SEMOP(2) System Calls SEMOP(2)
NAME
semop, semtimedop - semaphore operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop(
int semid,
struct sembuf *sops,
size_t nsops);
int semtimedop(
int semid,
struct sembuf *sops,
size_t nsops,
const struct timespec *timeout);
DESCRIPTION
The
semop() function is used to perform atomically an array of semaphore
operations on the set of semaphores associated with the semaphore
identifier specified by
semid. The
sops argument is a pointer to the
array of semaphore-operation structures. The
nsops argument is the number
of such structures in the array.
Each
sembuf structure contains the following members:
short sem_num; /* semaphore number */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
Each semaphore operation specified by
sem_op is performed on the
corresponding semaphore specified by
semid and
sem_num. 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:
00400 READ by user
00200 ALTER by user
00040 READ by group
00020 ALTER by group
00004 READ by others
00002 ALTER by others
See the
Semaphore Operation Permissions section of
Intro(2) for more
information.
A process maintains a value,
semadj, for each semaphore it modifies. This
value contains the cumulative effect of operations the process has
performed on an individual semaphore with the
SEM_UNDO flag set (so that
they can be undone if the process terminates unexpectedly). The value of
semadj can affect the behavior of calls to
semop(),
semtimedop(),
exit(),
and
_exit() (the latter two functions documented on
exit(2)), but is
otherwise unobservable. See below for details.
The
sem_op member specifies one of three semaphore operations:
1. The
sem_op member is a negative integer; {ALTER}
o If
semval (see
Intro(2)) is greater than or equal to the
absolute value of
sem_op, the absolute value of
sem_op is
subtracted from
semval. Also, if (
sem_flg&SEM_UNDO) is
true, the absolute value of
sem_op is added to the calling
process's
semadj value (see
exit(2)) for the specified
semaphore.
o If
semval is less than the absolute value of
sem_op and
(
sem_flg&IPC_NOWAIT) is true,
semop() returns immediately.
o If
semval is less than the absolute value of
sem_op and
(
sem_flg&IPC_NOWAIT) is false,
semop() increments the
semncnt associated with the specified semaphore and
suspends execution of the calling thread until one of the
following conditions occur:
o The value of
semval becomes greater than or equal to
the absolute value of
sem_op. When this occurs, the
value of
semncnt associated with the specified
semaphore is decremented, the absolute value of
sem_op is subtracted from
semval and, if (
sem_flg&SEM_UNDO)
is true, the absolute value of
sem_op is added to the
calling process's
semadj value for the specified
semaphore.
o The
semid for which the calling thread is awaiting
action is removed from the system (see
semctl(2)).
When this occurs,
errno is set to
EIDRM and
-1 is
returned.
o The calling thread receives a signal that is to be
caught. When this occurs, the value of
semncnt associated with the specified semaphore is
decremented, and the calling thread resumes execution
in the manner prescribed in
sigaction(2).
2. The
sem_op member is a positive integer; {ALTER}
The value of
sem_op is added to
semval and, if
(
sem_flg&SEM_UNDO) is true, the value of
sem_op is subtracted
from the calling process's
semadj value for the specified
semaphore.
3. The
sem_op member is 0; {READ}
o If
semval is 0,
semop() returns immediately.
o If
semval is not equal to 0 and (
sem_flg&IPC_NOWAIT) is
true,
semop() returns immediately.
o If
semval is not equal to 0 and (
sem_flg&IPC_NOWAIT) is
false,
semop() increments the
semzcnt associated with the
specified semaphore and suspends execution of the calling
thread until one of the following occurs:
o The value of
semval becomes 0, at which time the value
of
semzcnt associated with the specified semaphore is
set to 0 and all processes waiting on
semval to become
0 are awakened.
o The
semid for which the calling thread is awaiting
action is removed from the system. When this occurs,
errno is set to
EIDRM and
-1 is returned.
o The calling thread receives a signal that is to be
caught. When this occurs, the value of
semzcnt associated with the specified semaphore is
decremented, and the calling thread resumes execution
in the manner prescribed in
sigaction(2).
Upon successful completion, the value of
sempid for each semaphore
specified in the array pointed to by
sops is set to the process
ID of the
calling process.
The
semtimedop() function behaves as
semop() except when it must suspend
execution of the calling process to complete its operation. If
semtimedop() must suspend the calling process after the time interval
specified in
timeout expires, or if the timeout expires while the process
is suspended,
semtimedop() returns with an error. If the
timespec structure pointed to by
timeout is zero-valued and
semtimedop() needs to
suspend the calling process to complete the requested operation(s), it
returns immediately with an error. If
timeout is the
NULL pointer, the
behavior of
semtimedop() is identical to that of
semop().
RETURN VALUES
Upon successful completion,
0 is returned. Otherwise,
-1 is returned and
errno is set to indicate the error.
ERRORS
The
semop() and
semtimedop() functions will fail if:
E2BIG The
nsops argument is greater than the system-imposed maximum.
See NOTES.
EACCES Operation permission is denied to the calling process (see
Intro(2)).
EAGAIN The operation would result in suspension of the calling process
but (
sem_flg&IPC_NOWAIT) is true.
EFAULT The
sops argument points to an illegal address.
EFBIG The value of
sem_num is less than 0 or greater than or equal to
the number of semaphores in the set associated with
semid.
EIDRM A
semid was removed from the system.
EINTR A signal was received.
EINVAL The
semid argument is not a valid semaphore identifier, or the
number of individual semaphores for which the calling process
requests a
SEM_UNDO operation would exceed the system-imposed
limit. Solaris does not impose a limit on the number of
individual semaphores for which the calling process requests a
SEM_UNDO operation.
ENOSPC The limit on the number of individual processes requesting a
SEM_UNDO operation would be exceeded. Solaris does not impose a
limit on the number of individual processes requesting an
SEM_UNDO operation.
ERANGE An operation would cause a
semval or a
semadj value to overflow
the system-imposed limit.
The
semtimedop() function will fail if:
EAGAIN The timeout expired before the requested operation could be
completed.
The
semtimedop() function will fail if one of the following is detected:
EFAULT The
timeout argument points to an illegal address.
EINVAL The
timeout argument specified a
tv_sec or
tv_nsec value less
than 0, or a
tv_nsec value greater than or equal to 1000
million.
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+----------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+----------------------+
|Interface Stability |
semop() is Standard. |
+--------------------+----------------------+
SEE ALSO
ipcs(1),
Intro(2),
exec(2),
exit(2),
fork(2),
semctl(2),
semget(2),
setrctl(2),
sigaction(2),
attributes(7),
standards(7),
rctladm(8)NOTES
The system-imposed maximum on
nsops for a semaphore identifier is the
minimum enforced value of the
process.max-sem-ops resource control of the
creating process at the time
semget(2) was used to allocate the
identifier.
See
rctladm(8) and
setrctl(2) for information about using resource
controls.
May 12, 2006
SEMOP(2)