.\" 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 .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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 10:57:35 1993, Rik Faith .\" Modified Sun Nov 28 16:43:30 1993, Rik Faith .\" with material from Giorgio Ciucci .\" Portions Copyright 1993 Giorgio Ciucci .\" Modified Tue Oct 22 22:03:17 1996 by Eric S. Raymond .\" Modified, 8 Jan 2003, Michael Kerrisk, .\" Removed EIDRM from errors - that can't happen... .\" Modified, 27 May 2004, Michael Kerrisk .\" Added notes on capability requirements .\" Modified, 11 Nov 2004, Michael Kerrisk .\" Language and formatting clean-ups .\" Added notes on /proc files .\" .TH SHMGET 2 2012-05-31 "Linux" "Linux Programmer's Manual" .SH NAME shmget \- allocates a shared memory segment .SH SYNOPSIS .ad l .B #include .br .B #include .sp .BI "int shmget(key_t " key ", size_t " size ", int " shmflg ); .ad b .SH DESCRIPTION .BR shmget () returns the identifier of the shared memory segment associated with the value of the argument .IR key . A new shared memory segment, with size equal to the value of .I size rounded up to a multiple of .BR PAGE_SIZE , is created if .I key has the value .B IPC_PRIVATE or .I key isn't .BR IPC_PRIVATE , no shared memory segment corresponding to .I key exists, and .B IPC_CREAT is specified in .IR shmflg . .PP If .I shmflg specifies both .B IPC_CREAT and .B IPC_EXCL and a shared memory segment already exists for .IR key , then .BR shmget () fails with .I errno set to .BR EEXIST . (This is analogous to the effect of the combination .B O_CREAT | O_EXCL for .BR open (2).) .PP The value .I shmflg is composed of: .TP 12 .B IPC_CREAT to create a new segment. If this flag is not used, then .BR shmget () will find the segment associated with \fIkey\fP and check to see if the user has permission to access the segment. .TP .B IPC_EXCL used with \fBIPC_CREAT\fP to ensure failure if the segment already exists. .TP .I mode_flags (least significant 9 bits) specifying the permissions granted to the owner, group, and world. These bits have the same format, and the same meaning, as the .I mode argument of .BR open (2). Presently, the execute permissions are not used by the system. .TP .BR SHM_HUGETLB " (since Linux 2.6)" Allocate the segment using "huge pages." See the Linux kernel source file .I Documentation/vm/hugetlbpage.txt for further information. .TP .BR SHM_NORESERVE " (since Linux 2.6.15)" This flag serves the same purpose as the .BR mmap (2) .B MAP_NORESERVE flag. Do not reserve swap space for this segment. When swap space is reserved, one has the guarantee that it is possible to modify the segment. When swap space is not reserved one might get .B SIGSEGV upon a write if no physical memory is available. See also the discussion of the file .I /proc/sys/vm/overcommit_memory in .BR proc (5). .\" As at 2.6.17-rc2, this flag has no effect if SHM_HUGETLB was also .\" specified. .PP When a new shared memory segment is created, its contents are initialized to zero values, and its associated data structure, .I shmid_ds (see .BR shmctl (2)), is initialized as follows: .IP .I shm_perm.cuid and .I shm_perm.uid are set to the effective user ID of the calling process. .IP .I shm_perm.cgid and .I shm_perm.gid are set to the effective group ID of the calling process. .IP The least significant 9 bits of .I shm_perm.mode are set to the least significant 9 bit of .IR shmflg . .IP .I shm_segsz is set to the value of .IR size . .IP .IR shm_lpid , .IR shm_nattch , .I shm_atime and .I shm_dtime are set to 0. .IP .I shm_ctime is set to the current time. .PP If the shared memory segment already exists, the permissions are verified, and a check is made to see if it is marked for destruction. .SH "RETURN VALUE" A valid segment identifier, .IR shmid , is returned on success, \-1 on error. .SH ERRORS On failure, .I errno is set to one of the following: .TP .B EACCES The user does not have permission to access the shared memory segment, and does not have the .B CAP_IPC_OWNER capability. .TP .B EEXIST .B IPC_CREAT | IPC_EXCL was specified and the segment exists. .TP .B EINVAL A new segment was to be created and \fIsize\fP < \fBSHMMIN\fP or \fIsize\fP > \fBSHMMAX\fP, or no new segment was to be created, a segment with given key existed, but \fIsize\fP is greater than the size of that segment. .TP .B ENFILE .\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp() The system limit on the total number of open files has been reached. .TP .B ENOENT No segment exists for the given \fIkey\fP, and .B IPC_CREAT was not specified. .TP .B ENOMEM No memory could be allocated for segment overhead. .TP .B ENOSPC All possible shared memory IDs have been taken .RB ( SHMMNI ), or allocating a segment of the requested .I size would cause the system to exceed the system-wide limit on shared memory .RB ( SHMALL ). .TP .B EPERM The .B SHM_HUGETLB flag was specified, but the caller was not privileged (did not have the .B CAP_IPC_LOCK capability). .SH "CONFORMING TO" SVr4, POSIX.1-2001. .\" SVr4 documents an additional error condition EEXIST. .B SHM_HUGETLB is a nonportable Linux extension. .SH NOTES The inclusion of .I and .I isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files. .\" Like Linux, the FreeBSD man pages still document .\" the inclusion of these header files. .B IPC_PRIVATE isn't a flag field but a .I key_t type. If this special value is used for .IR key , the system call ignores everything but the least significant 9 bits of .I shmflg and creates a new shared memory segment (on success). .PP The following limits on shared memory segment resources affect the .BR shmget () call: .TP .B SHMALL System wide maximum of shared memory pages (on Linux, this limit can be read and modified via .IR /proc/sys/kernel/shmall ). .TP .B SHMMAX Maximum size in bytes for a shared memory segment: policy dependent (on Linux, this limit can be read and modified via .IR /proc/sys/kernel/shmmax ). .TP .B SHMMIN Minimum size in bytes for a shared memory segment: implementation dependent (currently 1 byte, though .B PAGE_SIZE is the effective minimum size). .TP .B SHMMNI System wide maximum number of shared memory segments: implementation dependent (currently 4096, was 128 before Linux 2.3.99; on Linux, this limit can be read and modified via .IR /proc/sys/kernel/shmmni ). .\" Kernels between 2.4.x and 2.6.8 had an off-by-one error that meant .\" that we could create one more segment than SHMMNI -- MTK .\" This /proc file is not available in Linux 2.2 and earlier -- MTK .PP The implementation has no specific limits for the per-process maximum number of shared memory segments .RB ( SHMSEG ). .SS Linux Notes Until version 2.3.30 Linux would return .B EIDRM for a .BR shmget () on a shared memory segment scheduled for deletion. .SH BUGS The name choice .B IPC_PRIVATE was perhaps unfortunate, .B IPC_NEW would more clearly show its function. .SH "SEE ALSO" .BR shmat (2), .BR shmctl (2), .BR shmdt (2), .BR ftok (3), .BR capabilities (7), .BR shm_overview (7), .BR svipc (7) .SH COLOPHON This page is part of release 3.44 of the Linux .I man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.