|SEMGET(3P)||POSIX Programmer's Manual||SEMGET(3P)|
int semget(key_t key, int nsems, int semflg);
The semget() function shall return the semaphore identifier associated with key.
A semaphore identifier with its associated semid_ds data structure and its associated set of nsems semaphores (see <sys/sem.h>) is created for key if one of the following is true:
- The argument key is equal to IPC_PRIVATE.
- The argument key does not already have a semaphore identifier associated with it and (semflg &IPC_CREAT) is non-zero.
Upon creation, the semid_ds data structure associated with the new semaphore identifier is initialized as follows:
- In the operation permissions structure sem_perm.cuid, sem_perm.uid, sem_perm.cgid, and sem_perm.gid shall be set to the effective user ID and effective group ID, respectively, of the calling process.
- The low-order 9 bits of sem_perm.mode shall be set to the low-order 9 bits of semflg.
- The variable sem_nsems shall be set to the value of nsems.
- The variable sem_otime shall be set to 0 and sem_ctime shall be set to the current time, as described in Section 2.7.1, IPC General Description.
- The data structure associated with each semaphore in the set need not be initialized. The semctl() function with the command SETVAL or SETALL can be used to initialize each semaphore.
- A semaphore identifier exists for key, but operation permission as specified by the low-order 9 bits of semflg would not be granted; see Section 2.7, XSI Interprocess Communication.
- A semaphore identifier exists for the argument key but ((semflg &IPC_CREAT) &&(semflg &IPC_EXCL)) is non-zero.
- The value of nsems is either less than or equal to 0 or greater than the system-imposed limit, or a semaphore identifier exists for the argument key, but the number of semaphores in the set associated with it is less than nsems and nsems is not equal to 0.
- A semaphore identifier does not exist for the argument key and (semflg &IPC_CREAT) is equal to 0.
- A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores system-wide would be exceeded.
The following sections are informative.
The Base Definitions volume of POSIX.1‐2017, Section 4.17, Semaphore, <sys_sem.h>
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|2017||IEEE/The Open Group|