semget(2)

1SEMGET(2)                  Linux Programmer's Manual                 SEMGET(2)
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NAME

6       semget - get a semaphore set identifier
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SYNOPSIS

9       #include <sys/types.h>
10       #include <sys/ipc.h>
11       #include <sys/sem.h>
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13       int semget(key_t key, int nsems, int semflg);
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DESCRIPTION

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.
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21       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).)
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26       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).
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33       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, initialises the semaphore values to 0, a  porta‐
36       ble  application  cannot  rely on this: it should explicitly initialise
37       the semaphores to the desired values.
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39       When creating a new semaphore set, semget() initialises the set's asso‐
40       ciated data structure, semid_ds (see semctl(2)), as follows:
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42              sem_perm.cuid  and sem_perm.uid are set to the effective user ID
43              of the calling process.
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45              sem_perm.cgid and sem_perm.gid are set to the effective group ID
46              of the calling process.
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48              The  least  significant  9  bits of sem_perm.mode are set to the
49              least significant 9 bits of semflg.
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51              sem_nsems is set to the value of nsems.
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53              sem_otime is set to 0.
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55              sem_ctime is set to the current time.
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57       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).
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61       If the semaphore set already exists, the permissions are verified.
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RETURN VALUE

64       If successful, the return value will be the semaphore set identifier (a
65       nonnegative  integer),  otherwise -1 is returned, with errno indicating
66       the error.
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ERRORS

69       On failure errno will be set to one of the following:
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71       EACCES     A semaphore set exists for key, but the calling process does
72                  not have permission to access the set, and does not have the
73                  CAP_IPC_OWNER capability.
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75       EEXIST     A semaphore set exists for key  and  semflg  specified  both
76                  IPC_CREAT and IPC_EXCL.
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78       EINVAL     nsems is less than 0 or greater than the limit on the number
79                  of semaphores per semaphore set (SEMMSL), or a semaphore set
80                  corresponding  to  key  already  exists, and nsems is larger
81                  than the number of semaphores in that set.
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83       ENOENT     No semaphore set exists for key and semflg did  not  specify
84                  IPC_CREAT.
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86       ENOMEM     A  semaphore  set  has to be created but the system does not
87                  have enough memory for the new data structure.
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89       ENOSPC     A semaphore set has to be created but the system  limit  for
90                  the maximum number of semaphore sets (SEMMNI), or the system
91                  wide  maximum  number  of  semaphores  (SEMMNS),  would   be
92                  exceeded.
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NOTES

95       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value
96       is used for key, the system call ignores everything but the least  sig‐
97       nificant 9 bits of semflg and creates a new semaphore set (on success).
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99       The  following  limits  on  semaphore set resources affect the semget()
100       call:
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102       SEMMNI     System wide maximum number of semaphore sets: policy  depen‐
103                  dent  (on Linux, this limit can be read and modified via the
104                  fourth field of /proc/sys/kernel/sem).
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106       SEMMSL     Maximum  number  of  semaphores  per  semid:  implementation
107                  dependent (on Linux, this limit can be read and modified via
108                  the first field of /proc/sys/kernel/sem).
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110       SEMMNS     System wide maximum number of semaphores:  policy  dependent
111                  (on  Linux, this limit can be read and modified via the sec‐
112                  ond field of  /proc/sys/kernel/sem).   Values  greater  than
113                  SEMMSL * SEMMNI makes it irrelevant.
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BUGS

116       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
117       clearly show its function.
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119       The semaphores in a set are not initialised by semget().  In  order  to
120       initialise  the  semaphores, semctl(2) must be used to perform a SETVAL
121       or a SETALL operation on the semaphore set.  (Where multiple  peers  do
122       not  know  who  will be the first to initialise the set, checking for a
123       non-zero sem_otime in the associated data structure retrieved by a sem‐
124       ctl() IPC_STAT operation can be used to avoid races.)
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CONFORMING TO

127       SVr4, POSIX.1-2001.
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