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)
