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man pages section 2: System Calls

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Updated: Wednesday, July 27, 2022

semget (2)


semget - get set of semaphores


#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int semget(key_t 
key, int nsems, 
int semflg);


The semget() function returns the semaphore identifier associated with key.

A semaphore identifier and associated data structure and set containing nsems semaphores (see Intro(2)) are created for key if one of the following is true:

  • key is equal to IPC_PRIVATE .

  • key does not already have a semaphore identifier associated with it, and (semflg & IPC_CREAT) is true.

On creation, the data structure associated with the new semaphore identifier is initialized as follows:

  • sem_perm.cuid, sem_perm.uid, sem_perm.cgid, and sem_perm.gid are set equal to the effective user ID and effective group ID, respectively, of the calling process.

  • The access permission bits of sem_perm.mode are set equal to the access permission bits of semflg.

  • sem_nsems is set equal to the value of nsems.

  • sem_otime is set equal to 0 and sem_ctime is set equal to the current time.

Return Values

Upon successful completion, a non-negative integer representing a semaphore identifier is returned. Otherwise, −1 is returned and errno is set to indicate the error.


The semget() function will fail if:


A semaphore identifier exists for key, but operation permission (see Intro(2)) as specified by the low-order 9 bits of semflg would not be granted.


A semaphore identifier exists for key but both (semflg & IPC_CREAT) and (semflg & IPC_EXCL) are both true.


The nsems argument is either less than or equal to 0 or greater than the system-imposed limit. See NOTES.

A semaphore identifier exists for 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 key and (semflg & IPC_CREAT) is false.


A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores or semaphore identifiers system-wide would be exceeded. See NOTES.


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

ipcrm(1), ipcs(1), Intro(2), semctl(2), semop(2), setrctl(2), ftok(3C), attributes(7), standards(7), rctladm(8)


The system-imposed limit on the value of the nsems argument is the maintained on a per-process basis using the process.max-sem-nsems resource control.

The system-imposed limit on the number of semaphore identifiers is maintained on a per-project basis using the project.max-sem-ids resource control. The zone.max-sem-ids resource control limits the total number of semaphore identifiers that can be allocated by a zone.

See rctladm(8) and setrctl(2) for information about using resource controls.