1SHMGET(2) Linux Programmer's Manual SHMGET(2)
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6 shmget - allocates a System V shared memory segment
7
9 #include <sys/ipc.h>
10 #include <sys/shm.h>
11 int shmget(key_t key, size_t size, int shmflg);
13
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 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 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 The value shmflg is composed of:
32 IPC_CREAT Create a new segment. If this flag is not used, then
34 shmget() will find the segment associated with key and
35 check to see if the user has permission to access the seg‐
36 ment.
37 IPC_EXCL This flag is used with IPC_CREAT to ensure that this call
39 creates the segment. If the segment already exists, the
40 call fails.
41 SHM_HUGETLB (since Linux 2.6)
43 Allocate the segment using "huge pages." See the Linux
44 kernel source file Documentation/admin-guide/mm/hugetlb‐
45 page.rst for further information.
46 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
50 that support multiple hugetlb page sizes.
51 More generally, the desired huge page size can be config‐
53 ured by encoding the base-2 logarithm of the desired page
54 size in the six bits at the offset SHM_HUGE_SHIFT. Thus,
55 the above two constants are defined as:
56 #define SHM_HUGE_2MB (21 << SHM_HUGE_SHIFT)
58 #define SHM_HUGE_1GB (30 << SHM_HUGE_SHIFT)
59 For some additional details, see the discussion of the sim‐
61 ilarly named constants in mmap(2).
62 SHM_NORESERVE (since Linux 2.6.15)
64 This flag serves the same purpose as the mmap(2) MAP_NORE‐
65 SERVE flag. Do not reserve swap space for this segment.
66 When swap space is reserved, one has the guarantee that it
67 is possible to modify the segment. When swap space is not
68 reserved one might get SIGSEGV upon a write if no physical
69 memory is available. See also the discussion of the file
70 /proc/sys/vm/overcommit_memory in proc(5).
71 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 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 shm_perm.cuid and shm_perm.uid are set to the effective user ID
82 of the calling process.
83 shm_perm.cgid and shm_perm.gid are set to the effective group ID
85 of the calling process.
86 The least significant 9 bits of shm_perm.mode are set to the
88 least significant 9 bit of shmflg.
89 shm_segsz is set to the value of size.
91 shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.
93 shm_ctime is set to the current time.
95 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.
98
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
104 On failure, errno is set to one of the following:
105 EACCES The user does not have permission to access the shared memory
107 segment, and does not have the CAP_IPC_OWNER capability in the
108 user namespace that governs its IPC namespace.
109 EEXIST IPC_CREAT and IPC_EXCL were specified in shmflg, but a shared
111 memory segment already exists for key.
112 EINVAL A new segment was to be created and size is less than SHMMIN or
114 greater than SHMMAX.
115 EINVAL A segment for the given key exists, but size is greater than the
117 size of that segment.
118 ENFILE The system-wide limit on the total number of open files has been
120 reached.
121 ENOENT No segment exists for the given key, and IPC_CREAT was not spec‐
123 ified.
124 ENOMEM No memory could be allocated for segment overhead.
126 ENOSPC All possible shared memory IDs have been taken (SHMMNI), or
128 allocating a segment of the requested size would cause the sys‐
129 tem to exceed the system-wide limit on shared memory (SHMALL).
130 EPERM The SHM_HUGETLB flag was specified, but the caller was not priv‐
132 ileged (did not have the CAP_IPC_LOCK capability).
133
135 POSIX.1-2001, POSIX.1-2008, SVr4.
136 SHM_HUGETLB and SHM_NORESERVE are Linux extensions.
138
140 The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
141 or by any version of POSIX. However, some old implementations required
142 the inclusion of these header files, and the SVID also documented their
143 inclusion. Applications intended to be portable to such old systems
144 may need to include these header files.
145 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 Shared memory limits
151 The following limits on shared memory segment resources affect the
152 shmget() call:
153 SHMALL System-wide limit on the total amount of shared memory, measured
155 in units of the system page size.
156 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 ULONG_MAX - 2^24
162 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 From Linux 2.4 up to Linux 3.15, the default value for this
172 limit was:
173 SHMMAX / PAGE_SIZE * (SHMMNI / 16)
175 If SHMMAX and SHMMNI were not modified, then multiplying the
177 result of this formula by the page size (to get a value in
178 bytes) yielded a value of 8 GB as the limit on the total memory
179 used by all shared memory segments.
180 SHMMAX Maximum size in bytes for a shared memory segment.
182 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 ULONG_MAX - 2^24
188 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 From Linux 2.2 up to Linux 3.15, the default value of this limit
195 was 0x2000000 (32 MB).
196 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 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 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 On Linux, this limit can be read and modified via /proc/sys/ker‐
212 nel/shmmni.
213 The implementation has no specific limits for the per-process maximum
215 number of shared memory segments (SHMSEG).
216 Linux notes
218 Until version 2.3.30, Linux would return EIDRM for a shmget() on a
219 shared memory segment scheduled for deletion.
220
222 The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
223 clearly show its function.
224
226 memfd_create(2), shmat(2), shmctl(2), shmdt(2), ftok(3), capabili‐
227 ties(7), shm_overview(7), sysvipc(7)
228
230 This page is part of release 5.02 of the Linux man-pages project. A
231 description of the project, information about reporting bugs, and the
232 latest version of this page, can be found at
233 https://www.kernel.org/doc/man-pages/.
234Linux 2019-08-02 SHMGET(2)