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semget(2) [opendarwin man page]

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

NAME
semget -- obtain a semaphore id LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> int semget(key_t key, int nsems, int flag); DESCRIPTION
Based on the values of key and flag, semget() returns the identifier of a newly created or previously existing set of semaphores. The key is analogous to a filename: it provides a handle that names an IPC object. There are three ways to specify a key: o IPC_PRIVATE may be specified, in which case a new IPC object will be created. o An integer constant may be specified. If no IPC object corresponding to key is specified and the IPC_CREAT bit is set in flag, a new one will be created. o The ftok(3) function may be used to generate a key from a pathname. The mode of a newly created IPC object is determined by OR'ing the following constants into the flag argument: SEM_R Read access for user. SEM_A Alter access for user. (SEM_R>>3) Read access for group. (SEM_A>>3) Alter access for group. (SEM_R>>6) Read access for other. (SEM_A>>6) Alter access for other. If a new set of semaphores is being created, nsems is used to indicate the number of semaphores the set should contain. Otherwise, nsems may be specified as 0. RETURN VALUES
The semget() system call returns the id of a semaphore set if successful; otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The semget() system call will fail if: [EACCES] Access permission failure. [EEXIST] IPC_CREAT and IPC_EXCL were specified, and a semaphore set corresponding to key already exists. [EINVAL] The number of semaphores requested exceeds the system imposed maximum per set. [ENOSPC] Insufficiently many semaphores are available. [ENOSPC] The kernel could not allocate a struct semid_ds. [ENOENT] No semaphore set was found corresponding to key, and IPC_CREAT was not specified. SEE ALSO
semctl(2), semop(2), ftok(3) BSD
September 12, 1995 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
This function 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 to key and IPC_CREAT is asserted in semflg (i.e. semflg & IPC_CREAT isn't zero). The presence in semflg of the fields IPC_CREAT and IPC_EXCL plays the same role, with respect to the existence of the semaphore set, as the presence of O_CREAT and O_EXCL in the mode argument of the open(2) system call: i.e. the semget function fails if semflg asserts both IPC_CREAT and IPC_EXCL and a semaphore set already exists for key. Upon creation, the low-order 9 bits of the argument semflg define the access permissions (for owner, group and others) for the semaphore set. These bits have the same format, and the same meaning, as the mode argument in the open(2) or creat(2) system calls (though the exe- cute permissions are not meaningful for semaphores, and write permissions mean permission to alter semaphore values). When creating a new semaphore set, semget initializes the semaphore set's associated data structure semid_ds 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 low-order 9 bits of sem_perm.mode are set to the low-order 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 access 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. EEXIST A semaphore set exists for key and semflg was asserting both IPC_CREAT and IPC_EXCL. ENOENT No semaphore set exists for key and semflg wasn't asserting IPC_CREAT. 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. ENOMEM A semaphore set has to be created but the system has not 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. 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 low- order 9 bits of semflg and creates a new semaphore set (on success). The followings are limits on semaphore set resources affecting a semget call: SEMMNI System wide maximum number of semaphore sets: policy dependent. SEMMSL Maximum number of semaphores per semid: implementation dependent (500 currently). SEMMNS System wide maximum number of semaphores: policy dependent. Values greater than SEMMSL * SEMMNI makes it irrelevant. BUGS
Use of IPC_PRIVATE doesn't inhibit to other processes the access to the allocated semaphore set. There is currently no intrinsic way for a process to ensure exclusive access to a semaphore set. Asserting both IPC_CREAT and IPC_EXCL in semflg only ensures (on success) that a new semaphore set will be created, it doesn't imply exclusive access to the semaphore set. The data structure associated with each semaphore in the set isn't initialized by the system call. In order to initialize those data structures, one has to execute a subsequent call to semctl(2) to perform a SETVAL or a SETALL command on the semaphore set. CONFORMING TO
SVr4, SVID. SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN, ERANGE, EFAULT. SEE ALSO
ftok(3), ipc(5), semctl(2), semop(2) Linux 2.5 2002-01-04 SEMGET(2)
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