1shmget(2)                     System Calls Manual                    shmget(2)
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NAME

6       shmget - allocates a System V shared memory segment
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LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <sys/shm.h>
13
14       int shmget(key_t key, size_t size, int shmflg);
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DESCRIPTION

17       shmget()  returns  the identifier of the System V shared memory segment
18       associated with the value of the argument key.  It may be  used  either
19       to  obtain the identifier of a previously created shared memory segment
20       (when shmflg is zero and key does not have the value  IPC_PRIVATE),  or
21       to create a new set.
22
23       A  new  shared  memory  segment,  with  size equal to the value of size
24       rounded up to a multiple of PAGE_SIZE, is created if key has the  value
25       IPC_PRIVATE  or  key isn't IPC_PRIVATE, no shared memory segment corre‐
26       sponding to key exists, and IPC_CREAT is specified in shmflg.
27
28       If shmflg specifies both IPC_CREAT and IPC_EXCL  and  a  shared  memory
29       segment  already  exists for key, then shmget() fails with errno set to
30       EEXIST.  (This is analogous to the effect of the combination O_CREAT  |
31       O_EXCL for open(2).)
32
33       The value shmflg is composed of:
34
35       IPC_CREAT
36              Create  a  new segment.  If this flag is not used, then shmget()
37              will find the segment associated with key and check  to  see  if
38              the user has permission to access the segment.
39
40       IPC_EXCL
41              This  flag  is used with IPC_CREAT to ensure that this call cre‐
42              ates the segment.  If  the  segment  already  exists,  the  call
43              fails.
44
45       SHM_HUGETLB (since Linux 2.6)
46              Allocate  the  segment using "huge" pages.  See the Linux kernel
47              source  file  Documentation/admin-guide/mm/hugetlbpage.rst   for
48              further information.
49
50       SHM_HUGE_2MB, SHM_HUGE_1GB (since Linux 3.8)
51              Used  in  conjunction  with  SHM_HUGETLB  to  select alternative
52              hugetlb page sizes (respectively, 2 MB and 1 GB) on systems that
53              support multiple hugetlb page sizes.
54
55              More  generally, the desired huge page size can be configured by
56              encoding the base-2 logarithm of the desired page  size  in  the
57              six bits at the offset SHM_HUGE_SHIFT.  Thus, the above two con‐
58              stants are defined as:
59
60                  #define SHM_HUGE_2MB    (21 << SHM_HUGE_SHIFT)
61                  #define SHM_HUGE_1GB    (30 << SHM_HUGE_SHIFT)
62
63              For some additional details, see the discussion of the similarly
64              named constants in mmap(2).
65
66       SHM_NORESERVE (since Linux 2.6.15)
67              This  flag  serves the same purpose as the mmap(2) MAP_NORESERVE
68              flag.  Do not reserve swap space for this  segment.   When  swap
69              space  is reserved, one has the guarantee that it is possible to
70              modify the segment.  When swap space is not reserved  one  might
71              get  SIGSEGV  upon  a  write if no physical memory is available.
72              See also the discussion of the file /proc/sys/vm/overcommit_mem‐
73              ory in proc(5).
74
75       In  addition to the above flags, the least significant 9 bits of shmflg
76       specify the permissions granted to the owner, group, and others.  These
77       bits  have  the same format, and the same meaning, as the mode argument
78       of open(2).  Presently, execute permissions are not used by the system.
79
80       When a new shared memory segment is created, its contents are  initial‐
81       ized  to  zero values, and its associated data structure, shmid_ds (see
82       shmctl(2)), is initialized as follows:
83
84shm_perm.cuid and shm_perm.uid are set to the effective user  ID  of
85          the calling process.
86
87shm_perm.cgid  and shm_perm.gid are set to the effective group ID of
88          the calling process.
89
90       •  The least significant 9 bits of shm_perm.mode are set to  the  least
91          significant 9 bit of shmflg.
92
93shm_segsz is set to the value of size.
94
95shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.
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97shm_ctime is set to the current time.
98
99       If  the shared memory segment already exists, the permissions are veri‐
100       fied, and a check is made to see if it is marked for destruction.
101

RETURN VALUE

103       On success, a valid shared memory identifier is returned.  On error, -1
104       is returned, and errno is set to indicate the error.
105

ERRORS

107       EACCES The  user  does  not have permission to access the shared memory
108              segment, and does not have the CAP_IPC_OWNER capability  in  the
109              user namespace that governs its IPC namespace.
110
111       EEXIST IPC_CREAT  and  IPC_EXCL  were specified in shmflg, but a shared
112              memory segment already exists for key.
113
114       EINVAL A new segment was to be created and size is less than SHMMIN  or
115              greater than SHMMAX.
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117       EINVAL A segment for the given key exists, but size is greater than the
118              size of that segment.
119
120       ENFILE The system-wide limit on the total number of open files has been
121              reached.
122
123       ENOENT No segment exists for the given key, and IPC_CREAT was not spec‐
124              ified.
125
126       ENOMEM No memory could be allocated for segment overhead.
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128       ENOSPC All possible shared memory IDs have been taken (SHMMNI), or  al‐
129              locating  a segment of the requested size would cause the system
130              to exceed the system-wide limit on shared memory (SHMALL).
131
132       EPERM  The SHM_HUGETLB flag was specified, but the caller was not priv‐
133              ileged  (did  not have the CAP_IPC_LOCK capability) and is not a
134              member of the sysctl_hugetlb_shm_group group; see  the  descrip‐
135              tion of /proc/sys/vm/sysctl_hugetlb_shm_group in proc(5).
136

STANDARDS

138       POSIX.1-2008.
139
140       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.
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HISTORY

143       POSIX.1-2001, SVr4.
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NOTES

146       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value
147       is used for key, the system call ignores all but the least  significant
148       9 bits of shmflg and creates a new shared memory segment.
149
150   Shared memory limits
151       The  following  limits  on  shared  memory segment resources affect the
152       shmget() call:
153
154       SHMALL System-wide limit on the total amount of shared memory, measured
155              in units of the system page size.
156
157              On Linux, this limit can be read and modified via /proc/sys/ker‐
158              nel/shmall.  Since Linux 3.16, the default value for this  limit
159              is:
160
161                  ULONG_MAX - 2^24
162
163              The  effect of this value (which is suitable for both 32-bit and
164              64-bit systems) is to impose no limitation on allocations.  This
165              value,  rather than ULONG_MAX, was chosen as the default to pre‐
166              vent some cases where historical applications simply raised  the
167              existing  limit  without first checking its current value.  Such
168              applications would cause the value to overflow if the limit  was
169              set at ULONG_MAX.
170
171              From  Linux  2.4  up  to  Linux 3.15, the default value for this
172              limit was:
173
174                  SHMMAX / PAGE_SIZE * (SHMMNI / 16)
175
176              If SHMMAX and SHMMNI were not modified, then multiplying the re‐
177              sult  of this formula by the page size (to get a value in bytes)
178              yielded a value of 8 GB as the limit on the total memory used by
179              all shared memory segments.
180
181       SHMMAX Maximum size in bytes for a shared memory segment.
182
183              On Linux, this limit can be read and modified via /proc/sys/ker‐
184              nel/shmmax.  Since Linux 3.16, the default value for this  limit
185              is:
186
187                  ULONG_MAX - 2^24
188
189              The  effect of this value (which is suitable for both 32-bit and
190              64-bit systems) is to impose no limitation on allocations.   See
191              the  description  of SHMALL for a discussion of why this default
192              value (rather than ULONG_MAX) is used.
193
194              From Linux 2.2 up to Linux 3.15, the default value of this limit
195              was 0x2000000 (32 MiB).
196
197              Because  it  is not possible to map just part of a shared memory
198              segment, the amount of virtual memory places  another  limit  on
199              the  maximum  size of a usable segment: for example, on i386 the
200              largest segments that can  be  mapped  have  a  size  of  around
201              2.8 GB, and on x86-64 the limit is around 127 TB.
202
203       SHMMIN Minimum  size  in bytes for a shared memory segment: implementa‐
204              tion dependent (currently 1 byte, though PAGE_SIZE is the effec‐
205              tive minimum size).
206
207       SHMMNI System-wide  limit  on the number of shared memory segments.  In
208              Linux 2.2, the default value for this limit was 128; since Linux
209              2.4, the default value is 4096.
210
211              On Linux, this limit can be read and modified via /proc/sys/ker‐
212              nel/shmmni.
213
214       The implementation has no specific limits for the  per-process  maximum
215       number of shared memory segments (SHMSEG).
216
217   Linux notes
218       Until Linux 2.3.30, Linux would return EIDRM for a shmget() on a shared
219       memory segment scheduled for deletion.
220

BUGS

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

226       See shmop(2).
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SEE ALSO

229       memfd_create(2),  shmat(2),  shmctl(2),  shmdt(2),  ftok(3),  capabili‐
230       ties(7), shm_overview(7), sysvipc(7)
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234Linux man-pages 6.05              2023-03-30                         shmget(2)
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