module Netsys_sem:sig..end
This module purely exists to also support "kind of anonymous"
sempahores on MacOS X (which only has named semaphores). On
other OS, it is just a wrapper for the functions in
Netsys_posix.
Note that on OS X named semaphores have a max name length of
31 characters (including the / at the beginning), and that
Netsys_sem uses 9 characters for its own purposes, leaving
22 characters for the prefix. (On other OS this is less restricted.)
val have_anon_semaphores : unit -> boolReturns true if anonymous semaphores are supported on this
system, either natively or emulated via named semaphores.
Constants.
val sem_value_max : intThe maximum value of a semaphore, but at most max_int
val sem_size : intThe size of an anonymous semaphore in bytes (sizeof(sem_t))
Types.
type
The container of the semaphore is the shared memory object
typeprefix =string
A name starting with a slash. Must not contain further slashes
type
typesem_open_flag =Netsys_posix.sem_open_flag=
| |
SEM_O_CREAT |
| |
SEM_O_EXCL |
Container functions.
val container : prefix -> containercontainer prefix: The prefix shall identify the container uniquely.
Once can e.g. use the path of the shared memory object. The prefix
is used to construct names for persistent objects.
Note that containers have kernel persistence! They are not
automatically deleted when the process finishes. Call drop
to delete containers, or create_container to force their
creation as fresh objects.
If the container does not exist yet, it is created. Otherwise the container is just opened.
val create_container : prefix -> containercreate_container prefix: Like container, but the container is
always created. A previous instance is first deleted.
val prefix : container -> prefixReturn the prefix
val drop : container -> unitDrop the semaphores in this container, and delete the container.
This function is a no-op if the OS supports anonymous semaphores directly (because in this case the deletion of the container will automatically destroy the semaphores).
Note that there is a general problem when a container is deleted or dropped by a process while it is still being used by another one. This will generally not generate errors, but can cause a lot of confusion, because the processes will partly access the same semaphores, and partly different semaphores.
val unlink : prefix -> unitUnlinks the identified container if it exists, and unlinks all possible named semaphores.
Semaphore functions.
val sem_init : container ->
Netsys_types.memory -> int -> bool -> int -> anon_semaphoresem_init cont mem pos pshared init_value: Initializes the memory
at position pos to pos + sem_size() - 1 as anonymous semaphore.
If pshared the semaphore is shared between processes.
init_value is the initial non-negative value (max is
sem_value_max).
val sem_destroy : container -> anon_semaphore -> unitDestroys the anonymous semaphore
val as_sem : container ->
Netsys_types.memory -> int -> anon_semaphoreas_sem mem pos: Interprets the memory at position pos
to pos + sem_size() - 1 as anonymous semaphore.
The memory region must already have been initialized.
val sem_getvalue : anon_semaphore -> intReturns the value of the semaphore. If the value is bigger than
what can be represented as int, an EINVAL error is returned.
The returned value is non-negative - if the underlying POSIX function reports a negative value zero is returned instead.
Unavailable on MacOS.
val sem_post : anon_semaphore -> unitUnlocks the semaphore (increases the value by 1)
typesem_wait_behavior =Netsys_posix.sem_wait_behavior=
| |
SEM_WAIT_BLOCK |
| |
SEM_WAIT_NONBLOCK |
val sem_wait : anon_semaphore -> sem_wait_behavior -> unitLocks the semaphore (decreases the value by 1). If the semaphore
value is already zero, and SEM_WAIT_BLOCK is given, the function
waits until another process or thread posts. If SEM_WAIT_NONBLOCK
the error EAGAIN is returned.
sem_wait may be interrupted by signals.
module Debug:sig..end
