1SHMGET(2)                  Linux Programmer's Manual                 SHMGET(2)
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NAME

6       shmget - allocates a System V shared memory segment
7

SYNOPSIS

9       #include <sys/shm.h>
10
11       int shmget(key_t key, size_t size, int shmflg);
12

DESCRIPTION

14       shmget()  returns  the identifier of the System V shared memory segment
15       associated with the value of the argument key.  It may be  used  either
16       to  obtain the identifier of a previously created shared memory segment
17       (when shmflg is zero and key does not have the value  IPC_PRIVATE),  or
18       to create a new set.
19
20       A  new  shared  memory  segment,  with  size equal to the value of size
21       rounded up to a multiple of PAGE_SIZE, is created if key has the  value
22       IPC_PRIVATE  or  key isn't IPC_PRIVATE, no shared memory segment corre‐
23       sponding to key exists, and IPC_CREAT is specified in shmflg.
24
25       If shmflg specifies both IPC_CREAT and IPC_EXCL  and  a  shared  memory
26       segment  already  exists for key, then shmget() fails with errno set to
27       EEXIST.  (This is analogous to the effect of the combination O_CREAT  |
28       O_EXCL for open(2).)
29
30       The value shmflg is composed of:
31
32       IPC_CREAT
33              Create  a  new segment.  If this flag is not used, then shmget()
34              will find the segment associated with key and check  to  see  if
35              the user has permission to access the segment.
36
37       IPC_EXCL
38              This  flag  is used with IPC_CREAT to ensure that this call cre‐
39              ates the segment.  If  the  segment  already  exists,  the  call
40              fails.
41
42       SHM_HUGETLB (since Linux 2.6)
43              Allocate  the  segment using "huge" pages.  See the Linux kernel
44              source  file  Documentation/admin-guide/mm/hugetlbpage.rst   for
45              further information.
46
47       SHM_HUGE_2MB, SHM_HUGE_1GB (since Linux 3.8)
48              Used  in  conjunction  with  SHM_HUGETLB  to  select alternative
49              hugetlb page sizes (respectively, 2 MB and 1 GB) on systems that
50              support multiple hugetlb page sizes.
51
52              More  generally, the desired huge page size can be configured by
53              encoding the base-2 logarithm of the desired page  size  in  the
54              six bits at the offset SHM_HUGE_SHIFT.  Thus, the above two con‐
55              stants are defined as:
56
57                  #define SHM_HUGE_2MB    (21 << SHM_HUGE_SHIFT)
58                  #define SHM_HUGE_1GB    (30 << SHM_HUGE_SHIFT)
59
60              For some additional details, see the discussion of the similarly
61              named constants in mmap(2).
62
63       SHM_NORESERVE (since Linux 2.6.15)
64              This  flag  serves the same purpose as the mmap(2) MAP_NORESERVE
65              flag.  Do not reserve swap space for this  segment.   When  swap
66              space  is reserved, one has the guarantee that it is possible to
67              modify the segment.  When swap space is not reserved  one  might
68              get  SIGSEGV  upon  a  write if no physical memory is available.
69              See also the discussion of the file /proc/sys/vm/overcommit_mem‐
70              ory in proc(5).
71
72       In  addition to the above flags, the least significant 9 bits of shmflg
73       specify the permissions granted to the owner, group, and others.  These
74       bits  have  the same format, and the same meaning, as the mode argument
75       of open(2).  Presently, execute permissions are not used by the system.
76
77       When a new shared memory segment is created, its contents are  initial‐
78       ized  to  zero values, and its associated data structure, shmid_ds (see
79       shmctl(2)), is initialized as follows:
80
81shm_perm.cuid and shm_perm.uid are set to the effective  user  ID  of
82         the calling process.
83
84shm_perm.cgid  and  shm_perm.gid are set to the effective group ID of
85         the calling process.
86
87       • The least significant 9 bits of shm_perm.mode are set  to  the  least
88         significant 9 bit of shmflg.
89
90shm_segsz is set to the value of size.
91
92shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.
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94shm_ctime is set to the current time.
95
96       If  the shared memory segment already exists, the permissions are veri‐
97       fied, and a check is made to see if it is marked for destruction.
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RETURN VALUE

100       On success, a valid shared memory identifier is returned.  On error, -1
101       is returned, and errno is set to indicate the error.
102

ERRORS

104       EACCES The  user  does  not have permission to access the shared memory
105              segment, and does not have the CAP_IPC_OWNER capability  in  the
106              user namespace that governs its IPC namespace.
107
108       EEXIST IPC_CREAT  and  IPC_EXCL  were specified in shmflg, but a shared
109              memory segment already exists for key.
110
111       EINVAL A new segment was to be created and size is less than SHMMIN  or
112              greater than SHMMAX.
113
114       EINVAL A segment for the given key exists, but size is greater than the
115              size of that segment.
116
117       ENFILE The system-wide limit on the total number of open files has been
118              reached.
119
120       ENOENT No segment exists for the given key, and IPC_CREAT was not spec‐
121              ified.
122
123       ENOMEM No memory could be allocated for segment overhead.
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125       ENOSPC All possible shared memory IDs have been taken (SHMMNI), or  al‐
126              locating  a segment of the requested size would cause the system
127              to exceed the system-wide limit on shared memory (SHMALL).
128
129       EPERM  The SHM_HUGETLB flag was specified, but the caller was not priv‐
130              ileged  (did  not have the CAP_IPC_LOCK capability) and is not a
131              member of the sysctl_hugetlb_shm_group group; see  the  descrip‐
132              tion of /proc/sys/vm/sysctl_hugetlb_shm_group in proc(5).
133

CONFORMING TO

135       POSIX.1-2001, POSIX.1-2008, SVr4.
136
137       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.
138

NOTES

140       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value
141       is used for key, the system call ignores all but the least  significant
142       9 bits of shmflg and creates a new shared memory segment.
143
144   Shared memory limits
145       The  following  limits  on  shared  memory segment resources affect the
146       shmget() call:
147
148       SHMALL System-wide limit on the total amount of shared memory, measured
149              in units of the system page size.
150
151              On Linux, this limit can be read and modified via /proc/sys/ker‐
152              nel/shmall.  Since Linux 3.16, the default value for this  limit
153              is:
154
155                  ULONG_MAX - 2^24
156
157              The  effect of this value (which is suitable for both 32-bit and
158              64-bit systems) is to impose no limitation on allocations.  This
159              value,  rather than ULONG_MAX, was chosen as the default to pre‐
160              vent some cases where historical applications simply raised  the
161              existing  limit  without first checking its current value.  Such
162              applications would cause the value to overflow if the limit  was
163              set at ULONG_MAX.
164
165              From  Linux  2.4  up  to  Linux 3.15, the default value for this
166              limit was:
167
168                  SHMMAX / PAGE_SIZE * (SHMMNI / 16)
169
170              If SHMMAX and SHMMNI were not modified, then multiplying the re‐
171              sult  of this formula by the page size (to get a value in bytes)
172              yielded a value of 8 GB as the limit on the total memory used by
173              all shared memory segments.
174
175       SHMMAX Maximum size in bytes for a shared memory segment.
176
177              On Linux, this limit can be read and modified via /proc/sys/ker‐
178              nel/shmmax.  Since Linux 3.16, the default value for this  limit
179              is:
180
181                  ULONG_MAX - 2^24
182
183              The  effect of this value (which is suitable for both 32-bit and
184              64-bit systems) is to impose no limitation on allocations.   See
185              the  description  of SHMALL for a discussion of why this default
186              value (rather than ULONG_MAX) is used.
187
188              From Linux 2.2 up to Linux 3.15, the default value of this limit
189              was 0x2000000 (32 MB).
190
191              Because  it  is not possible to map just part of a shared memory
192              segment, the amount of virtual memory places  another  limit  on
193              the  maximum  size of a usable segment: for example, on i386 the
194              largest segments that can  be  mapped  have  a  size  of  around
195              2.8 GB, and on x86-64 the limit is around 127 TB.
196
197       SHMMIN Minimum  size  in bytes for a shared memory segment: implementa‐
198              tion dependent (currently 1 byte, though PAGE_SIZE is the effec‐
199              tive minimum size).
200
201       SHMMNI System-wide  limit  on the number of shared memory segments.  In
202              Linux 2.2, the default value for this limit was 128; since Linux
203              2.4, the default value is 4096.
204
205              On Linux, this limit can be read and modified via /proc/sys/ker‐
206              nel/shmmni.
207
208       The implementation has no specific limits for the  per-process  maximum
209       number of shared memory segments (SHMSEG).
210
211   Linux notes
212       Until  version  2.3.30,  Linux  would  return EIDRM for a shmget() on a
213       shared memory segment scheduled for deletion.
214

BUGS

216       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
217       clearly show its function.
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EXAMPLES

220       See shmop(2).
221

SEE ALSO

223       memfd_create(2),  shmat(2),  shmctl(2),  shmdt(2),  ftok(3),  capabili‐
224       ties(7), shm_overview(7), sysvipc(7)
225

COLOPHON

227       This page is part of release 5.13 of the Linux  man-pages  project.   A
228       description  of  the project, information about reporting bugs, and the
229       latest    version    of    this    page,    can     be     found     at
230       https://www.kernel.org/doc/man-pages/.
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234Linux                             2021-03-22                         SHMGET(2)
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