1shmget(2) System Calls Manual shmget(2)
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6 shmget - allocates a System V shared memory segment
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9 Standard C library (libc, -lc)
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12 #include <sys/shm.h>
13 int shmget(key_t key, size_t size, int shmflg);
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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 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 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 The value shmflg is composed of:
34 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 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 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 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 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 #define SHM_HUGE_2MB (21 << SHM_HUGE_SHIFT)
61 #define SHM_HUGE_1GB (30 << SHM_HUGE_SHIFT)
62 For some additional details, see the discussion of the similarly
64 named constants in mmap(2).
65 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 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 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 • shm_perm.cuid and shm_perm.uid are set to the effective user ID of
85 the calling process.
86 • shm_perm.cgid and shm_perm.gid are set to the effective group ID of
88 the calling process.
89 • The least significant 9 bits of shm_perm.mode are set to the least
91 significant 9 bit of shmflg.
92 • shm_segsz is set to the value of size.
94 • shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.
96 • shm_ctime is set to the current time.
98 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
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
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 EEXIST IPC_CREAT and IPC_EXCL were specified in shmflg, but a shared
112 memory segment already exists for key.
113 EINVAL A new segment was to be created and size is less than SHMMIN or
115 greater than SHMMAX.
116 EINVAL A segment for the given key exists, but size is greater than the
118 size of that segment.
119 ENFILE The system-wide limit on the total number of open files has been
121 reached.
122 ENOENT No segment exists for the given key, and IPC_CREAT was not spec‐
124 ified.
125 ENOMEM No memory could be allocated for segment overhead.
127 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 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
138 POSIX.1-2008.
139 SHM_HUGETLB and SHM_NORESERVE are Linux extensions.
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143 POSIX.1-2001, SVr4.
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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 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 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 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 MiB).
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 Linux 2.3.30, Linux would return EIDRM for a shmget() on a shared
219 memory segment scheduled for deletion.
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222 The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
223 clearly show its function.
224
226 See shmop(2).
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229 memfd_create(2), shmat(2), shmctl(2), shmdt(2), ftok(3), capabili‐
230 ties(7), shm_overview(7), sysvipc(7)
231Linux man-pages 6.05 2023-03-30 shmget(2)