System Calls semop(2)
NAME
semop, semtimedop - semaphore operations
SYNOPSIS
#include
#include
#include
int semop(int semid, struct sembuf *sops, sizet nsops);
int semtimedop(int semid, struct sembuf *sops, sizet 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 struc-
tures in the array.
Each sembuf structure contains the following members:
short semnum; /* semaphore number */
short semop; /* semaphore operation */
short semflg; /* operation flags */
Each semaphore operation specified by semop is performed on
the corresponding semaphore specified by semid and semnum.
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.
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System Calls semop(2)
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 sema-
phore with the SEMUNDO 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(), sem-
timedop(), exit(), and exit() (the latter two functions
documented on exit(2)), but is otherwise unobservable. See
below for details.
The semop member specifies one of three semaphore opera-
tions:
1. The semop member is a negative integer; {ALTER}
o If semval (see Intro(2)) is greater than or
equal to the absolute value of semop, the
absolute value of semop is subtracted from
semval. Also, if (semflg&SEMUNDO) is true,
the absolute value of semop 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
semop and (semflg&IPCNOWAIT) is true,
semop() returns immediately.
o If semval is less than the absolute value of
semop and (semflg&IPCNOWAIT) is false,
semop() increments the semncnt associated with
the specified semaphore and suspends execution
of the calling thread until one of the follow-
ing conditions occur:
o The value of semval becomes greater than or
equal to the absolute value of semop. When
this occurs, the value of semncnt associ-
ated with the specified semaphore is decre-
mented, the absolute value of semop is
subtracted from semval and, if
(semflg&SEMUNDO) is true, the absolute
value of semop 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
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System Calls semop(2)
is to be caught. When this occurs, the
value of semncnt associated with the speci-
fied semaphore is decremented, and the cal-
ling thread resumes execution in the manner
prescribed in sigaction(2).
2. The semop member is a positive integer; {ALTER}
The value of semop is added to semval and, if
(semflg&SEMUNDO) is true, the value of semop is
subtracted from the calling process's semadj value
for the specified semaphore.
3. The semop member is 0; {READ}
o If semval is 0, semop() returns immediately.
o If semval is not equal to 0 and
(semflg&IPCNOWAIT) is true, semop() returns
immediately.
o If semval is not equal to 0 and
(semflg&IPCNOWAIT) is false, semop() incre-
ments 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 speci-
fied semaphore is decremented, and the cal-
ling 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.
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System Calls semop(2)
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 com-
plete the requested operation(s), it returns immediately
with an error. If timeout is the NUL 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.
ERORS
The semop() and semtimedop() functions will fail if:
E2BIG The nsops argument is greater than the system-
imposed maximum. See NOTES.
EACES Operation permission is denied to the calling pro-
cess (see Intro(2)).
EAGAIN The operation would result in suspension of the
calling process but (semflg&IPCNOWAIT) is true.
EFAULT The sops argument points to an illegal address.
EFBIG The value of semnum 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 iden-
tifier, or the number of individual semaphores for
which the calling process requests a SEMUNDO
operation would exceed the system-imposed limit.
Solaris does not impose a limit on the number of
individual semaphores for which the calling pro-
cess requests a SEMUNDO operation.
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System Calls semop(2)
ENOSPC The limit on the number of individual processes
requesting a SEMUNDO operation would be exceeded.
Solaris does not impose a limit on the number of
individual processes requesting an SEMUNDO opera-
tion.
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 tvsec or tvnsec
value less than 0, or a tvnsec value greater than
or equal to 1000 million.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE 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), attri-
butes(5), standards(5)
NOTES
The system-imposed maximum on nsops for a semaphore identif-
ier is the minimum enforced value of the process.max-sem-ops
resource control of the creating process at the time
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System Calls semop(2)
semget(2) was used to allocate the identifier.
See rctladm(1M) and setrctl(2) for information about using
resource controls.
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