SEMCTL(S) XENIX System V SEMCTL(S)
Name
semctl - Controls semaphore operations.
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semctl (semid, semnum, cmd, arg)
int semid, cmd;
int semnum;
union semun {
int 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(S)).
SETVAL Set the value of semval to arg.val. 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 array pointed to by
arg.array.
SETALL Set semvals according to the array pointed
to by arg.array. 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
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SEMCTL(S) XENIX System V SEMCTL(S)
the data structure associated with semid
into the structure pointed to by arg.buf.
The contents of this structure are defined
in intro(S).
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 the super-user or to the
value of 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 the super-user or to the
value of sem_perm.uid in the data structure
associated with semid.
semctl will fail if one or more of the following are
true:
semid is not a valid semaphore identifier.
[EINVAL]
semnum is less than zero or greater than
sem_nsems. [EINVAL]
cmd is not a valid command. [EINVAL]
cmd is equal to GETALL or IPC_STAT and arg points
to an address in read-only shared data. [EINVAL]
Operation permission is denied to the calling
process (see intro(S)). [EACCES]
cmd is SETVAL or SETALL and the value to which
semval is to be set is greater than the system
imposed maximum. [ERANGE]
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 and it is not equal to
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SEMCTL(S) XENIX System V SEMCTL(S)
the value of sem_perm.uid in the data structure
associated with semid. [EPERM]
arg.buf points to an illegal address. [EFAULT]
arg.array points to an illegal address. [EFAULT]
Return Value
Upon successful completion, the value returned 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 A value of 0.
Otherwise, a value of -1 is returned and errno is set to
indicate the error.
See Also
intro(S), semget(S), semop(S)
Notes
Programs using this function must be compiled with the -Me
compiler option.
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