NAME

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}

GETNCNT 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 either that of super-user, or to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid.

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 only be executed only by a process that has an effective user ID equal to either that of super-user, or to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid.

semctl fails if one or more of the following are true:

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 that of super-user, or to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid.

EFAULT arg.buf points to an illegal address. 

SEE ALSO

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

DIAGNOSTICS

Upon successful completion, the value returned depends on cmd as follows:

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

Otherwise, a value of −1 is returned and errno is set to indicate the error.