semop(2)semop(2)NAMEsemop(), semtimedop() - semaphore operations
SYNOPSISDESCRIPTION
is used to atomically perform an array of semaphore operations on the
set of semaphores associated with the semaphore identifier specified by
semid. sops is a pointer to the array of semaphore-operation struc‐
tures. nsops is the number of such structures in the array. The con‐
tents of each structure includes the following members:
Each semaphore operation specified by sem_op is performed on the corre‐
sponding semaphore specified by semid and sem_num. Semaphore array
operations are atomic in that none of the semaphore operations are per‐
formed until blocking conditions on all of the semaphores in the array
have been removed.
sem_op specifies one of three semaphore operations as follows:
· If sem_op is a negative integer, one of the following occurs:
If semaphore value (see semaphore identifier in glossary(9))
is greater than or equal to the absolute value of sem_op, the
absolute value of sem_op is subtracted from semaphore value.
Also, if (sem_flg & is "true", the absolute value of sem_op
is added to the calling process's semadj value (see glos‐
sary(9) and exit(2)) for the specified semaphore.
If semaphore value is less than the absolute value of sem_op
and (sem_flg & is "true", returns immediately.
If semaphore value is less than the absolute value of sem_op
and (sem_flg & is ``false'', increments the internal sema‐
phore waiter count associated with the specified semaphore
and suspend execution of the calling process until one of the
following conditions occur:
Semaphore value becomes greater than or equal to the
absolute value of sem_op. When this occurs, the value of
internal semaphore waiter count associated with the spec‐
ified semaphore is decremented, the absolute value of
sem_op is subtracted from semaphore value and, if
(sem_flg & is "true", the absolute value of sem_op is
added to the calling process's semadj value for the spec‐
ified semaphore.
The semid for which the calling process is awaiting
action is removed from the system (see semctl(2)). When
this occurs, is set equal to and a value of −1 is
returned.
The calling process receives a signal that is to be
caught. When this occurs, the value of internal sema‐
phore waiter count associated with the specified sema‐
phore is decremented, and the calling process resumes
execution in the manner prescribed in signal(5).
· If sem_op is a positive integer, the value of sem_op is added
to semaphore value and, if (sem_flg & is "true", the value of
sem_op is subtracted from the calling process's semadj value
for the specified semaphore.
· If sem_op is zero, one of the following occurs:
If semaphore value is zero, proceeds to the next semaphore
operation specified by sops , or returns immediately if this
is the last operation.
If semaphore value is not equal to zero and (sem_flg & is
"true", returns immediately.
If semaphore value is not equal to zero and (sem_flg & is
``false'', increments the internal zero waiter count associ‐
ated with the specified semaphore and suspends execution of
the calling process until one of the following occurs:
Semaphore value becomes zero, at which time the value of
internal zero waiter count associated with the specified
semaphore is decremented.
The semid for which the calling process is awaiting
action is removed from the system. When this occurs, is
set equal to and a value of −1 is returned.
The calling process receives a signal that is to be
caught. When this occurs, the value of internal zero
waiter count associated with the specified semaphore is
decremented, and the calling process resumes execution in
the manner prescribed in signal(5).
is identical to except that an additional constraint is provided by the
timeout parameter. If must suspend the calling process longer than the
time indicated by timeout, or if the timeout expires while the calling
process is suspended in the call returns an error.
If the structure pointed to by the timeout parameter is zero-valued,
returns with an error immediately if it would otherwise have to suspend
the calling program.
If the timeout parameter is the NULL pointer, behaves exactly like
EXAMPLES
The following call to atomically performs a "P" or "get" operation on
the second semaphore in the semaphore set and a "V" or "release" opera‐
tion on the third semaphore in the set. This example assumes the
process has a valid semid which represents a set of 4 semaphores as
shown on the semget(2) manual page. It also assumes that the semaphore
values of the semaphores in the set have been initialized as shown in
the semctl(2) manual entry.
RETURN VALUE
If or returns due to the receipt of a signal, a value of −1 is returned
to the calling process and is set to If it returns due to the removal
of a semid from the system, a value of −1 is returned and is set to
Upon successful completion, a non-negative value is returned. Other‐
wise, a value of −1 is returned and is set to indicate the error.
ERRORS
fails if one or more of the following is true for any of the semaphore
operations specified by sops:
semid is not a valid semaphore identifier.
sem_num is less than zero or greater than or equal to the
number of semaphores in the set associated with
semid.
nsops is greater than the system-imposed maximum.
Operation permission is denied to the calling process (see
glossary(9)).
The operation would result in suspension of the calling process
but
(sem_flg & is "true".
The limit on the number of individual processes requesting an
would be exceeded.
The number of individual semaphores for which the calling
process
requests a would exceed the limit.
An operation would cause a
semaphore value to overflow the system-imposed
limit.
An operation would cause a
semadj value to overflow the system-imposed limit.
sops points to an illegal address. The reliable detec‐
tion of this error will be implementation depen‐
dent.
fails if one or more of the following is true:
The timeout argument points to an illegal address.
The timeout argument has one or more invalid fields.
For example, tv_sec or tv_nsec is less than 0 or
tv_nsec is greater than or equal to one billion.
The timeout expired before the semaphore operations could be
completed.
Upon successful completion, each semaphore specified in the array
pointed to sops will be updated to record the current time as the last
operation time and the process ID of the calling process as the last to
have modified that semaphore.
SEE ALSOipcs(1), exec(2), exit(2), fork(2), semctl(2), semget(2), stdipc(3C),
privileges(5), signal(5).
STANDARDS CONFORMANCEsemop(2)