1SEMGET(2) Linux Programmer's Manual SEMGET(2)
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6 semget - get a semaphore set identifier
7
9 #include <sys/types.h>
10 #include <sys/ipc.h>
11 #include <sys/sem.h>
12 int semget(key_t key, int nsems, int semflg);
14
16 The semget() system call returns the semaphore set identifier associ‐
17 ated with the argument key. A new set of nsems semaphores is created
18 if key has the value IPC_PRIVATE or if no existing semaphore set is
19 associated with key and IPC_CREAT is specified in semflg.
20 If semflg specifies both IPC_CREAT and IPC_EXCL and a semaphore set
22 already exists for key, then semget() fails with errno set to EEXIST.
23 (This is analogous to the effect of the combination O_CREAT | O_EXCL
24 for open(2).)
25 Upon creation, the least significant 9 bits of the argument semflg
27 define the permissions (for owner, group and others) for the semaphore
28 set. These bits have the same format, and the same meaning, as the
29 mode argument of open(2) (though the execute permissions are not mean‐
30 ingful for semaphores, and write permissions mean permission to alter
31 semaphore values).
32 The values of the semaphores in a newly created set are indeterminate.
34 (POSIX.1-2001 is explicit on this point.) Although Linux, like many
35 other implementations, initializes the semaphore values to 0, a porta‐
36 ble application cannot rely on this: it should explicitly initialize
37 the semaphores to the desired values.
38 When creating a new semaphore set, semget() initializes the set's asso‐
40 ciated data structure, semid_ds (see semctl(2)), as follows:
41 sem_perm.cuid and sem_perm.uid are set to the effective user ID
43 of the calling process.
44 sem_perm.cgid and sem_perm.gid are set to the effective group ID
46 of the calling process.
47 The least significant 9 bits of sem_perm.mode are set to the
49 least significant 9 bits of semflg.
50 sem_nsems is set to the value of nsems.
52 sem_otime is set to 0.
54 sem_ctime is set to the current time.
56 The argument nsems can be 0 (a don't care) when a semaphore set is not
58 being created. Otherwise nsems must be greater than 0 and less than or
59 equal to the maximum number of semaphores per semaphore set (SEMMSL).
60 If the semaphore set already exists, the permissions are verified.
62
64 If successful, the return value will be the semaphore set identifier (a
65 non-negative integer), otherwise -1 is returned, with errno indicating
66 the error.
67
69 On failure errno will be set to one of the following:
70 EACCES A semaphore set exists for key, but the calling process does not
72 have permission to access the set, and does not have the
73 CAP_IPC_OWNER capability.
74 EEXIST A semaphore set exists for key and semflg specified both
76 IPC_CREAT and IPC_EXCL.
77 EINVAL nsems is less than 0 or greater than the limit on the number of
79 semaphores per semaphore set (SEMMSL), or a semaphore set corre‐
80 sponding to key already exists, and nsems is larger than the
81 number of semaphores in that set.
82 ENOENT No semaphore set exists for key and semflg did not specify
84 IPC_CREAT.
85 ENOMEM A semaphore set has to be created but the system does not have
87 enough memory for the new data structure.
88 ENOSPC A semaphore set has to be created but the system limit for the
90 maximum number of semaphore sets (SEMMNI), or the system wide
91 maximum number of semaphores (SEMMNS), would be exceeded.
92
94 SVr4, POSIX.1-2001.
95
97 IPC_PRIVATE isn't a flag field but a key_t type. If this special value
98 is used for key, the system call ignores everything but the least sig‐
99 nificant 9 bits of semflg and creates a new semaphore set (on success).
100 The following limits on semaphore set resources affect the semget()
102 call:
103 SEMMNI System wide maximum number of semaphore sets: policy dependent
105 (on Linux, this limit can be read and modified via the fourth
106 field of /proc/sys/kernel/sem).
107 SEMMSL Maximum number of semaphores per semid: implementation dependent
109 (on Linux, this limit can be read and modified via the first
110 field of /proc/sys/kernel/sem).
111 SEMMNS System wide maximum number of semaphores: policy dependent (on
113 Linux, this limit can be read and modified via the second field
114 of /proc/sys/kernel/sem). Values greater than SEMMSL * SEMMNI
115 makes it irrelevant.
116
118 The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
119 clearly show its function.
120 The semaphores in a set are not initialized by semget(). In order to
122 initialize the semaphores, semctl(2) must be used to perform a SETVAL
123 or a SETALL operation on the semaphore set. (Where multiple peers do
124 not know who will be the first to initialize the set, checking for a
125 non-zero sem_otime in the associated data structure retrieved by a sem‐
126 ctl(2) IPC_STAT operation can be used to avoid races.)
127
129 semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7),
130 svipc(7)
131
133 This page is part of release 3.22 of the Linux man-pages project. A
134 description of the project, and information about reporting bugs, can
135 be found at http://www.kernel.org/doc/man-pages/.
136Linux 2004-05-27 SEMGET(2)