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. A new shared memory
17 segment, with size equal to the value of size rounded up to a multiple
18 of PAGE_SIZE, is created if key has the value IPC_PRIVATE or key isn't
19 IPC_PRIVATE, no shared memory segment corresponding to key exists, and
20 IPC_CREAT is specified in shmflg.
21 If shmflg specifies both IPC_CREAT and IPC_EXCL and a shared memory
23 segment already exists for key, then shmget() fails with errno set to
24 EEXIST. (This is analogous to the effect of the combination O_CREAT |
25 O_EXCL for open(2).)
26 The value shmflg is composed of:
28 IPC_CREAT to create a new segment. If this flag is not used, then
30 shmget() will find the segment associated with key and
31 check to see if the user has permission to access the seg‐
32 ment.
33 IPC_EXCL used with IPC_CREAT to ensure failure if the segment
35 already exists.
36 mode_flags (least significant 9 bits) specifying the permissions
38 granted to the owner, group, and world. These bits have
39 the same format, and the same meaning, as the mode argument
40 of open(2). Presently, the execute permissions are not
41 used by the system.
42 SHM_HUGETLB (since Linux 2.6)
44 Allocate the segment using "huge pages." See the Linux
45 kernel source file Documentation/vm/hugetlbpage.txt for
46 further information.
47 SHM_NORESERVE (since Linux 2.6.15)
49 This flag serves the same purpose as the mmap(2) MAP_NORE‐
50 SERVE flag. Do not reserve swap space for this segment.
51 When swap space is reserved, one has the guarantee that it
52 is possible to modify the segment. When swap space is not
53 reserved one might get SIGSEGV upon a write if no physical
54 memory is available. See also the discussion of the file
55 /proc/sys/vm/overcommit_memory in proc(5).
56 When a new shared memory segment is created, its contents are initial‐
58 ized to zero values, and its associated data structure, shmid_ds (see
59 shmctl(2)), is initialized as follows:
60 shm_perm.cuid and shm_perm.uid are set to the effective user ID
62 of the calling process.
63 shm_perm.cgid and shm_perm.gid are set to the effective group ID
65 of the calling process.
66 The least significant 9 bits of shm_perm.mode are set to the
68 least significant 9 bit of shmflg.
69 shm_segsz is set to the value of size.
71 shm_lpid, shm_nattch, shm_atime and shm_dtime are set to 0.
73 shm_ctime is set to the current time.
75 If the shared memory segment already exists, the permissions are veri‐
77 fied, and a check is made to see if it is marked for destruction.
78
80 On success, a valid shared memory identifier is returned. On errir, -1
81 is returned, and errno is set to indicate the error.
82
84 On failure, errno is set to one of the following:
85 EACCES The user does not have permission to access the shared memory
87 segment, and does not have the CAP_IPC_OWNER capability.
88 EEXIST IPC_CREAT | IPC_EXCL was specified and the segment exists.
90 EINVAL A new segment was to be created and size < SHMMIN or size > SHM‐
92 MAX, or no new segment was to be created, a segment with given
93 key existed, but size is greater than the size of that segment.
94 ENFILE The system limit on the total number of open files has been
96 reached.
97 ENOENT No segment exists for the given key, and IPC_CREAT was not spec‐
99 ified.
100 ENOMEM No memory could be allocated for segment overhead.
102 ENOSPC All possible shared memory IDs have been taken (SHMMNI), or
104 allocating a segment of the requested size would cause the sys‐
105 tem to exceed the system-wide limit on shared memory (SHMALL).
106 EPERM The SHM_HUGETLB flag was specified, but the caller was not priv‐
108 ileged (did not have the CAP_IPC_LOCK capability).
109
111 SVr4, POSIX.1-2001.
112 SHM_HUGETLB is a nonportable Linux extension.
114
116 The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
117 or by any version of POSIX. However, some old implementations required
118 the inclusion of these header files, and the SVID also documented their
119 inclusion. Applications intended to be portable to such old systems
120 may need to include these header files.
121 IPC_PRIVATE isn't a flag field but a key_t type. If this special value
123 is used for key, the system call ignores everything but the least sig‐
124 nificant 9 bits of shmflg and creates a new shared memory segment (on
125 success).
126 The following limits on shared memory segment resources affect the
128 shmget() call:
129 SHMALL System wide maximum of shared memory pages (on Linux, this limit
131 can be read and modified via /proc/sys/kernel/shmall).
132 SHMMAX Maximum size in bytes for a shared memory segment: policy depen‐
134 dent (on Linux, this limit can be read and modified via
135 /proc/sys/kernel/shmmax).
136 SHMMIN Minimum size in bytes for a shared memory segment: implementa‐
138 tion dependent (currently 1 byte, though PAGE_SIZE is the effec‐
139 tive minimum size).
140 SHMMNI System wide maximum number of shared memory segments: implemen‐
142 tation dependent (currently 4096, was 128 before Linux 2.3.99;
143 on Linux, this limit can be read and modified via /proc/sys/ker‐
144 nel/shmmni).
145 The implementation has no specific limits for the per-process maximum
147 number of shared memory segments (SHMSEG).
148 Linux notes
150 Until version 2.3.30 Linux would return EIDRM for a shmget() on a
151 shared memory segment scheduled for deletion.
152
154 The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
155 clearly show its function.
156
158 shmat(2), shmctl(2), shmdt(2), ftok(3), capabilities(7), shm_over‐
159 view(7), svipc(7)
160
162 This page is part of release 3.53 of the Linux man-pages project. A
163 description of the project, and information about reporting bugs, can
164 be found at http://www.kernel.org/doc/man-pages/.
165Linux 2013-04-19 SHMGET(2)