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semget(2) [opensolaris man page]

semget(2)							   System Calls 							 semget(2)

NAME
semget - get set of semaphores SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> int semget(key_t key, int nsems, int semflg); DESCRIPTION
The semget() function returns the semaphore identifier associated with key. A semaphore identifier and associated data structure and set containing nsems semaphores (see Intro(2)) are created for key if one of the following is true: o key is equal to IPC_PRIVATE. o key does not already have a semaphore identifier associated with it, and (semflg&IPC_CREAT) is true. On creation, the data structure associated with the new semaphore identifier is initialized as follows: o sem_perm.cuid, sem_perm.uid, sem_perm.cgid, and sem_perm.gid are set equal to the effective user ID and effective group ID, respectively, of the calling process. o The access permission bits of sem_perm.mode are set equal to the access permission bits of semflg. o sem_nsems is set equal to the value of nsems. o sem_otime is set equal to 0 and sem_ctime is set equal to the current time. RETURN VALUES
Upon successful completion, a non-negative integer representing a semaphore identifier is returned. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The semget() function will fail if: EACCES A semaphore identifier exists for key, but operation permission (see Intro(2)) as specified by the low-order 9 bits of semflg would not be granted. EEXIST A semaphore identifier exists for key but both (semflg&IPC_CREAT) and (semflg&IPC_EXCL) are both true. EINVAL The nsems argument is either less than or equal to 0 or greater than the system-imposed limit. See NOTES. A semaphore identifier exists for key, but the number of semaphores in the set associated with it is less than nsems and nsems is not equal to 0. ENOENT A semaphore identifier does not exist for key and (semflg&IPC_CREAT) is false. ENOSPC A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores or semaphore identifiers system-wide would be exceeded. See NOTES. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ SEE ALSO
ipcrm(1), ipcs(1), rctladm(1M), Intro(2), semctl(2), semop(2), setrctl(2), ftok(3C), attributes(5), standards(5) NOTES
The system-imposed limit on the value of the nsems argument is the maintained on a per-process basis using the process.max-sem-nsems resource control. The system-imposed limit on the number of semaphore identifiers is maintained on a per-project basis using the project.max-sem-ids resource control. The zone.max-sem-ids resource control limis the total number of semaphore identifiers that can be allocated by a zone. See rctladm(1M) and setrctl(2) for information about using resource controls. SunOS 5.11 14 Aug 2006 semget(2)

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

NAME
semget - get a semaphore set identifier SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> int semget(key_t key, int nsems, int semflg); DESCRIPTION
The semget() system call returns the semaphore set identifier associated with the argument key. A new set of nsems semaphores is created if key has the value IPC_PRIVATE or if no existing semaphore set is associated with key and IPC_CREAT is specified in semflg. If semflg specifies both IPC_CREAT and IPC_EXCL and a semaphore set already exists for key, then semget() fails with errno set to EEXIST. (This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).) Upon creation, the least significant 9 bits of the argument semflg define the permissions (for owner, group and others) for the semaphore set. These bits have the same format, and the same meaning, as the mode argument of open(2) (though the execute permissions are not mean- ingful for semaphores, and write permissions mean permission to alter semaphore values). The values of the semaphores in a newly created set are indeterminate. (POSIX.1-2001 is explicit on this point.) Although Linux, like many other implementations, initializes the semaphore values to 0, a portable application cannot rely on this: it should explicitly ini- tialize the semaphores to the desired values. When creating a new semaphore set, semget() initializes the set's associated data structure, semid_ds (see semctl(2)), as follows: sem_perm.cuid and sem_perm.uid are set to the effective user ID of the calling process. sem_perm.cgid and sem_perm.gid are set to the effective group ID of the calling process. The least significant 9 bits of sem_perm.mode are set to the least significant 9 bits of semflg. sem_nsems is set to the value of nsems. sem_otime is set to 0. sem_ctime is set to the current time. The argument nsems can be 0 (a don't care) when a semaphore set is not being created. Otherwise nsems must be greater than 0 and less than or equal to the maximum number of semaphores per semaphore set (SEMMSL). If the semaphore set already exists, the permissions are verified. RETURN VALUE
If successful, the return value will be the semaphore set identifier (a nonnegative integer), otherwise -1 is returned, with errno indicat- ing the error. ERRORS
On failure errno will be set to one of the following: EACCES A semaphore set exists for key, but the calling process does not have permission to access the set, and does not have the CAP_IPC_OWNER capability. EEXIST A semaphore set exists for key and semflg specified both IPC_CREAT and IPC_EXCL. EINVAL nsems is less than 0 or greater than the limit on the number of semaphores per semaphore set (SEMMSL), or a semaphore set corre- sponding to key already exists, and nsems is larger than the number of semaphores in that set. ENOENT No semaphore set exists for key and semflg did not specify IPC_CREAT. ENOMEM A semaphore set has to be created but the system does not have enough memory for the new data structure. ENOSPC A semaphore set has to be created but the system limit for the maximum number of semaphore sets (SEMMNI), or the system wide maximum number of semaphores (SEMMNS), would be exceeded. CONFORMING TO
SVr4, POSIX.1-2001. NOTES
IPC_PRIVATE isn't a flag field but a key_t type. If this special value is used for key, the system call ignores everything but the least significant 9 bits of semflg and creates a new semaphore set (on success). The following limits on semaphore set resources affect the semget() call: SEMMNI System wide maximum number of semaphore sets: policy dependent (on Linux, this limit can be read and modified via the fourth field of /proc/sys/kernel/sem). SEMMSL Maximum number of semaphores per semid: implementation dependent (on Linux, this limit can be read and modified via the first field of /proc/sys/kernel/sem). SEMMNS System wide maximum number of semaphores: policy dependent (on Linux, this limit can be read and modified via the second field of /proc/sys/kernel/sem). Values greater than SEMMSL * SEMMNI makes it irrelevant. BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more clearly show its function. The semaphores in a set are not initialized by semget(). In order to initialize the semaphores, semctl(2) must be used to perform a SETVAL or a SETALL operation on the semaphore set. (Where multiple peers do not know who will be the first to initialize the set, checking for a nonzero sem_otime in the associated data structure retrieved by a semctl(2) IPC_STAT operation can be used to avoid races.) SEE ALSO
semctl(2), semop(2), ftok(3), capabilities(7), sem_overview(7), svipc(7) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2004-05-27 SEMGET(2)
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