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(5) for descriptions of the following attributes:


+--------------------+----------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+----------------------+
|Interface Stability | semop() is Standard. |
+--------------------+----------------------+

SEE ALSO


ipcs(1), rctladm(1M), Intro(2), exec(2), exit(2), fork(2), semctl(2),
semget(2), setrctl(2), sigaction(2), attributes(5), standards(5)

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(1M) and setrctl(2) for information about using resource
controls.


May 12, 2006 SEMOP(2)