.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it) .\" .\" %%%LICENSE_START(VERBATIM) .\" 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 .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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. .\" %%%LICENSE_END .\" .\" Modified Sun Nov 28 17:06:19 1993, Rik Faith (faith@cs.unc.edu) .\" with material from Luigi P. Bai (lpb@softint.com) .\" Portions Copyright 1993 Luigi P. Bai .\" Modified Tue Oct 22 22:04:23 1996 by Eric S. Raymond .\" Modified, 5 Jan 2002, Michael Kerrisk .\" Modified, 19 Sep 2002, Michael Kerrisk .\" Added SHM_REMAP flag description .\" Modified, 27 May 2004, Michael Kerrisk .\" Added notes on capability requirements .\" Modified, 11 Nov 2004, Michael Kerrisk .\" Language and formatting clean-ups .\" Changed wording and placement of sentence regarding attachment .\" of segments marked for destruction .\" .\" FIXME . Add an example program to this page. .\" .TH SHMOP 2 2016-10-08 "Linux" "Linux Programmer's Manual" .SH NAME shmat, shmdt \- System V shared memory operations .SH SYNOPSIS .nf .B #include .B #include .BI "void *shmat(int " shmid ", const void *" shmaddr ", int " shmflg ); .BI "int shmdt(const void *" shmaddr ); .fi .SH DESCRIPTION .SS shmat() .BR shmat () attaches the System\ V shared memory segment identified by .I shmid to the address space of the calling process. The attaching address is specified by .I shmaddr with one of the following criteria: .IP * 3 If .I shmaddr is NULL, the system chooses a suitable (unused) address at which to attach the segment. .IP * If .I shmaddr isn't NULL and .B SHM_RND is specified in .IR shmflg , the attach occurs at the address equal to .I shmaddr rounded down to the nearest multiple of .BR SHMLBA . .IP * Otherwise, .I shmaddr must be a page-aligned address at which the attach occurs. .PP In addition to .BR SHM_RND , the following flags may be specified in the .I shmflg bit-mask argument: .TP .BR SHM_EXEC " (Linux-specific; since Linux 2.6.9)" Allow the contents of the segment to be executed. The caller must have execute permission on the segment. .TP .BR SHM_RDONLY Attach the segment for read-only access. The process must have read permission for the segment. If this flag is not specified, the segment is attached for read and write access, and the process must have read and write permission for the segment. There is no notion of a write-only shared memory segment. .TP .BR SHM_REMAP " (Linux-specific)" This flag specifies that the mapping of the segment should replace any existing mapping in the range starting at .I shmaddr and continuing for the size of the segment. (Normally, an .B EINVAL error would result if a mapping already exists in this address range.) In this case, .I shmaddr must not be NULL. .PP The .BR brk (2) value of the calling process is not altered by the attach. The segment will automatically be detached at process exit. The same segment may be attached as a read and as a read-write one, and more than once, in the process's address space. .PP A successful .BR shmat () call updates the members of the .I shmid_ds structure (see .BR shmctl (2)) associated with the shared memory segment as follows: .IP .I shm_atime is set to the current time. .IP .I shm_lpid is set to the process-ID of the calling process. .IP .I shm_nattch is incremented by one. .\" .SS shmdt() .BR shmdt () detaches the shared memory segment located at the address specified by .I shmaddr from the address space of the calling process. The to-be-detached segment must be currently attached with .I shmaddr equal to the value returned by the attaching .BR shmat () call. .PP On a successful .BR shmdt () call, the system updates the members of the .I shmid_ds structure associated with the shared memory segment as follows: .IP .I shm_dtime is set to the current time. .IP .I shm_lpid is set to the process-ID of the calling process. .IP .I shm_nattch is decremented by one. If it becomes 0 and the segment is marked for deletion, the segment is deleted. .SH RETURN VALUE On success, .BR shmat () returns the address of the attached shared memory segment; on error, .I (void\ *)\ \-1 is returned, and .I errno is set to indicate the cause of the error. On success, .BR shmdt () returns 0; on error \-1 is returned, and .I errno is set to indicate the cause of the error. .SH ERRORS When .BR shmat () fails, .I errno is set to one of the following: .TP .B EACCES The calling process does not have the required permissions for the requested attach type, and does not have the .B CAP_IPC_OWNER capability in the user namespace that governs its IPC namespace. .TP .B EIDRM \fIshmid\fP points to a removed identifier. .TP .B EINVAL Invalid .I shmid value, unaligned (i.e., not page-aligned and \fBSHM_RND\fP was not specified) or invalid .I shmaddr value, or can't attach segment at .IR shmaddr , or .B SHM_REMAP was specified and .I shmaddr was NULL. .TP .B ENOMEM Could not allocate memory for the descriptor or for the page tables. .PP When .BR shmdt () fails, .I errno is set as follows: .TP .B EINVAL There is no shared memory segment attached at .IR shmaddr ; or, .\" The following since 2.6.17-rc1: .I shmaddr is not aligned on a page boundary. .SH CONFORMING TO POSIX.1-2001, POSIX.1-2008, SVr4. .\" SVr4 documents an additional error condition EMFILE. In SVID 3 (or perhaps earlier), the type of the \fIshmaddr\fP argument was changed from .I "char\ *" into .IR "const void\ *" , and the returned type of .BR shmat () from .I "char\ *" into .IR "void\ *" . .SH NOTES .PP After a .BR fork (2), the child inherits the attached shared memory segments. After an .BR execve (2), all attached shared memory segments are detached from the process. Upon .BR _exit (2), all attached shared memory segments are detached from the process. Using .BR shmat () with .I shmaddr equal to NULL is the preferred, portable way of attaching a shared memory segment. Be aware that the shared memory segment attached in this way may be attached at different addresses in different processes. Therefore, any pointers maintained within the shared memory must be made relative (typically to the starting address of the segment), rather than absolute. .PP On Linux, it is possible to attach a shared memory segment even if it is already marked to be deleted. However, POSIX.1 does not specify this behavior and many other implementations do not support it. .LP The following system parameter affects .BR shmat (): .TP .B SHMLBA Segment low boundary address multiple. When explicitly specifying an attach address in a call to .BR shmat (), the caller should ensure that the address is a multiple of this value. This is necessary on some architectures, in order either to ensure good CPU cache performance or to ensure that different attaches of the same segment have consistent views within the CPU cache. .B SHMLBA is normally some multiple of the system page size (on many Linux architectures, it is the same as the system page size). .PP The implementation places no intrinsic per-process limit on the number of shared memory segments .RB ( SHMSEG ). .SH SEE ALSO .BR brk (2), .BR mmap (2), .BR shmctl (2), .BR shmget (2), .BR capabilities (7), .BR shm_overview (7), .BR svipc (7) .SH COLOPHON This page is part of release 4.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.