semctl(2)

semctl -- semaphore control operations

Synopsis

   #include <sys/types.h>
   #include <sys/ipc.h>
   #include <sys/sem.h>
   
   union semun {
        int val;
        struct semid_ds buf;
        ushort array;
   };
   

   int semctl(int semid, int semnum, int cmd, . . . / union semun arg /);

Description

semctl provides a variety of semaphore control operations as specified by cmd.

The following cmds are executed with respect to the semaphore specified by semid and semnum:

GETVAL: Return the value of semval [see intro(2)]. {READ}
SETVAL: Set the value of semval to arg.val. {ALTER}. When this command is successfully executed, the semadj value corresponding to the specified semaphore in all processes is cleared.
GETPID: Return the value of (int) sempid. {READ} Return the value of semncnt. {READ}
GETZCNT: Return the value of semzcnt. {READ}

The following cmds return and set, respectively, every semval in the set of semaphores.

GETALL: Place semvals into array pointed to by arg.array. {READ}
SETALL: Set semvals according to the array pointed to by arg.array. {ALTER}. When this cmd is successfully executed, the semadj values corresponding to each specified semaphore in all processes are cleared.

The following cmds are also available:

IPC_STAT

Place the current value of each member of the data structure associated with semid into the structure pointed to by arg.buf. The contents of this structure are defined in intro(2). {READ}

IPC_SET

Set the value of the following members of the data structure associated with semid to the corresponding value found in the structure pointed to by arg.buf:

   sem_perm.uid
   sem_perm.gid
   sem_perm.mode / only access permission bits /

This command can be executed only by a process that has an effective user ID equal to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid, or by a process that has the P_OWNER privilege.

IPC_RMID

Remove the semaphore identifier specified by semid from the system and destroy the set of semaphores and data structure associated with it. This command can be executed only by a process that has an effective user ID equal to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid, or by a process that has the P_OWNER privilege.

Return values

On success, semctl returns a value that depends on cmd:

GETVAL: the value of semval
GETPID: the value of (int) sempid
GETNCNT: the value of semncnt
GETZCNT: the value of semzcnt
all others: a value of 0

On failure, semctl returns -1 and sets errno to identify the error.

Errors

In the following conditions, semctl fails and sets errno to:

EACCES: Operation permission is denied to the calling process [see intro(2)].
EINVAL: semid is not a valid semaphore identifier.
EINVAL: semnum is less than 0 or greater than sem_nsems.
EINVAL: cmd is not a valid command.
EINVAL: cmd is IPC_SET and sem_perm.uid or sem_perm.gid is not valid.
EOVERFLOW: cmd is IPC_STAT and uid or gid is too large to be stored in the structure pointed to by arg.buf.
ERANGE: cmd is SETVAL or SETALL and the value to which semval is to be set is greater than the system imposed maximum.
EPERM: cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid and the calling process does not have P_OWNER privilege.
EFAULT: arg.buf points to an illegal address.

References

intro(2), semget(2), semop(2)