#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.
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; or 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.