semget(2) System Calls Manual semget(2)
NAME
semget - Returns (and possibly creates) 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 programs
are being developed for multiple platforms. The additional #include statements are not required on Tru64 UNIX systems or by ISO or X/Open
standards, but may be required on other vendors' systems that conform to these standards.
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
semget(): XPG4, XPG4-UNIX
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 flags. 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 semaphore 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 param-
eter.
The semget() function creates a semaphore set containing nsems semaphores and returns its ID in the following situations: The key parameter
is IPC_PRIVATE. The key parameter does not already exist as an entry in the semaphore table and the IPC_CREAT flag is set.
To create a semaphore set, the semget() function creates and initializes 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 indi-
vidual semaphores are initialized when you call the semctl function with the SETVAL or SETALL command.
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 following 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 semaphore ID exceeded the system-wide limit on the size of the
semaphore table.
RELATED INFORMATION
Functions: semctl(2), semop(2), table(2), ftok(3)
Files: semid_ds(4), sysconfigtab(4)
Standards: standards(5) delim off
semget(2)