semget(2)semget(2)NAMEsemget - Return (and possibly create) a semaphore ID
SYNOPSIS
#include <sys/sem.h>
int semget(
key_t key,
int nsems,
int semflg );
Application developers may want to specify #include statements for
<sys/types.h> and <sys/ipc.h> before the one for <sys/sem.h> if pro‐
grams are being developed for multiple platforms. The additional
#include statements are not required on Tru64 UNIX systems or by ISO or
XSH specifications, but may be required on other vendors' systems that
conform to these standards.
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
semget(): XPG4, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
Specifies the key that identifies the semaphore set. The value for the
key parameter can be IPC_PRIVATE or a random number other than 0
(zero). To ensure that you receive a new, unused entry in the semaphore
table, specify IPC_PRIVATE as the value of key. Specifies the number
of semaphores to create in the semaphore set. Specifies the creation
options. Possible values are as follows: If the key specified does not
exist, the semget() function creates a semaphore ID using the specified
key.
If the specified key exists, and IPC_EXCL is not set, the sema‐
phore id for the specified key is returned.
If the specified key exists and IPC_EXCL is set, the semget()
function fails and returns an error. Specifies that you want
exclusive access to the semaphore set.
If the specified key already exists, the semget() function fails
and returns an error notification.
DESCRIPTION
The system defines sets of semaphores in a system-wide table, with each
set being an entry in the table. The semget() function returns an ID
that identifies the semaphore set's entry in the table. You determine
which semaphore set's ID is returned by specifying the key parameter.
The semget() function creates a semaphore set containing nsems sema‐
phores and returns its ID in the following situations: The key parame‐
ter is IPC_PRIVATE. The key parameter does not already exist as an
entry in the semaphore table and the IPC_CREAT option is set.
To create a semaphore set, the semget() function creates and initial‐
izes a structure of type semid_ds, which is defined as follows:
struct semid_ds {
struct ipc_perm sem_perm;
struct sem *sem_base;
ushort_t sem_nsems;
time_t sem_otime;
time_t sem_ctime; };
For a complete description of this structure, see semid_ds(4). The
semget() function initializes the structure as follows: The
sem_perm.cuid and sem_perm.uid fields are set equal to the effective
user ID of the calling process. The sem_perm.cgid and sem_perm.gid
fields are set equal to the effective group ID of the calling process.
The low-order nine bits of sem_perm.mode are set equal to the low-order
nine bits of semflg. The sem_nsems field is set equal to the value of
nsems. The sem_otime field is set equal to zero (0) and the sem_ctime
field is set equal to the current time.
The individual semaphores within a set are implemented using the sem
structure. (For more information about this structure, see the
<sys/sem.h> header file.) The semget() function does not initialize the
sem structure associated with each semaphore in the set. The individual
semaphores are initialized when you call the semctl() function with the
SETVAL or SETALL request.
NOTES
The librt library contains alternative interfaces for interprocess com‐
munication. The names of these routines adhere to the format sem_* and
their reference pages are listed in SEE ALSO.
RETURN VALUES
Upon successful completion, the semget() function returns a semaphore
identifier. If the semget() function fails, it returns a value of -1
and sets errno to indicate the error.
ERRORS
The semget() function sets errno to the specified values for the fol‐
lowing conditions: A semaphore ID already exists for the key parameter,
but operation permission as specified by the low-order nine bits of the
semflg parameter was not granted. A semaphore ID already exists for
the key parameter, but IPC_CREAT and IPC_EXCL were used for the semflg
parameter. The value of the nsems parameter is less than or equal to 0
(zero) or greater than the system-defined limit. Or, a semaphore ID
already exists for the key parameter, but the number of semaphores in
the set is less than the nsems parameter, and the nsems parameter is
not equal to 0 (zero). A semaphore ID does not exist for the key
parameter and IPC_CREAT was not set. An attempt to create a new sema‐
phore ID exceeded the system-wide limit on the size of the semaphore
table.
SEE ALSO
Functions: semctl(2), semop(2), table(2), ftok(3), sem_close(3),
sem_destroy(3), sem_getvalue(3),
sem_init(3), sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3)
Files: semid_ds(4), sysconfigtab(4)
Standards: standards(5)semget(2)
