.\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993 .\" .\" 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this one .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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 Wed Jul 28 11:03:24 1993, Rik Faith .\" Modified Sun Nov 28 16:43:30 1993, Rik Faith .\" with material from Giorgio Ciucci .\" Portions Copyright 1993 Giorgio Ciucci .\" Modified Fri Jan 31 17:24:31 1997 by Eric S. Raymond .\" .TH SHMCTL 2 "November 28, 1993" "Linux 0.99.11" "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 user to receive information on a shared memory segment, set the owner, group, and permissions of a shared memory segment, or destroy a segment. The information about the segment identified by \fIshmid\fP is returned in a \fIshmid_ds\fP structure: .PP .in +0.5i .nf struct shmid_ds { struct ipc_perm shm_perm; /* operation perms */ int 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 */ unsigned short shm_cpid; /* pid of creator */ unsigned short shm_lpid; /* pid of last operator */ short shm_nattch; /* no. of current attaches */ /* the following are private */ unsigned short shm_npages; /* size of segment (pages) */ unsigned long *shm_pages; struct shm_desc *attaches; /* descriptors for attaches */ }; .fi .in -0.5i .PP The fields in the member \fIshm_perm\fP can be set: .PP .in +0.5i .nf struct ipc_perm { key_t key; \fBushort uid\fP; /* \fBowner\fP euid and egid */ \fBushort gid\fP; ushort cuid; /* creator euid and egid */ ushort cgid; \fBushort mode\fP; /* lower 9 bits of access modes */ ushort seq; /* sequence number */ }; .fi .PP The following \fIcmds\fP are available: .br .TP 12 .B IPC_STAT is used to copy the information about the shared memory segment into the buffer \fIbuf\fP. The user must have \fBread\fP access to the shared memory segment. .TP .B IPC_SET is used to apply the changes the user has made to the \fIuid\fP, \fIgid\fP, or \fImode\fP members of the \fIshm_perms\fP field. Only the lowest 9 bits of \fImode\fP are used. The .I shm_ctime member is also updated. The user must be the owner, creator, or the super-user. .TP .B IPC_RMID is used to mark the segment as destroyed. It will actually be destroyed after the last detach. (I.e., when the .I shm_nattch member of the associated structure .I shmid_ds is zero.) The user must be the owner, creator, or the super-user. .PP The user \fImust\fP ensure that a segment is eventually destroyed; otherwise its pages that were faulted in will remain in memory or swap. .PP In addition, the \fBsuper-user\fP can prevent or allow swapping of a shared memory segment with the following \fIcmds\fP: (Linux only) .br .TP 12 .B SHM_LOCK prevents swapping of a shared memory segment. The user must fault in any pages that are required to be present after locking is enabled. .TP .B SHM_UNLOCK allows the shared memory segment 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. In the future, these man be modified as needed or moved to a proc file system interface. .PP .SH "SYSTEM CALLS" .TP .B fork() After a .B fork() the child inherits the attached shared memory segments. .TP .B exec() After an .B exec() all attached shared memory segments are detached (not destroyed). .TP .B exit() Upon .B exit() all attached shared memory segments are detached (not destroyed). .PP .SH "RETURN VALUE" 0 is returned on success, \-1 on error. .SH ERRORS On error, .B errno will be set to one of the following: .TP 12 .B EACCES is returned if \fBIPC_STAT\fP is requested and \fIshm_perm.modes\fP does not allow read access for .IR msqid . .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 EINVAL is returned if \fIshmid\fP is not a valid identifier, or \fIcmd\fP is not a valid command. .TP .B EIDRM is returned if \fIshmid\fP points to a removed identifier. .TP .B EPERM is returned if \fBIPC_SET\fP or \fBIPC_RMID\fP is attempted, and the user is not the creator, the owner, or the super-user, and the user does not have permission granted to their group or to the world. .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 shmget "(2), " shmop (2)