semctl - semaphore control operations
# include
<sys/types.h>
# include <sys/ipc.h>
# include <sys/sem.h>
int semctl ( int semid, int semnun, int cmd, union semun arg )
The function performs the control operation specified by cmd on the semaphore set (or on the sumun-nth semaphore of the set) identified by semid. The first semaphore of the set is indicated by a value 0 for semun.
The type of arg is the union
union semun {
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int val; /* used for SETVAL only */ |
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struct semid_ds *buf; /* for IPC_STAT and IPC_SET */ |
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ushort *array; |
/* used for GETALL and SETALL */ |
};
Legal values for cmd are
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IPC_STAT |
Copy info from the semaphore set data structure into the structure pointed to by arg.buf. The argument semnum is ignored. The calling process must have read access privileges on the semaphore set. | ||
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IPC_SET |
Write the values of some members of the semid_ds structure pointed to by arg.buf to the semaphore set data structure, updating also its sem_ctime member. Considered members from the user supplied struct semid_ds pointed to by arg.buf are |
sem_perm.uid
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sem_perm.gid |
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sem_perm.mode |
/* only lowest 9-bits */ |
The calling process effective user-ID must be one among super-user, creator or owner of the semaphore set. The argument semnum is ignored.
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IPC_RMID |
Remove immediately the semaphore set and its data structures awakening all waiting processes (with an error return and errno set to EIDRM). The calling process effective user-ID must be one among super-user, creator or owner of the semaphore set. The argument semnum is ignored. | ||
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GETALL |
Return semval for all semaphores of the set into arg.array. The argument semnum is ignored. The calling process must have read access privileges on the semaphore set. | ||
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GETNCNT |
The system call returns the value of semncnt for the semno-th semaphore of the set (i.e. the number of processes waiting for an increase of semval for the semno-th semaphore of the set). The calling process must have read access privileges on the semaphore set. | ||
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GETPID |
The system call returns the value of sempid for the semno-th semaphore of the set (i.e. the pid of the process that executed the last semop call for the semno-th semaphore of the set). The calling process must have read access privileges on the semaphore set. | ||
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GETVAL |
The system call returns the value of semval for the semno-th semaphore of the set. The calling process must have read access privileges on the semaphore set. | ||
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GETZCNT |
The system call returns the value of semzcnt for the semno-th semaphore of the set (i.e. the number of processes waiting for semval of the semno-th semaphore of the set to become 0). The calling process must have read access privileges on the semaphore set. | ||
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SETALL |
Set semval for all semaphores of the set using arg.array, updating also the sem_ctime member of the semid_ds structure associated to the set. Undo entries are cleared for altered semaphores in all processes. Processes sleeping on the wait queue are awakened if some semval becomes 0 or increases. The argument semnum is ignored. The calling process must have alter access privileges on the semaphore set. | ||
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SETVAL |
Set the value of semval to arg.val for the semnum-th semaphore of the set, updating also the sem_ctime member of the semid_ds structure associated to the set. Undo entry is cleared for altered semaphore in all processes. Processes sleeping on the wait queue are awakened if semval becomes 0 or increases. The calling process must have alter access privileges on the semaphore set. |
On fail the system call returns -1 with errno indicating the error. Otherwise the system call returns a nonnegative value depending on cmd as follows:
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GETNCNT |
the value of semncnt. |
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GETPID |
the value of sempid. |
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GETVAL |
the value of semval. |
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GETZCNT |
the value of semzcnt. |
For a failing return, errno will be set to one among the following values:
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EACCESS |
The calling process has no access permissions needed to execute cmd. | ||
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EFAULT |
The address pointed to by arg.buf or arg.array isn’t accessible. | ||
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EIDRM |
The semaphore set was removed. | ||
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EINVAL |
Invalid value for cmd or semid. | ||
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EPERM |
The argument cmd has value IPC_SET or IPC_RMID but the calling process effective user-ID has insufficient privileges to execute the command. | ||
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ERANGE |
The argument cmd has value SETALL or SETVAL and the value to which semval has to be set (for some semaphore of the set) is less than 0 or greater than the implementation value SEMVMX. |
The IPC_INFO, SEM_STAT and SEM_INFO control calls are used by the ipcs(1) program to provide information on allocated resources. In the future these can be modified as needed or moved to a proc file system interface.
The following system limit on semaphore sets affects a semctl call:
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SEMVMX |
Maximum value for semval: implementation dependent (32767). |
ipc(5), shmget(2), shmat(2), shmdt(2).