| .\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993 |
| .\" |
| .\" %%%LICENSE_START(VERBATIM) |
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| .\" Since the Linux kernel and libraries are constantly changing, this |
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| .\" Modified Wed Jul 28 10:57:35 1993, Rik Faith <faith@cs.unc.edu> |
| .\" Modified Sun Nov 28 16:43:30 1993, Rik Faith <faith@cs.unc.edu> |
| .\" with material from Giorgio Ciucci <giorgio@crcc.it> |
| .\" Portions Copyright 1993 Giorgio Ciucci <giorgio@crcc.it> |
| .\" Modified Tue Oct 22 22:03:17 1996 by Eric S. Raymond <esr@thyrsus.com> |
| .\" Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpages@gmail.com> |
| .\" Removed EIDRM from errors - that can't happen... |
| .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com> |
| .\" Added notes on capability requirements |
| .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com> |
| .\" Language and formatting clean-ups |
| .\" Added notes on /proc files |
| .\" |
| .TH SHMGET 2 2020-04-11 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| shmget \- allocates a System V shared memory segment |
| .SH SYNOPSIS |
| .ad l |
| .B #include <sys/ipc.h> |
| .br |
| .B #include <sys/shm.h> |
| .PP |
| .BI "int shmget(key_t " key ", size_t " size ", int " shmflg ); |
| .ad b |
| .SH DESCRIPTION |
| .BR shmget () |
| returns the identifier of the System\ V shared memory segment |
| associated with the value of the argument |
| .IR key . |
| It may be used either to obtain the identifier of a previously created |
| shared memory segment (when |
| .I shmflg |
| is zero and |
| .I key |
| does not have the value |
| .BR IPC_PRIVATE ), |
| or to create a new set. |
| .PP |
| 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 |
| .B IPC_CREAT |
| 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 |
| This flag is used with |
| .B IPC_CREAT |
| to ensure that this call creates the segment. |
| If the segment already exists, the call fails. |
| .TP |
| .BR SHM_HUGETLB " (since Linux 2.6)" |
| Allocate the segment using "huge pages." |
| See the Linux kernel source file |
| .I Documentation/admin-guide/mm/hugetlbpage.rst |
| for further information. |
| .TP |
| .BR SHM_HUGE_2MB ", " SHM_HUGE_1GB " (since Linux 3.8)" |
| .\" See https://lwn.net/Articles/533499/ |
| Used in conjunction with |
| .B SHM_HUGETLB |
| to select alternative hugetlb page sizes (respectively, 2\ MB and 1\ GB) |
| on systems that support multiple hugetlb page sizes. |
| .IP |
| More generally, the desired huge page size can be configured by encoding |
| the base-2 logarithm of the desired page size in the six bits at the offset |
| .BR SHM_HUGE_SHIFT . |
| Thus, the above two constants are defined as: |
| .IP |
| .in +4 |
| .EX |
| #define SHM_HUGE_2MB (21 << SHM_HUGE_SHIFT) |
| #define SHM_HUGE_1GB (30 << SHM_HUGE_SHIFT) |
| .EE |
| .in |
| .IP |
| For some additional details, |
| see the discussion of the similarly named constants in |
| .BR mmap (2). |
| .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 |
| In addition to the above flags, the least significant 9 bits of |
| .I shmflg |
| specify the permissions granted to the owner, group, and others. |
| These bits have the same format, and the same |
| meaning, as the |
| .I mode |
| argument of |
| .BR open (2). |
| Presently, execute permissions are not used by the system. |
| .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 \(bu 2 |
| .I shm_perm.cuid |
| and |
| .I shm_perm.uid |
| are set to the effective user ID of the calling process. |
| .IP \(bu |
| .I shm_perm.cgid |
| and |
| .I shm_perm.gid |
| are set to the effective group ID of the calling process. |
| .IP \(bu |
| The least significant 9 bits of |
| .I shm_perm.mode |
| are set to the least significant 9 bit of |
| .IR shmflg . |
| .IP \(bu |
| .I shm_segsz |
| is set to the value of |
| .IR size . |
| .IP \(bu |
| .IR shm_lpid , |
| .IR shm_nattch , |
| .IR shm_atime , |
| and |
| .I shm_dtime |
| are set to 0. |
| .IP \(bu |
| .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 |
| On success, a valid shared memory identifier is returned. |
| On error, \-1 is returned, and |
| .I errno |
| is set to indicate the 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 in the user namespace that governs its IPC namespace. |
| .TP |
| .B EEXIST |
| .BR IPC_CREAT |
| and |
| .BR IPC_EXCL |
| were specified in |
| .IR shmflg , |
| but a shared memory segment already exists for |
| .IR key . |
| .TP |
| .B EINVAL |
| A new segment was to be created and |
| .I size |
| is less than |
| .B SHMMIN |
| or greater than |
| .BR SHMMAX . |
| .TP |
| .B EINVAL |
| A segment for the given |
| .I key |
| exists, 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-wide 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 |
| POSIX.1-2001, POSIX.1-2008, SVr4. |
| .\" SVr4 documents an additional error condition EEXIST. |
| .PP |
| .B SHM_HUGETLB |
| and |
| .B SHM_NORESERVE |
| are Linux extensions. |
| .SH NOTES |
| The inclusion of |
| .I <sys/types.h> |
| and |
| .I <sys/ipc.h> |
| 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. |
| .PP |
| .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 all but the least significant 9 bits of |
| .I shmflg |
| and creates a new shared memory segment. |
| .\" |
| .SS Shared memory limits |
| The following limits on shared memory segment resources affect the |
| .BR shmget () |
| call: |
| .TP |
| .B SHMALL |
| System-wide limit on the total amount of shared memory, |
| measured in units of the system page size. |
| .IP |
| On Linux, this limit can be read and modified via |
| .IR /proc/sys/kernel/shmall . |
| Since Linux 3.16, |
| .\" commit 060028bac94bf60a65415d1d55a359c3a17d5c31 |
| the default value for this limit is: |
| .IP |
| ULONG_MAX - 2^24 |
| .IP |
| The effect of this value |
| (which is suitable for both 32-bit and 64-bit systems) |
| is to impose no limitation on allocations. |
| This value, rather than |
| .BR ULONG_MAX , |
| was chosen as the default to prevent some cases where historical |
| applications simply raised the existing limit without first checking |
| its current value. |
| Such applications would cause the value to overflow if the limit was set at |
| .BR ULONG_MAX . |
| .IP |
| From Linux 2.4 up to Linux 3.15, |
| the default value for this limit was: |
| .IP |
| SHMMAX / PAGE_SIZE * (SHMMNI / 16) |
| .IP |
| If |
| .B SHMMAX |
| and |
| .B SHMMNI |
| were not modified, then multiplying the result of this formula |
| by the page size (to get a value in bytes) yielded a value of 8\ GB |
| as the limit on the total memory used by all shared memory segments. |
| .TP |
| .B SHMMAX |
| Maximum size in bytes for a shared memory segment. |
| .IP |
| On Linux, this limit can be read and modified via |
| .IR /proc/sys/kernel/shmmax . |
| Since Linux 3.16, |
| .\" commit 060028bac94bf60a65415d1d55a359c3a17d5c31 |
| the default value for this limit is: |
| .IP |
| ULONG_MAX - 2^24 |
| .IP |
| The effect of this value |
| (which is suitable for both 32-bit and 64-bit systems) |
| is to impose no limitation on allocations. |
| See the description of |
| .BR SHMALL |
| for a discussion of why this default value (rather than |
| .BR ULONG_MAX ) |
| is used. |
| .IP |
| From Linux 2.2 up to Linux 3.15, the default value of |
| this limit was 0x2000000 (32\ MB). |
| .IP |
| Because it is not possible to map just part of a shared memory segment, |
| the amount of virtual memory places another limit on the maximum size of a |
| usable segment: |
| for example, on i386 the largest segments that can be mapped have a |
| size of around 2.8\ GB, and on x86-64 the limit is around 127 TB. |
| .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 limit on the number of shared memory segments. |
| In Linux 2.2, the default value for this limit was 128; |
| since Linux 2.4, the default value is 4096. |
| .IP |
| 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 EXAMPLES |
| See |
| .BR shmop (2). |
| .SH SEE ALSO |
| .BR memfd_create (2), |
| .BR shmat (2), |
| .BR shmctl (2), |
| .BR shmdt (2), |
| .BR ftok (3), |
| .BR capabilities (7), |
| .BR shm_overview (7), |
| .BR sysvipc (7) |