semop(2) System Calls Manual semop(2)
NAME
semop(), semtimedop() - semaphore operations
SYNOPSIS
DESCRIPTION
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 structures. nsops is the number of such structures in the array. The
contents of each structure includes the following members:
Each semaphore operation specified by sem_op is performed on the corresponding semaphore specified by semid and sem_num. Semaphore array
operations are atomic in that none of the semaphore operations are performed until blocking conditions on all of the semaphores in the
array have been removed.
sem_op specifies one of three semaphore operations as follows:
o 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 abso-
lute 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 glossary(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 semaphore
waiter count associated with the specified semaphore and suspend execution of the calling process until one of the following con-
ditions occur:
Semaphore value becomes greater than or equal to the absolute value of sem_op. When this occurs, the value of internal sema-
phore waiter count associated with the specified 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 specified 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 semaphore waiter count
associated with the specified semaphore is decremented, and the calling process resumes execution in the manner prescribed in
signal(5).
o 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.
o 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 associated 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 asso-
ciated 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 sus-
pend 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"
operation 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. Otherwise, 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 detection of this error will be implementation dependent.
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 ALSO
ipcs(1), exec(2), exit(2), fork(2), semctl(2), semget(2), stdipc(3C), privileges(5), signal(5).
STANDARDS CONFORMANCE
semop(2)