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semget(2) [netbsd man page]

SEMGET(2)						      BSD System Calls Manual							 SEMGET(2)

NAME
semget -- get set of semaphores LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/sem.h> int semget(key_t key, int nsems, int semflg); DESCRIPTION
The semget() system call returns the semaphore identifier associated with key. A new set containing nsems semaphores is created if either key is equal to IPC_PRIVATE, or key does not have a semaphore set associated with it and the IPC_CREAT bit is set in semflg. If both the IPC_CREAT bit and the IPC_EXCL bit are set in semflg, and key has a semaphore set associated with it already, the operation will fail. If a new set of semaphores is created, the data structure associated with it (the semid_ds structure, see semctl(2)) is initialized as fol- lows: o sem_perm.cuid and sem_perm.uid are set to the effective uid of the calling process. o sem_perm.gid and sem_perm.cgid are set to the effective gid of the calling process. o sem_perm.mode is set to the lower 9 bits of semflg. o sem_nsems is set to the value of nsems. o sem_ctime is set to the current time. o sem_otime is set to 0. RETURN VALUES
semget() returns a non-negative semaphore identifier if successful. Otherwise, -1 is returned and errno is set to reflect the error. ERRORS
[EACCES] The caller has no permission to access a semaphore set already associated with key. [EEXIST] Both IPC_CREAT and IPC_EXCL are set in semflg, and a semaphore set is already associated with key. [EINVAL] nsems is less than 0 or greater than the system limit for the number in a semaphore set. A semaphore set associated with key exists, but has fewer semaphores than the number specified in nsems. [ENOSPC] A new set of semaphores could not be created because the system limit for the number of semaphores or the number of sema- phore sets has been reached. [ENOENT] IPC_CREAT is not set in semflg and no semaphore set associated with key was found. SEE ALSO
ipcs(1), semctl(2), semop(2), ftok(3) STANDARDS
The semget system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5''). HISTORY
Semaphores appeared in the first release of AT&T System V UNIX. BSD
May 13, 2004 BSD

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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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