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/ipc.h>
10       #include <sys/shm.h>
11
12       int shmget(key_t key, size_t size, int shmflg);
13

DESCRIPTION

15       shmget()  returns  the identifier of the System V shared memory segment
16       associated with the value of the argument key.  It may be  used  either
17       to  obtain the identifier of a previously created shared memory segment
18       (when shmflg is zero and key does not have the value  IPC_PRIVATE),  or
19       to create a new set.
20
21       A  new  shared  memory  segment,  with  size equal to the value of size
22       rounded up to a multiple of PAGE_SIZE, is created if key has the  value
23       IPC_PRIVATE  or  key isn't IPC_PRIVATE, no shared memory segment corre‐
24       sponding to key exists, and IPC_CREAT is specified in shmflg.
25
26       If shmflg specifies both IPC_CREAT and IPC_EXCL  and  a  shared  memory
27       segment  already  exists for key, then shmget() fails with errno set to
28       EEXIST.  (This is analogous to the effect of the combination O_CREAT  |
29       O_EXCL for open(2).)
30
31       The value shmflg is composed of:
32
33       IPC_CREAT
34              Create  a  new segment.  If this flag is not used, then shmget()
35              will find the segment associated with key and check  to  see  if
36              the user has permission to access the segment.
37
38       IPC_EXCL
39              This  flag  is used with IPC_CREAT to ensure that this call cre‐
40              ates the segment.  If  the  segment  already  exists,  the  call
41              fails.
42
43       SHM_HUGETLB (since Linux 2.6)
44              Allocate  the  segment using "huge pages."  See the Linux kernel
45              source  file  Documentation/admin-guide/mm/hugetlbpage.rst   for
46              further information.
47
48       SHM_HUGE_2MB, SHM_HUGE_1GB (since Linux 3.8)
49              Used  in  conjunction  with  SHM_HUGETLB  to  select alternative
50              hugetlb page sizes (respectively, 2 MB and 1 GB) on systems that
51              support multiple hugetlb page sizes.
52
53              More  generally, the desired huge page size can be configured by
54              encoding the base-2 logarithm of the desired page  size  in  the
55              six bits at the offset SHM_HUGE_SHIFT.  Thus, the above two con‐
56              stants are defined as:
57
58                  #define SHM_HUGE_2MB    (21 << SHM_HUGE_SHIFT)
59                  #define SHM_HUGE_1GB    (30 << SHM_HUGE_SHIFT)
60
61              For some additional details, see the discussion of the similarly
62              named constants in mmap(2).
63
64       SHM_NORESERVE (since Linux 2.6.15)
65              This  flag  serves the same purpose as the mmap(2) MAP_NORESERVE
66              flag.  Do not reserve swap space for this  segment.   When  swap
67              space  is reserved, one has the guarantee that it is possible to
68              modify the segment.  When swap space is not reserved  one  might
69              get  SIGSEGV  upon  a  write if no physical memory is available.
70              See also the discussion of the file /proc/sys/vm/overcommit_mem‐
71              ory in proc(5).
72
73       In  addition to the above flags, the least significant 9 bits of shmflg
74       specify the permissions granted to the owner, group, and others.  These
75       bits  have  the same format, and the same meaning, as the mode argument
76       of open(2).  Presently, execute permissions are not used by the system.
77
78       When a new shared memory segment is created, its contents are  initial‐
79       ized  to  zero values, and its associated data structure, shmid_ds (see
80       shmctl(2)), is initialized as follows:
81
82       · shm_perm.cuid and shm_perm.uid are set to the effective  user  ID  of
83         the calling process.
84
85       · shm_perm.cgid  and  shm_perm.gid are set to the effective group ID of
86         the calling process.
87
88       · The least significant 9 bits of shm_perm.mode are set  to  the  least
89         significant 9 bit of shmflg.
90
91       · shm_segsz is set to the value of size.
92
93       · shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.
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95       · shm_ctime is set to the current time.
96
97       If  the shared memory segment already exists, the permissions are veri‐
98       fied, and a check is made to see if it is marked for destruction.
99

RETURN VALUE

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

ERRORS

105       On failure, errno is set to one of the following:
106
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.
116
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
129              allocating  a segment of the requested size would cause the sys‐
130              tem 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).
134

CONFORMING TO

136       POSIX.1-2001, POSIX.1-2008, SVr4.
137
138       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.
139

NOTES

141       The  inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
142       or by any version of POSIX.  However, some old implementations required
143       the inclusion of these header files, and the SVID also documented their
144       inclusion.  Applications intended to be portable to  such  old  systems
145       may need to include these header files.
146
147       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value
148       is used for key, the system call ignores all but the least  significant
149       9 bits of shmflg and creates a new shared memory segment.
150
151   Shared memory limits
152       The  following  limits  on  shared  memory segment resources affect the
153       shmget() call:
154
155       SHMALL System-wide limit on the total amount of shared memory, measured
156              in units of the system page size.
157
158              On Linux, this limit can be read and modified via /proc/sys/ker‐
159              nel/shmall.  Since Linux 3.16, the default value for this  limit
160              is:
161
162                  ULONG_MAX - 2^24
163
164              The  effect of this value (which is suitable for both 32-bit and
165              64-bit systems) is to impose no limitation on allocations.  This
166              value,  rather than ULONG_MAX, was chosen as the default to pre‐
167              vent some cases where historical applications simply raised  the
168              existing  limit  without first checking its current value.  Such
169              applications would cause the value to overflow if the limit  was
170              set at ULONG_MAX.
171
172              From  Linux  2.4  up  to  Linux 3.15, the default value for this
173              limit was:
174
175                  SHMMAX / PAGE_SIZE * (SHMMNI / 16)
176
177              If SHMMAX and SHMMNI were not  modified,  then  multiplying  the
178              result  of  this  formula  by  the  page size (to get a value in
179              bytes) yielded a value of 8 GB as the limit on the total  memory
180              used by all shared memory segments.
181
182       SHMMAX Maximum size in bytes for a shared memory segment.
183
184              On Linux, this limit can be read and modified via /proc/sys/ker‐
185              nel/shmmax.  Since Linux 3.16, the default value for this  limit
186              is:
187
188                  ULONG_MAX - 2^24
189
190              The  effect of this value (which is suitable for both 32-bit and
191              64-bit systems) is to impose no limitation on allocations.   See
192              the  description  of SHMALL for a discussion of why this default
193              value (rather than ULONG_MAX) is used.
194
195              From Linux 2.2 up to Linux 3.15, the default value of this limit
196              was 0x2000000 (32 MB).
197
198              Because  it  is not possible to map just part of a shared memory
199              segment, the amount of virtual memory places  another  limit  on
200              the  maximum  size of a usable segment: for example, on i386 the
201              largest segments that can  be  mapped  have  a  size  of  around
202              2.8 GB, and on x86-64 the limit is around 127 TB.
203
204       SHMMIN Minimum  size  in bytes for a shared memory segment: implementa‐
205              tion dependent (currently 1 byte, though PAGE_SIZE is the effec‐
206              tive minimum size).
207
208       SHMMNI System-wide  limit  on the number of shared memory segments.  In
209              Linux 2.2, the default value for this limit was 128; since Linux
210              2.4, the default value is 4096.
211
212              On Linux, this limit can be read and modified via /proc/sys/ker‐
213              nel/shmmni.
214
215       The implementation has no specific limits for the  per-process  maximum
216       number of shared memory segments (SHMSEG).
217
218   Linux notes
219       Until  version  2.3.30,  Linux  would  return EIDRM for a shmget() on a
220       shared memory segment scheduled for deletion.
221

BUGS

223       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
224       clearly show its function.
225

EXAMPLES

227       See shmop(2).
228

SEE ALSO

230       memfd_create(2),  shmat(2),  shmctl(2),  shmdt(2),  ftok(3),  capabili‐
231       ties(7), shm_overview(7), sysvipc(7)
232

COLOPHON

234       This page is part of release 5.07 of the Linux  man-pages  project.   A
235       description  of  the project, information about reporting bugs, and the
236       latest    version    of    this    page,    can     be     found     at
237       https://www.kernel.org/doc/man-pages/.
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241Linux                             2020-04-11                         SHMGET(2)
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