SEMCTL(2) BSD System Calls Manual SEMCTL(2)NAMEsemctl — semaphore control operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/sem.h>
int
semctl(int semid, int semnum, int cmd, ...);
DESCRIPTION
The semctl() system call provides a number of control operations on the
semaphore specified by semnum and semid. The operation to be performed
is specified in cmd (see below). The fourth argument is optional and
depends upon the operation requested. If required, it is a union of the
following fields:
int val; /* value for SETVAL */
struct semid_ds *buf; /* buffer for IPC_{STAT,SET} */
u_short *array; /* array for GETALL & SETALL */
The semid_ds structure used in the IPC_SET and IPC_STAT commands is
defined in <sys/sem.h> and contains the following members:
struct ipc_perm sem_perm; /* operation permissions */
unsigned short sem_nsems; /* number of sems in set */
time_t sem_otime; /* last operation time */
time_t sem_ctime; /* last change time */
The ipc_perm structure used inside the semid_ds structure is defined in
<sys/ipc.h> and contains the following members:
uid_t cuid; /* creator user id */
gid_t cgid; /* creator group id */
uid_t uid; /* user id */
gid_t gid; /* group id */
mode_t mode; /* permission (lower 9 bits) */
semctl() provides the following operations:
GETVAL Return the value of the semaphore.
SETVAL Set the value of the semaphore to arg.val, where arg is the
fourth argument to semctl().
GETPID Return the pid of the last process that did an operation on
this semaphore.
GETNCNT Return the number of processes waiting to acquire the sema‐
phore.
GETZCNT Return the number of processes waiting for the value of the
semaphore to reach 0.
GETALL Return the values of all the semaphores associated with semid.
SETALL Set the values of all the semaphores that are associated with
the semaphore identifier semid to the corresponding values in
arg.array, where arg is the fourth argument to semctl().
IPC_STAT Gather information about a semaphore and place the information
in the structure pointed to by arg.buf, where arg is the
fourth argument to semctl().
IPC_SET Set the value of the sem_perm.uid, sem_perm.gid and
sem_perm.mode fields in the structure associated with the sem‐
aphore. The values are taken from the corresponding fields in
the structure pointed to by arg.buf, there arg is the fourth
argument to semctl(). This operation can only be executed by
the super-user, or a process that has an effective user id
equal to either sem_perm.cuid or sem_perm.uid in the data
structure associated with the semaphore.
IPC_RMID Remove the semaphores associated with semid from the system
and destroy the data structures associated with it. Only the
super-user or a process with an effective uid equal to the
sem_perm.cuid or sem_perm.uid values in the data structure
associated with the semaphore can do this.
The permission to read or change a semaphore (see semop(2)) is determined
by the sem_perm.mode field in the same way as is done with files (see
chmod(2)), but the effective uid can match either the sem_perm.cuid field
or the sem_perm.uid field, and the effective gid can match either
sem_perm.cgid or sem_perm.gid.
RETURN VALUES
For the GETVAL, GETPID, GETNCNT, and GETZCNT operations, semctl() returns
one of the values described above if successful. All other operations
will make semctl() return 0 if no errors occur. Otherwise -1 is returned
and errno set to reflect the error.
ERRORSsemctl() will fail if:
[EACCES] The caller has no operation permission for this sema‐
phore.
[EFAULT] arg.buf or arg.array specifies an invalid address.
[EINVAL] semid is not a valid message semaphore identifier.
cmd is not a valid command.
[EPERM] cmd is equal to IPC_SET or IPC_RMID and the caller is
not the super-user, nor does the effective uid match
either the sem_perm.uid or sem_perm.cuid fields of the
data structure associated with the semaphore.
[ERANGE] cmd is equal to SETVAL or SETALL and the value to be
set is greater than the system semaphore maximum
value.
SEE ALSOsemget(2), semop(2)STANDARDS
The semctl system call conforms to X/Open System Interfaces and Headers
Issue 5 (“XSH5”).
HISTORY
Semaphores appeared in the first release of AT&T System V UNIX.
BSD August 25, 1999 BSD
