SEMCTL(2) — SYSTEM CALLS

NAME

semctl − semaphore control operations

SYNOPSIS

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

int semctl(semid, semnum, cmd, arg)
int semid, semnum, cmd;
union semun {
val;
struct semid_ds ∗buf;
ushort ∗array;
} 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 cmd is successfully executed, the semadj value corresponding to the specified semaphore in all processes is cleared.

GETPID Return the value of 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 the 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 low 9 bits ∗/

This cmd 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.

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 cmd 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.

In the semop(2) and semctl(2) system call descriptions, the permission required for an operation is given as "[token]", where “token” is the type of permission needed interpreted as follows:

00400 Read by user

00200 Alter by user

00060 Read, Alter by group

00006 Read, Alter by others

Read and Alter permissions on a semid are granted to a process if one or more of the following are true:

The effective user ID of the process is super-user.

The effective user ID of the process matches sem_perm.[c]uid in the data structure associated with semid and the appropriate bit of the “user” portion (0600) of sem_perm.mode is set.

The effective user ID of the process does not match sem_perm.[c]uid and the effective group ID of the process matches sem_perm.[c]gid and the appropriate bit of the “group” portion (060) of sem_perm.mode is set.

The effective user ID of the process does not match sem_perm.[c]uid and the effective group ID of the process does not match sem_perm.[c]gid and the appropriate bit of the “other” portion (06) of sem_perm.mode is set.

Otherwise, the corresponding permissions are denied.

RETURN VALUES

On success, the value returned by semctl() depends on cmd as follows:

GETVAL The value of semval.

GETPID The value of sempid.

GETNCNT The value of semncnt.

GETZCNT The value of semzcnt.

All others 0.

On failure, semctl() returns −1 and sets errno to indicate the error.

ERRORS

EACCES Operation permission is denied to the calling process (see intro(2)).

EFAULT arg.buf points to an illegal address.

EINVAL semid is not a valid semaphore identifier.

semnum is less than zero or greater than sem_nsems.

cmd is not a valid command.

EPERM cmd is IPC_RMID or IPC_SET and the effective user ID of the calling process is not super-user.

cmd is IPC_RMID or IPC_SET and the effective user ID of the calling process is not the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid.

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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