ pid_t shm_cpid; /* PID of creator */ pid_t shm_lpid; /* PID of last stmat()/shmdt() */ shmatt_t shm_nattch; /* No. of current attaches */ ... }; .fi .in -4n .PP The .I ipc_perm structure is defined in as follows (the highlighted fields are settable using .BR IPC_SET ): .PP .in +4n .nf struct ipc_perm { key_t key; /* Key supplied to shmget() */ uid_t \fBuid\fP; /* Effective UID of owner */ gid_t \fBgid\fP; /* Effective GID of owner */ uid_t cuid; /* Effective UID of creator */ gid_t cgid; /* Effective GID of creator */ unsigned short \fBmode\fP; /* \fBPermissions\fP + SHM_DEST and SHM_LOCKED flags */ unsigned short seq; /* Sequence number */ }; .fi .in -4n .PP Valid values for .I cmd are: .br .TP 12 .B IPC_STAT Copy information from the kernel data structure associated with .I shmid into the .I shmid_ds structure pointed to by \fIbuf\fP. The caller must have read permission on the shared memory segment. .TP .B IPC_SET Write the values of some members of the .I shmid_ds structure pointed to by .I arg.buf to the kernel data structure associated with this shared memory segment, updating also its .I shm_ctime member. The following fields can be changed: \fIshm_perms.uid\fP, \fIshm_perms.gid\fP, and (the least significant 9 bits of) \fIshm_perms.mode\fP. The effective UID of the calling process must match the owner .RI ( shm_perm.uid ) or creator .RI ( shm_perm.cuid ) of the shared memory segment, or the caller must be privileged. .TP .B IPC_RMID Mark the segment to be destroyed. The segment will only actually be destroyed after the last process detaches it (i.e., when the .I shm_nattch member of the associated structure .I shmid_ds is zero). The caller must be the owner or creator, or be privileged. If a segment has been marked for destruction, then the (non-standard) .B SHM_DEST flag of the .I shm_perm.mode field in the associated data structure retrieved by .B IPC_STAT will be set. .PP The caller \fImust\fP ensure that a segment is eventually destroyed; otherwise its pages that were faulted in will remain in memory or swap. .SS "Linux additions" A privileged caller can prevent or allow swapping of a shared memory segment with the following \fIcmd\fP values: .br .TP 12 .B SHM_LOCK Prevent swapping of the shared memory segment. The caller must fault in any pages that are required to be present after locking is enabled. If a segment has been locked, then the (non-standard) .B SHM_LOCKED flag of the .I shm_perm.mode field in the associated data structure retrieved by .B IPC_STAT will be set. .TP .B SHM_UNLOCK Unlock the segment, allowing it to be swapped out. .PP The .BR IPC_INFO , .BR SHM_STAT and .B SHM_INFO control calls are used by the .BR ipcs (8) program to provide information on allocated resources. .SH "RETURN VALUE" On success, zero is returned. On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP 11 .B EACCES \fBIPC_STAT\fP or \fBSHM_STAT\fP is requested and \fIshm_perm.modes\fP does not allow read access for .IR shmid , and the calling process does not have the .BR CAP_IPC_OWNER capability. .TP .B EFAULT The argument .I cmd has value .B IPC_SET or .B IPC_STAT but the address pointed to by .I buf isn't accessible. .TP .B EIDRM \fIshmid\fP points to a removed identifier. .TP .B EINVAL \fIshmid\fP is not a valid identifier, or \fIcmd\fP is not a valid command. .TP .B ENOMEM (In kernels since 2.6.9), .B SHM_LOCK was specified and the size of the to-be-locked segment would mean that the total bytes in locked shared memory segments would exceed the limit for the real user ID of the calling process. This limit is defined by the .BR RLIMIT_MEMLOCK soft resource limit (see .BR setrlimit (2)). .TP .B EOVERFLOW \fBIPC_STAT\fP is attempted, and the gid or uid value is too large to be stored in the structure pointed to by .IR buf . .TP .B EPERM \fBIPC_SET\fP or \fBIPC_RMID\fP is attempted, and the effective user ID of the calling process is not that of the creator (found in .IR shm_perm.cuid ), or the owner (found in .IR shm_perm.uid ), and the process was not privileged (Linux: did not have the .B CAP_SYS_ADMIN capability). Or (in kernels before 2.6.9), .B SHM_LOCK or .B SHM_UNLOCK was specified, but the process was not privileged (Linux: did not have the .B CAP_IPC_LOCK capability). (Since Linux 2.6.9, this error can also occur if the .BR RLIMIT_MEMLOCK is 0 and the caller is not privileged.) .SH NOTE Various fields in a \fIstruct shmid_ds\fP were shorts under Linux 2.2 and have become longs under Linux 2.4. To take advantage of this, a recompilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by an IPC_64 flag in .IR cmd .) .SH "CONFORMING TO" SVr4, SVID. SVr4 documents additional error conditions EINVAL, ENOENT, ENOSPC, ENOMEM, EEXIST. Neither SVr4 nor SVID documents an EIDRM error condition. .SH "SEE ALSO" .BR mlock (2), .BR setrlimit (2), .BR shmget (2), .BR shmop (2), .BR capabilities (7)