semctl(2)

NAME

semctl − semaphore control operations

SYNOPSIS

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

int semctl(int semid, int semnum, int cmd, ...);

DESCRIPTION

semctl() provides a variety of semaphore control operations as specified by cmd. The fourth argument is optional, depending upon the operation requested. If required it is of type union semun, which must be explicitly declared by the application program.

union semun {

int val;
struct semid_ds ∗buf;
ushort ∗array

} arg;

The permission required for a semaphore operation is given as {token}, where token is the type of permission needed. The types of permission are interpreted as follows:

00400READ by user
00200ALTER by user
00040READ by group
00020ALTER by group
00004READ by others
00002ALTER by others

See the Semaphore Operation Permissions subsection of the DEFINITIONS section of intro(2) for more information. The following semaphore operations as specified by cmd 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 operations 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 operations 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 can only be executed 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.

RETURN VALUES

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 other successful completions return 0; otherwise, −1 is returned and errno is set to indicate the error.

ERRORS

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 −1.

EINVAL cmd is not a valid command.

EINVAL cmd is IPC_SET and sem_perm.uid or sem_perm.gid is not valid.

EPERM cmd is equal to IPC_RMID or IPC_SET and the effective user of the calling process is not super-user, or to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid.

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.

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