1SHMGET(2) Linux Programmer's Manual SHMGET(2)
2
6 shmget - allocates a shared memory segment
7
9 #include <sys/ipc.h>
10 #include <sys/shm.h>
12 int shmget(key_t key, size_t size, int shmflg);
14
16 shmget() returns the identifier of the shared memory segment associated
17 with the value of the argument key. A new shared memory segment, with
18 size equal to the value of size rounded up to a multiple of PAGE_SIZE,
19 is created if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE,
20 no shared memory segment corresponding to key exists, and IPC_CREAT is
21 specified in shmflg.
22 If shmflg specifies both IPC_CREAT and IPC_EXCL and a shared memory
24 segment already exists for key, then shmget() fails with errno set to
25 EEXIST. (This is analogous to the effect of the combination O_CREAT |
26 O_EXCL for open(2).)
27 The value shmflg is composed of:
29 IPC_CREAT to create a new segment. If this flag is not used, then
31 shmget() will find the segment associated with key and
32 check to see if the user has permission to access the seg‐
33 ment.
34 IPC_EXCL used with IPC_CREAT to ensure failure if the segment
36 already exists.
37 mode_flags (least significant 9 bits) specifying the permissions
39 granted to the owner, group, and world. These bits have
40 the same format, and the same meaning, as the mode argument
41 of open(2). Presently, the execute permissions are not
42 used by the system.
43 SHM_HUGETLB (since Linux 2.6)
45 Allocate the segment using "huge pages." See the kernel
46 source file Documentation/vm/hugetlbpage.txt for further
47 information.
48 SHM_NORESERVE (since Linux 2.6.15)
50 This flag serves the same purpose as the mmap(2) MAP_NORE‐
51 SERVE flag. Do not reserve swap space for this segment.
52 When swap space is reserved, one has the guarantee that it
53 is possible to modify the segment. When swap space is not
54 reserved one might get SIGSEGV upon a write if no physical
55 memory is available. See also the discussion of the file
56 /proc/sys/vm/overcommit_memory in proc(5).
57 When a new shared memory segment is created, its contents are ini‐
59 tialised to zero values, and its associated data structure, shmid_ds
60 (see shmctl(2)), is initialised as follows:
61 shm_perm.cuid and shm_perm.uid are set to the effective user ID
63 of the calling process.
64 shm_perm.cgid and shm_perm.gid are set to the effective group ID
66 of the calling process.
67 The least significant 9 bits of shm_perm.mode are set to the
69 least significant 9 bit of shmflg.
70 shm_segsz is set to the value of size.
72 shm_lpid, shm_nattch, shm_atime and shm_dtime are set to 0.
74 shm_ctime is set to the current time.
76 If the shared memory segment already exists, the permissions are veri‐
78 fied, and a check is made to see if it is marked for destruction.
79
81 fork() After a fork() the child inherits the attached shared memory
82 segments.
83 exec() After an exec() all attached shared memory segments are detached
85 (not destroyed).
86 exit() Upon exit() all attached shared memory segments are detached
88 (not destroyed).
89
91 A valid segment identifier, shmid, is returned on success, -1 on error.
92
94 On failure, errno is set to one of the following:
95 EACCES The user does not have permission to access the shared mem‐
97 ory segment, and does not have the CAP_IPC_OWNER capabil‐
98 ity.
99 EEXIST IPC_CREAT | IPC_EXCL was specified and the segment exists.
101 EINVAL A new segment was to be created and size < SHMMIN or size >
103 SHMMAX, or no new segment was to be created, a segment with
104 given key existed, but size is greater than the size of
105 that segment.
106 ENFILE The system limit on the total number of open files has been
108 reached.
109 ENOENT No segment exists for the given key, and IPC_CREAT was not
111 specified.
112 ENOMEM No memory could be allocated for segment overhead.
114 ENOSPC All possible shared memory IDs have been taken (SHMMNI), or
116 allocating a segment of the requested size would cause the
117 system to exceed the system-wide limit on shared memory
118 (SHMALL).
119 EPERM The SHM_HUGETLB flag was specified, but the caller was not
121 privileged (did not have the CAP_IPC_LOCK capability).
122
124 IPC_PRIVATE isn't a flag field but a key_t type. If this special value
125 is used for key, the system call ignores everything but the least sig‐
126 nificant 9 bits of shmflg and creates a new shared memory segment (on
127 success).
128 The following limits on shared memory segment resources affect the
130 shmget() call:
131 SHMALL System wide maximum of shared memory pages (on Linux, this
133 limit can be read and modified via /proc/sys/kernel/shmall).
134 SHMMAX Maximum size in bytes for a shared memory segment: policy
136 dependent (on Linux, this limit can be read and modified via
137 /proc/sys/kernel/shmmax).
138 SHMMIN Minimum size in bytes for a shared memory segment: implemen‐
140 tation dependent (currently 1 byte, though PAGE_SIZE is the
141 effective minimum size).
142 SHMMNI System wide maximum number of shared memory segments: imple‐
144 mentation dependent (currently 4096, was 128 before Linux
145 2.3.99; on Linux, this limit can be read and modified via
146 /proc/sys/kernel/shmmni).
147 The implementation has no specific limits for the per process maximum
149 number of shared memory segments (SHMSEG).
150
152 The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
153 clearly show its function.
154
156 SVr4, POSIX.1-2001.
157
159 Until version 2.3.30 Linux would return EIDRM for a shmget() on a
160 shared memory segment scheduled for deletion.
161 SHM_HUGETLB is a non-portable Linux extension.
163
165 shmat(2), shmctl(2), shmdt(2), ftok(3), capabilities(7), svipc(7)
166Linux 2.6.15 2006-05-02 SHMGET(2)