Originální popis anglicky:
semop, semtimedop - semaphore operations
Návod, kniha: Linux Programmer's Manual
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop(int semid, struct sembuf
*sops, unsigned nsops);
int semtimedop(int semid, struct sembuf
*sops, unsigned nsops, struct
timespec *timeout);
A semaphore is represented by an anonymous structure including the following
members:
unsigned short semval; /* semaphore value */
unsigned short semzcnt; /* # waiting for zero */
unsigned short semncnt; /* # waiting for increase */
pid_t sempid; /* process that did last op */
The function
semop performs operations on selected members of the
semaphore set indicated by
semid. Each of the
nsops elements in
the array pointed to by
sops specifies an operation to be performed on
a semaphore by a
struct sembuf including the following members:
unsigned short sem_num; /* semaphore number */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
Flags recognized in
sem_flg are
IPC_NOWAIT and
SEM_UNDO. If
an operation asserts
SEM_UNDO, it will be undone when the process
exits.
The set of operations contained in
sops is performed
atomically,
that is, the operations are performed at the same time, and only if they can
all be simultaneously performed. The behaviour of the system call if not all
operations can be performed immediately depends on the presence of the
IPC_NOWAIT flag in the individual
sem_flg fields, as noted
below.
Each operation is performed on the
sem_num-th semaphore of the semaphore
set, where the first semaphore of the set is semaphore
0. There are
three types of operation, distinguished by the value of
sem_op.
If
sem_op is a positive integer, the operation adds this value to the
semaphore value (
semval). Furthermore, if
SEM_UNDO is asserted
for this operation, the system updates the process undo count (
semadj)
for this semaphore. This operation can always proceed - it never forces a
process to wait. The calling process must have alter permission on the
semaphore set.
If
sem_op is zero, the process must have read access permission on the
semaphore set. This is a "wait-for-zero" operation: if
semval
is zero, the operation can immediately proceed. Otherwise, if
IPC_NOWAIT is asserted in
sem_flg, the system call fails with
errno set to
EAGAIN (and none of the operations in
sops
is performed). Otherwise
semzcnt (the count of processes waiting until
this semaphore's value becomes zero) is incremented by one and the process
sleeps until one of the following occurs:
- •
- semval becomes 0, at which time the value of
semzcnt is decremented.
- •
- The semaphore set is removed: the system call fails, with
errno set to EIDRM.
- •
- The calling process catches a signal: the value of
semzcnt is decremented and the system call fails, with errno
set to EINTR.
- •
- The time limit specified by timeout in a
semtimedop call expires: the system call fails, with errno
set to EAGAIN.
If
sem_op is less than zero, the process must have alter permission on
the semaphore set. If
semval is greater than or equal to the absolute
value of
sem_op, the operation can proceed immediately: the absolute
value of
sem_op is subtracted from
semval, and, if
SEM_UNDO is asserted for this operation, the system updates the process
undo count (
semadj) for this semaphore. If the absolute value of
sem_op is greater than
semval, and
IPC_NOWAIT is asserted
in
sem_flg, the system call fails, with
errno set to
EAGAIN (and none of the operations in
sops is performed).
Otherwise
semncnt (the counter of processes waiting for this
semaphore's value to increase) is incremented by one and the process sleeps
until one of the following occurs:
- •
- semval becomes greater than or equal to the absolute
value of sem_op, at which time the value of semncnt is
decremented, the absolute value of sem_op is subtracted from
semval and, if SEM_UNDO is asserted for this operation, the
system updates the process undo count (semadj) for this
semaphore.
- •
- The semaphore set is removed from the system: the system
call fails with errno set to EIDRM.
- •
- The calling process catches a signal: the value of
semncnt is decremented and the system call fails with errno
set to EINTR.
- •
- The time limit specified by timeout in a
semtimedop call expires: the system call fails, with errno
set to EAGAIN.
On successful completion, the
sempid value for each semaphore specified
in the array pointed to by
sops is set to the process ID of the calling
process. In addition, the
sem_otime is set to the current time.
The function
semtimedop behaves identically to the function
semop
except that in those cases were the calling process would sleep, the duration
of that sleep is limited by the amount of elapsed time specified by the
timespec structure whose address is passed in the
timeout
parameter. If the specified time limit has been reached, the system call fails
with
errno set to
EAGAIN (and none of the operations in
sops is performed). If the
timeout parameter is
NULL,
then
semtimedop behaves exactly like
semop.
If successful the system call returns
0, otherwise it returns
-1
with
errno indicating the error.
On failure,
errno is set to one of the following:
- E2BIG
- The argument nsops is greater than SEMOPM,
the maximum number of operations allowed per system call.
- EACCES
- The calling process does not have the permissions required
to perform the specified semaphore operations, and does not have the
CAP_IPC_OWNER capability.
- EAGAIN
- An operation could not proceed immediately and either
IPC_NOWAIT was asserted in its sem_flg or the time limit
specified in timeout expired.
- EFAULT
- An address specified in either the sops or
timeout parameters isn't accessible.
- EFBIG
- For some operation the value of sem_num is less than
0 or greater than or equal to the number of semaphores in the set.
- EIDRM
- The semaphore set was removed.
- EINTR
- While blocked in this system call, the process caught a
signal.
- EINVAL
- The semaphore set doesn't exist, or semid is less
than zero, or nsops has a non-positive value.
- ENOMEM
- The sem_flg of some operation asserted
SEM_UNDO and the system does not have enough memory to allocate the
undo structure.
- ERANGE
- For some operation sem_op+semval is greater than
SEMVMX, the implementation dependent maximum value for
semval.
The
sem_undo structures of a process aren't inherited across a
fork(2) system call, but they are inherited across a
execve(2)
system call.
semop is never automatically restarted after being interrupted by a
signal handler, regardless of the setting of the
SA_RESTART flags when
establishing a signal handler.
semadj is a per-process integer which is simply the (negative) count of
all semaphore operations performed specifying the
SEM_UNDO flag. When a
semaphore's value is directly set using the
SETVAL or
SETALL
request to
semctl(2), the corresponding
semadj values in all
processes are cleared.
The
semval,
sempid,
semzcnt, and
semnct values for a
semaphore can all be retrieved using appropriate
semctl(2) calls.
The followings are limits on semaphore set resources affecting a
semop
call:
- SEMOPM
- Maximum number of operations allowed for one semop
call (32).
- SEMVMX
- Maximum allowable value for semval: implementation
dependent (32767).
The implementation has no intrinsic limits for the adjust on exit maximum value
(
SEMAEM), the system wide maximum number of undo structures
(
SEMMNU) and the per-process maximum number of undo entries system
parameters.
When a process terminates, its set of associated
semadj structures is
used to undo the effect of all of the semaphore operations it performed with
the
SEM_UNDO flag. This raises a difficulty: if one (or more) of these
semaphore adjustments would result in an attempt to decrease a semaphore's
value below zero, what should an implementation do? One possible approach
would be to block until all the semaphore adjustments could be performed. This
is however undesirable since it could force process termination to block for
arbitrarily long periods. Another possibility is that such semaphore
adjustments could be ignored altogether (somewhat analogously to failing when
IPC_NOWAIT is specified for a semaphore operation). Linux adopts a
third approach: decreasing the semaphore value as far as possible (i.e., to
zero) and allowing process termination to proceed immediately.
SVr4, SVID. SVr4 documents additional error conditions EINVAL, EFBIG, ENOSPC.
semctl(2),
semget(2),
sigaction(2),
ipc(5),
capabilities(7)
