Ocamlnet 4 Reference Manual : Netsys_posix

# Module Netsys_posix

    module Netsys_posix: sig .. end

POSIX-specific system calls missing in the `Unix` module, and further API's from POSIX-style operating systems.

# Files, Processes, TTYs, Users, Groups

    val int_of_file_descr : Unix.file_descr -> int

Return the file descriptor as integer. See also `Netsys.int64_of_file_descr` which works for all OS.

    val file_descr_of_int : int -> Unix.file_descr

Make a file descriptor from an integer

    val sysconf_open_max : unit -> int

Return the maximum number of open file descriptor per process. It is also ensured that for every file descriptor `fd`: `fd < sysconf_open_max()`

    val get_nonblock : Unix.file_descr -> bool

Returns whether the nonblock flag is set

    val fchdir : Unix.file_descr -> unit

Set the current directory to the directory referenced by the file descriptor

    val fdopendir : Unix.file_descr -> Unix.dir_handle

Make a directory handle from a file descriptor. The descriptor is then "owned" by the directory handle, and will be closed by `Unix.closedir`.

This function is useful in conjunction with `Netsys_posix.openat` to read directories relative to a parent directory.

This is a recent addition to the POSIX standard; be prepared to get `Invalid_argument` because it is unavailable.

    val realpath : string -> string

Returns a pathname pointing to the same filesystem object so that the pathname does not include "." or ".." or symbolic links.

    val getpgid : int -> int

Return the process group ID of the process with the passed PID. For the number 0, the process group ID of the current process is returned.

    val getpgrp : unit -> int

Same as `getpgid 0`, i.e. returns the process group ID of the current process.

    val setpgid : int -> int -> unit

`setpgid pid pgid`: Set the process group ID of the process `pid` to `pgid`. If `pid = 0`, the process group ID of the current process is changed. If `pgid = 0`, as process group ID the process ID of the process referenced by `pid` is used.

It is only possible for a process to join a process group if both belong to the same session.

    val setpgrp : unit -> unit

Same as `setpgid 0 0`: A new process group ID is created, and the current process becomes its sole member.

    val tcgetpgrp : Unix.file_descr -> int

Return the process group ID of the foreground process group of the session associated with the file descriptor, which must be a tty.

    val tcsetpgrp : Unix.file_descr -> int -> unit

Sets the foreground process group ID of the session associated with the file descriptor, which must be a tty.

    val ctermid : unit -> string

Returns the name of the controlling tty of the current process as pathname to a device file

    val ttyname : Unix.file_descr -> string

Returns the name of the controlling tty referred to by the file descriptor.

    val getsid : int -> int

Returns the session ID of the process with the passed PID. For the PID 0, the session ID of the current process is returned.

    val with_tty : (Unix.file_descr -> unit) -> unit

`with_tty f`: Runs `f fd` where `fd` is the terminal of the process. If the process does not have a terminal (because it is a daemon) `with_tty` will fail.

    val tty_read_password : ?tty:Unix.file_descr -> string -> string

`tty_read_password prompt`: If `tty` is a terminal, the `prompt` is printed, and a password is read from the terminal (echo off). If `tty` is not a terminal, no `prompt` is printed, and just a line is read from the `tty` descriptor (non-interactive case).

`tty` defaults to `Unix.stdin`. If this function is used in a program where stdin is not redirected, and the program is started in a terminal, it will read the password with prompt and echo disabled. If stdin is redirected, it is assumed that the program is used in a script, and the password is piped into it.

Use in conjunction with `with_tty` to ensure that `tty` is the terminal even if a redirection is in effect, e.g.

     with_tty (fun tty -> tty_read_password ~tty prompt)

Raises `Sys.Break` if the user triggers SIGINT (i.e. presses CTRL-C) to abort the input of a password.

    val posix_openpt : bool -> Unix.file_descr

`posix_openpt noctty`: Opens an unused PTY master.

`noctty`: If true, the descriptor will not become the controlling terminal.

If this function is not provided by the OS, an emulation is used.

On some OS, System V style PTY's are unavailable (but they get rare).

    val grantpt : Unix.file_descr -> unit

Grant access to this PTY

    val unlockpt : Unix.file_descr -> unit

Unlock a PTY master/slave pair

    val ptsname : Unix.file_descr -> string

Get the name of the slave PTY

    type node_type =

`|` | `S_IFREG`
`|` | `S_IFCHR of `int``
`|` | `S_IFBLK of `int``
`|` | `S_IFIFO`
`|` | `S_IFSOCK`

    val mknod : string -> int -> node_type -> unit

Creates the node with the given permissions and the given type

    val setreuid : int -> int -> unit

Changes both the real and the effective user ID of the current process.

    val setregid : int -> int -> unit

Changes both the real and the effective group ID of the current process.

    val initgroups : string -> int -> unit

See initgroups(3). This is a non-POSIX function but widely available.

# The "at" variants of system calls

Note that a few "at" calls have been omitted because the same functionality can be achieved by first opening the file with `openat` and then by using a function that references the file by descriptor. An example for this is `fstatat`: After the `openat` call one can use `fstat` to get the stat record of the file.

    val have_at : unit -> bool

Whether the `*at` functions are available (they were only recently standardized and cannot be expected on all OS yet)

    val at_fdcwd : Unix.file_descr

Pseudo descriptor value to be used as first argument of `*at` functions

    type at_flag =

`|` | `AT_EACCESS`
`|` | `AT_SYMLINK_NOFOLLOW`
`|` | `AT_SYMLINK_FOLLOW`
`|` | `AT_REMOVEDIR` | `(*` | Flags one can pass to "at" functions. Not all functions support all flags | `*)`

    val openat : Unix.file_descr ->
           string -> Unix.open_flag list -> Unix.file_perm -> Unix.file_descr

Same as `Unix.openfile` but open relative to the directory given by first argument

    val faccessat : Unix.file_descr ->
           string -> Unix.access_permission list -> at_flag list -> unit

Same as `Unix.access` but the file is taken relative to the directory given by first argument

    val mkdirat : Unix.file_descr -> string -> int -> unit

Same as `Unix.mkdir` but the file is taken relative to the directory given by first argument

    val renameat : Unix.file_descr -> string -> Unix.file_descr -> string -> unit

`renameat olddirfd oldpath newdirfd newpath`

    val linkat : Unix.file_descr ->
           string -> Unix.file_descr -> string -> at_flag list -> unit

`linkat olddirfd oldpath newdirfd newpath flags`

    val unlinkat : Unix.file_descr -> string -> at_flag list -> unit

Same as `Unix.unlink` but unlink the file relative to the directory given by first argument

    val symlinkat : string -> Unix.file_descr -> string -> unit

`symlinkat oldpath newdirfd newpath flags`

    val mkfifoat : Unix.file_descr -> string -> int -> unit

`mkfifoat dirfd path mode`

NB. MacOS 10.10 doesn't support mkfifoat although the other "at" functions are implemented. Be prepared to get `Invalid_argument`.

    val readlinkat : Unix.file_descr -> string -> string

`readlinkat dirfd path`

# File descriptor polling

    type poll_array

The array of `poll_cell` entries

    type poll_req_events

    type poll_act_events

Poll events. `poll_req_events` is used to request that certain event types are observed. `poll_act_event` shows which event types are actually possible

    type poll_cell = {

`` | `mutable poll_fd : `Unix.file_descr`;`
`` | `mutable poll_req_events : `poll_req_events`;`
`` | `mutable poll_act_events : `poll_act_events`;`

}
The poll cell refers to the descriptor `poll_fd`. The `poll_req_events` are the events the descriptor is polled for. The `poll_act_events` are the actually reported events.

    val have_poll : unit -> bool

Whether there is a native `poll` implementation on this OS

    val poll_req_events : bool -> bool -> bool -> poll_req_events

`poll_req_events rd wr pri`: Create a set of in events consisting of the bits `rd`, `wr`, and `pri`. `rd` means to poll for input data, `wr` to poll for output data, and `pri` to poll for urgent input data.

    val poll_req_triple : poll_req_events -> bool * bool * bool

Looks into a `poll_req_events` value, and returns the triple `(rd,wr,pri)`.

    val poll_null_events : unit -> poll_act_events

Create an empty set of `poll_act_events`, for initilization of poll cells.

    val poll_result : poll_act_events -> bool

Look whether there is any event in `poll_out_events`

    val poll_rd_result : poll_act_events -> bool

    val poll_wr_result : poll_act_events -> bool

    val poll_pri_result : poll_act_events -> bool

    val poll_err_result : poll_act_events -> bool

    val poll_hup_result : poll_act_events -> bool

    val poll_nval_result : poll_act_events -> bool

Look for the bit in `poll_act_events` and return the status

    val create_poll_array : int -> poll_array

Create a poll array with the given size. The `poll_fd` member is initialized with `Unix.stdin`, and the two event members are empty.

    val set_poll_cell : poll_array -> int -> poll_cell -> unit

`set_poll_cell a k c`: Sets the poll cell `k` to `c`. The index `k` must be in the range from `0` to `N-1` when `N` is the length of the poll array.

    val get_poll_cell : poll_array -> int -> poll_cell

`get_poll_cell a k`: Returns the poll cell `k`. The index `k` must be in the range from `0` to `N-1` when `N` is the length of the poll array.

    val blit_poll_array : poll_array ->
           int -> poll_array -> int -> int -> unit

`blit_poll_array a1 p1 a2 p2 len`: Copies the `len` cells at index `p1` from `a1` to `a2` at index `p2`.

    val poll_array_length : poll_array -> int

Return the number of cells in the poll array

    val poll : poll_array -> int -> float -> int

`poll a n tmo`: Poll for the events of the cells 0 to `n-1` of poll array `a`, and set the `poll_act_events` member of all cells. Wait for at most `tmo` seconds (a negative value means there is no timeout). Returns the number of ready file descriptors.

On platforms without native support for `poll` the function is emulated using `Unix.select`. Note, however, that there is a performance penalty for the emulation, and that the output flags `poll_error_result`, `poll_hangup_result`, and `poll_invalid_result` are not emulated.

    val restarting_poll : poll_array -> int -> float -> int

A wrapper around `poll` that handles the `EINTR` condition

    val poll_single : Unix.file_descr -> bool -> bool -> bool -> float -> bool

`poll_single fd rd wr pri tmo`: Polls a single descriptor for the events given by `rd`, `wr`, and `pri`. In `tmo` the timeout can be passed. Returns `true` if one of the requested events is indicated for the descriptor. The `EINTR` case is not handled.
Actually, `poll_req_events` and `poll_act_events` are integers that are bitmasks of some constants. The following functions allow access to this detail.

    val int_of_req_events : poll_req_events -> int

    val int_of_act_events : poll_act_events -> int

    val req_events_of_int : int -> poll_req_events

    val act_events_of_int : int -> poll_act_events

    val const_rd_event : int

    val const_wr_event : int

    val const_pri_event : int

    val const_err_event : int

    val const_hup_event : int

    val const_nval_event : int

# Event aggregation

Support for "high-speed" poll implementations. Currently, only `epoll` for Linux is supported.

The model exhibited in this API is the smallest common denominator of Linux epoll, BSD kqueue, and Solaris ports. The `event_aggregator` represents the set of monitored event sources. There is, so far, only one source, namely file descriptors, i.e. one can check whether a descriptor is readable or writable (like `poll`). The source can be added to the `event_aggregator` to monitor the source.

By calling `poll_event_sources` one can determine sources that are currently active (i.e. in signalling state).

It is undefined what happens when a file descriptor is closed while being member of the aggregator.

    type event_aggregator

    type event_source

    val have_event_aggregation : unit -> bool

Whether there is an implementation for this OS

    val create_event_aggregator : bool -> event_aggregator

`create_event_aggregator is_interruptible`: Creates a new aggregator, and allocates the required OS resources.

If `is_interruptible`, the aggregator can be interrupted from a different thread. See `interrupt_event_aggregator` below.

    val destroy_event_aggregator : event_aggregator -> unit

Frees all OS resources

    val fd_event_source : Unix.file_descr -> poll_req_events -> event_source

Wraps a file descriptor as event_source, and monitors the events in `poll_req_events`.

The `event_source` contains state about the relation to the aggregator, and because of this, the `event_source` should only be used together with one aggregator (at a time).

    val modify_fd_event_source : event_source -> poll_req_events -> unit

Modifies the set of events monitored at this event source

    val get_fd_of_event_source : event_source -> Unix.file_descr

Get the file descriptor wrapped by this event source

    val act_events_of_event_source : event_source -> poll_act_events

Return the actual events of the source. This is updated when `poll_event_sources` returns the source.

    val add_event_source : event_aggregator -> event_source -> unit

Adds the event source to the aggregator

    val del_event_source : event_aggregator -> event_source -> unit

Removes the source from the aggregator

    val interrupt_event_aggregator : event_aggregator -> unit

If `create_event_aggregator` was called with `true` as argument, the aggregator is interruptible, and this function interrupts it. The effect is that a currently running `poll_event_sources`, or, if it is not running, the next invocation of `poll_event_sources` returns immediately.

If the aggregator is not interruptible, this function is a NOP.

    val push_event_updates : event_aggregator -> unit

Pushes all modifications of the sources to the kernel

    val poll_event_sources : event_aggregator -> float -> event_source list

`poll_event_sources ea tmo`: First, all modifications are pushed to the kernel, and polling is set up to get events. If no events can currently be delivered, the function waits up to `tmo` seconds (or endlessly if negative) for events. The function returns only a limited number of events at a time. It is allowed that the function returns fewer events than are currently in signalled state, even none.

Call the function with `tmo=0.0` for non-blocking behavior.

Note that this is the "level-triggered" behavior: If a source remains active it will be reported again by the next `poll_event_sources`, just as `poll` would do.

    val event_aggregator_fd : event_aggregator -> Unix.file_descr

Returns the underlying file descriptor. It is implementation-defined whether this descriptor can also be polled for events. Generally, you should run `push_event_updates` before polling from the descriptor.

# Fork helpers

Ocamlnet invokes `Unix.fork` at some places to create child processes for doing real work. The following functions allow it to register a handler that is run in the forked child process. Note that this is done by the O'caml code calling `fork`, and not via the POSIX `atfork()` facility.

The handler should release OS resources like file descriptors that are by default shared with the parent process.

The handler are not invoked when the only purpose of the `fork` is to `exec` a different process.

    class type post_fork_handler = object .. end

A `post_fork_handler` is a named function `unit -> unit`

    val register_post_fork_handler : post_fork_handler -> unit

Registers a new post fork handler (MT-Safe)

    val remove_post_fork_handler : post_fork_handler -> unit

Removes a post fork handler from the registry (MT-Safe)

    val run_post_fork_handlers : unit -> unit

Runs all post fork handlers. Exceptions are caught and printed to stderr.

# Fork+exec

The following function has some similarity with posix_spawn, but is extended to our needs, Only special (although frequent) cases are implemented with posix_spawn.

    type wd_spec =

`|` | `Wd_keep` | `(*` | Keep the current working directory in the spawned process | `*)`
`|` | `Wd_chdir of `string`` | `(*` | Change to this directory in the spawned process | `*)`
`|` | `Wd_fchdir of `Unix.file_descr`` | `(*` | Change to the directory which has been previously been opened | `*)`

    type pg_spec =

`|` | `Pg_keep` | `(*` | The new process will be member of the same process group as this process | `*)`
`|` | `Pg_new_bg_group` | `(*` | A new background process group is created, and the spawned process will be its single member | `*)`
`|` | `Pg_new_fg_group` | `(*` | A new foreground process group is created, and the spawned process will be its single member | `*)`
`|` | `Pg_join_group of `int`` | `(*` | The spawned process will be member of this process group | `*)`

    type fd_action =

`|` | `Fda_close of `Unix.file_descr`` | `(*` | Close the descriptor | `*)`
`|` | `Fda_close_ignore of `Unix.file_descr`` | `(*` | Close the descriptor but ignore `EBADF` errors | `*)`
`|` | `Fda_close_except of `bool array`` | `(*` | Closes all descriptors except those for which `except.(k)` is true where `k = int_of_file_descr fd`. Descriptors outside the array index range are closed. | `*)`
`|` | `Fda_dup2 of `Unix.file_descr * Unix.file_descr`` | `(*` | Duplicate the first descriptor to the second as `dup2` does | `*)`

    type sig_action =

`|` | `Sig_default of `int`` | `(*` | Resets this signal to default behavior in the spawned process | `*)`
`|` | `Sig_ignore of `int`` | `(*` | Ignores the signal in the spawned process | `*)`
`|` | `Sig_mask of `int list`` | `(*` | Set the signal mask in the spawned process | `*)`

    val spawn : ?chdir:wd_spec ->
           ?pg:pg_spec ->
           ?fd_actions:fd_action list ->
           ?sig_actions:sig_action list ->
           ?env:string array -> ?no_posix_spawn:bool -> string -> string array -> int

`spawn cmd args`: Fork the process and exec `cmd` which gets the arguments `args`. On success, the PID of the new process is returned. This function does not wait for the completion of the process; use `Unix.waitpid` for this purpose.

- `chdir`: If set, the new process starts with this working directory (this is done before anything else)
- `pg`: If set, the new process will be a member of this process group
- `fd_actions`: If set, these descriptor actions are executed sequentially
- `sig_actions`: If set, these signal actions are executed sequentially
- `env`: If set, the process gets this environment instead of the current one
- `no_posix_spawn`: If set, the `posix_spawn` family of library functions is not used to spawn even if possible, and always a `fork/exec` approach is taken. This may be slower, but there is normally better error reporting.

Any exceptions in the subprocess are detected, and reported. However, if `Fda_close_ignore` leads to `EBADF` for a descriptor, this error is ignored.

If `pg=Pg_new_fg_group`, one should include `Sig_ignore Sys.sigttou` in `sig_actions`.

There are two implementations for `spawn`: One calls `fork` and `exec` directly, and one uses the `posix_spawn` family of library functions. The latter is faster on certain conditions, but this is very OS-specific. A number of features are not supported by `posix_spawn` and will force that `fork/exec` is used: `Wd_chdir`, `Wd_fchdir`, `Pg_new_fg_group`, and `Sig_ignore`. However, note some implementations of `posix_spawn` also fall back to `fork/exec` internally for some combinations of flags, and it is hard to predict which spawn calls can actually be accelerated. The tendency, though, is that recent OS have sped up `posix_spawn` so far possible (e.g. by using `vfork` internally, or even by making `posix_spawn` a system call).

# Notification via file descriptor events

Often, it is advantageous to report asynchronous events via file descriptors. On Linux, this is available via the `eventfd` system call. On other platforms, pipes are used for emulation.

A `not_event` can have two states: off and on. Initially, the `not_event` is off. By signalling it, the state changes to on, and the underlying real file descriptor becomes readable. By consuming the event, the state is switched back to off.

Note that a similar API exists for Win32: See `Netsys_win32.w32_event`.

    type not_event

    val create_event : unit -> not_event

Creates a new event file descriptor.

    val set_nonblock_event : not_event -> unit

Sets the event fd to non-blocking mode

    val get_event_fd : not_event -> Unix.file_descr

Returns a duplicate of the underlying file descriptor. This should only be used for one thing: checking whether the desciptor becomes readable. As this is a duplicate, the caller has to close the descriptor.

    val set_event : not_event -> unit

Signals the event

    val wait_event : not_event -> unit

If the event fd is not signalled, the function blocks until it gets signalled, even in non-blocking mode.

    val consume_event : not_event -> unit

Consumes the event, and switches the event fd to off again. If the event fd is not signalled, the function blocks until it gets signalled (in blocking mode), or it raises `EAGAIN` or `EWOULDBLOCK` (in non-blocking mode).

This is effectively an atomic "wait-and-reset" operation.

    val destroy_event : not_event -> unit

Releases the OS resources. Note that there can be a hidden second file descriptor, so closing the descriptor returned by `get_event_fd` is not sufficient.

    val report_signal_as_event : not_event -> int -> unit

`report_signal_as_event ev sig` Installs a new signal handler for signal `sig` so that `ev` is signalled when a signal arrives.

# Notification queues

Unimplemented, but a spec exists. Notification queues are intended for forwarding events from C level to OCaml level. Possible uses:

- POSIX timers
- Realtime signals
- Subprocess monitoring
- AIO completion

# Notification queues

Unimplemented, but a spec exists. Notification queues are intended for forwarding events from C level to OCaml level. Possible uses:

- POSIX timers
- Realtime signals
- Subprocess monitoring
- AIO completion

# Subprocesses and signals

# Notification queues

Unimplemented, but a spec exists. Notification queues are intended for forwarding events from C level to OCaml level. Possible uses:

- POSIX timers
- Realtime signals
- Subprocess monitoring
- AIO completion

# Subprocesses and signals

Watching subprocesses requires that the right signal handler is installed: `install_subprocess_handler`

    type watched_subprocess

    val watch_subprocess : int -> int -> bool -> Unix.file_descr * watched_subprocess

`let fd, ws = watch_subprocess pid pgid kill_flag`: Enters the subprocess `pid` into the watch list. If `pgid > 0`, the process group ID is `pgid` (for `killpg_subprocess` and `killpg_all_subprocesses`). The `kill_flag` controls the process selection of `kill_all_subprocesses` and `killpg_all_subprocesses`.

The returned descriptor `fd` is open for reading and will indicate EOF when the subprocess is terminated. Via `ws` it is possible to query information about the subprocess. The installed signal handler will `wait` for the subprocess and put the process status into `ws`.

The caller has to close `fd` after the termination is signaled.

    val ignore_subprocess : watched_subprocess -> unit

Changes the arrangement so that the termination of the subprocess is no longer reported by the file descriptor. The file descriptor indicates EOF immediately (and can be closed by the caller). Nevertheless, the signal handler still `wait`s for the subprocess to avoid zombies.

Any further access to `ws` will fail.

    val forget_subprocess : watched_subprocess -> unit

Frees OS resources. Any further access to the `ws` will fail.

    val get_subprocess_status : watched_subprocess -> Unix.process_status option

If the subprocess is terminated, this function returns the status. Otherwise `None` is returned

    val kill_subprocess : int -> watched_subprocess -> unit

Sends this signal to the subprocess if this process still exists. Never throws an exception.

    val killpg_subprocess : int -> watched_subprocess -> unit

Sends this signal to the process group of the subprocess if there is still a watched subprocess belonging to this group. Never throws an exception.

    val kill_all_subprocesses : int -> bool -> bool -> unit

`kill_all_subprocess signal override nogroup`: Sends a signal to potentially all subprocesses. The signal is sent to a watched process if the process still exists, and these two conditions hold both:

- `not nogroup || pgid = 0`: Processes with `pgid > 0` are excluded if `nogroup` is set
- `kill_flag || override`: A process needs to have `kill_flag` set, or `override` is specified

Never throws an exception if the signal handler is installed.

    val killpg_all_subprocesses : int -> bool -> unit

`killpg_all_subprocess signal override`: Sends a signal to potentially all subprocesses belonging to a process group (i.e. `pgid>0`). . The signal is sent to a process group if there are still watched subprocesses belonging to the group, and if either the `kill_flag` of any of the subprocesses process was set to `true`, or `override` is `true`.

Never throws an exception if the signal handler is installed.

    val install_subprocess_handler : unit -> unit

Installs a SIGCHLD handler for watching subprocesses. Note that only processes are `wait`ed for that are registered with `watch_subprocess`.

The handler works both in the single-threaded and the multi-threaded case. `install_subprocess_handler` can safely called several times. The handler is installed every time the function is called, but the required data structures are only initialized at the first call.

    val register_subprocess_handler : unit -> unit

Uses the `Netsys_signal` framework to manage the installation of the SIGCHLD handler.

This is the preferred method of installing the SIGCHLD handler.

Further notes.
Further notes.
The subprocess handler and `fork()`: The subprocess handler uses pipes for notification, and because of this it is sensitive to unpredicted duplicates of the pipe descriptors. `fork()` duplicates these pipe descriptors. If nothing is done about this issue, it can happen that the notification does not work anymore as it relies on detecting closed pipes.

If `fork()` is immediately followed by `exec()` (as it is done to run subcommands), the problem does not occur, because the relevant descriptors are closed at `exec()` time.

If `fork()` is used to start worker processes, however, we have to be careful. The descriptors need to be closed, so that the parent can continue to monitor subprocesses, and to allow the worker processes to use this mechanism. This module defines post fork handlers (see above), and a handler is automatically added that cleans the descriptors up. All user code has to do is to call `run_post_fork_handlers` immediately after `fork()` has spawned the new child, from the new child. This completely resets everything.
Further notes.
The subprocess handler and `fork()`: The subprocess handler uses pipes for notification, and because of this it is sensitive to unpredicted duplicates of the pipe descriptors. `fork()` duplicates these pipe descriptors. If nothing is done about this issue, it can happen that the notification does not work anymore as it relies on detecting closed pipes.

If `fork()` is immediately followed by `exec()` (as it is done to run subcommands), the problem does not occur, because the relevant descriptors are closed at `exec()` time.

If `fork()` is used to start worker processes, however, we have to be careful. The descriptors need to be closed, so that the parent can continue to monitor subprocesses, and to allow the worker processes to use this mechanism. This module defines post fork handlers (see above), and a handler is automatically added that cleans the descriptors up. All user code has to do is to call `run_post_fork_handlers` immediately after `fork()` has spawned the new child, from the new child. This completely resets everything.
The subprocess handler and multi-threading: The handler has been carefully designed, and works even in multi-threaded programs. However, one should know that multi-threading and `fork()` do not interact well with each other. Again, the problems do not occur if `fork()` is followed by `exec()`. There is no solution for the case that worker processes are started with `fork()`, though. The (very generic) problem is that the state of mutexes and other multi-threading primitives is not well-defined after a `fork()`.
Further notes.
The subprocess handler and `fork()`: The subprocess handler uses pipes for notification, and because of this it is sensitive to unpredicted duplicates of the pipe descriptors. `fork()` duplicates these pipe descriptors. If nothing is done about this issue, it can happen that the notification does not work anymore as it relies on detecting closed pipes.

If `fork()` is immediately followed by `exec()` (as it is done to run subcommands), the problem does not occur, because the relevant descriptors are closed at `exec()` time.

If `fork()` is used to start worker processes, however, we have to be careful. The descriptors need to be closed, so that the parent can continue to monitor subprocesses, and to allow the worker processes to use this mechanism. This module defines post fork handlers (see above), and a handler is automatically added that cleans the descriptors up. All user code has to do is to call `run_post_fork_handlers` immediately after `fork()` has spawned the new child, from the new child. This completely resets everything.
The subprocess handler and multi-threading: The handler has been carefully designed, and works even in multi-threaded programs. However, one should know that multi-threading and `fork()` do not interact well with each other. Again, the problems do not occur if `fork()` is followed by `exec()`. There is no solution for the case that worker processes are started with `fork()`, though. The (very generic) problem is that the state of mutexes and other multi-threading primitives is not well-defined after a `fork()`.

# Syslog

    type level = Netlog.level

The log levels

    type syslog_facility = [ `Authpriv
           | `Cron
           | `Daemon
           | `Default
           | `Ftp
           | `Kern
           | `Local0
           | `Local1
           | `Local2
           | `Local3
           | `Local4
           | `Local5
           | `Local6
           | `Local7
           | `Lpr
           | `Mail
           | `News
           | `Syslog
           | `User
           | `Uucp ]

The facilities. Only ``User` and ``Local0` to ``Local7` are standard POSIX. If a facility is unavailable it is silently substituted by ``Local0`. The value ``Default` leaves this unspecified.

    type syslog_option = [ `Cons | `Ndelay | `Nowait | `Odelay | `Pid ]

The syslog options:

- ``Cons`: Fall back to console logging if syslog is unavailable
- ``Ndelay`: Open the connection immediately
- ``Odelay`: Open the connection at the first call `syslog` (default)
- ``Nowait`: Do not wait until it is ensured that the message is sent
- ``Pid`: Log the PID with every message

    val openlog : string option ->
           syslog_option list -> syslog_facility -> unit

`openlog ident options facility`: Opens a log stream. `ident` is prepended to every message if given (usually the program name). The `facility` is the default facility for `syslog` calls.

    val syslog : syslog_facility -> level -> string -> unit

`syslog facility level message`: Logs `message` at `level` for `facility`

    val closelog : unit -> unit

Closes the log stream
Usually, the log stream is redirected to syslog by either:

- setting `Netlog.current_logger` to `syslog facility`, e.g.

       Netlog.current_logger := Netsys_posix.syslog `User

- using the Netplex class for sending message to syslog (XXX)

Usually, the log stream is redirected to syslog by either:

- setting `Netlog.current_logger` to `syslog facility`, e.g.

       Netlog.current_logger := Netsys_posix.syslog `User

- using the Netplex class for sending message to syslog (XXX)

# Sync

    val fsync : Unix.file_descr -> unit

Sync data and metadata to disk

    val fdatasync : Unix.file_descr -> unit

Syncs only data to disk. If this is not implemented, same effect as `fsync`

# Sending file descriptors over Unix domain sockets

These functions can be used to send file descriptors from one process to another one. The descriptor `sock` must be a connected Unix domain socket.

The functionality backing this is non-standard but widely available.

Not yet implemented, but spec exists.

# Sending file descriptors over Unix domain sockets

These functions can be used to send file descriptors from one process to another one. The descriptor `sock` must be a connected Unix domain socket.

The functionality backing this is non-standard but widely available.

Not yet implemented, but spec exists.

# Optional POSIX functions

    val have_fadvise : unit -> bool

Returns whether the OS supports the fadvise POSIX option

    type advice =

`|` | `POSIX_FADV_NORMAL`
`|` | `POSIX_FADV_SEQUENTIAL`
`|` | `POSIX_FADV_RANDOM`
`|` | `POSIX_FADV_NOREUSE`
`|` | `POSIX_FADV_WILLNEED`
`|` | `POSIX_FADV_DONTNEED`
`|` | `FADV_NORMAL`
`|` | `FADV_SEQUENTIAL`
`|` | `FADV_RANDOM`
`|` | `FADV_NOREUSE`
`|` | `FADV_WILLNEED`
`|` | `FADV_DONTNEED` | `(*` | Possible advices for fadvise. The names starting with "POSIX_" and the ones lacking the prefix have the same meaning. In new code, the names starting with "POSIX_" should be preferred (for better compaibility with other libraries). | `*)`

    val fadvise : Unix.file_descr -> int64 -> int64 -> advice -> unit

Advises to load pages into the page table from the file, or to remove such pages.

    val have_fallocate : unit -> bool

Returns whether the OS supports the fallocate POSIX option

    val fallocate : Unix.file_descr -> int64 -> int64 -> unit

Allocate space for the file and the specified file region

# POSIX Shared Memory

    val have_posix_shm : unit -> bool

Returns whether the OS supports POSIX shared memory

    type shm_open_flag =

`|` | `SHM_O_RDONLY`
`|` | `SHM_O_RDWR`
`|` | `SHM_O_CREAT`
`|` | `SHM_O_EXCL`
`|` | `SHM_O_TRUNC`

    val shm_open : string -> shm_open_flag list -> int -> Unix.file_descr

Opens a shared memory object. The first arg is the name of the object. The name must begin with a slash, but there must be no further slash in it (e.g. "/sample"). The second arg are the open flags. The third arg are the permission bits.

The open flags are interpreted as follows:

- `SHM_O_RDONLY`: Open the object for read access
- `SHM_O_RDWR`: Open the object for read-write access
- `SHM_O_CREAT`: Create the object if it does not exist
- `SHM_O_EXCL`: If `SHM_O_CREAT` was also specified, and a an object with the given name already exists, return an error (`Unix.EEXIST`).
- `SHM_O_TRUNC`: If the object already exists, truncate it to zero bytes

One of `SHM_O_RDONLY` or `SHM_O_RDWR` must be given.

On success, the function returns a file descriptor representing the object. To access the object, one has to memory-map this file use one of the `map_file` functions in the `Bigarray` module, or in `Netsys_mem`). Use `Unix.ftruncate` to resize the object.

Note that it is unspecified whether this file pops up somewhere in the file system, and if so, where.

If a system error occurs, the function raises a `Unix.Unix_error` exception.

    val shm_unlink : string -> unit

Unlinks the name for a shared memory object

    val shm_create : string -> int -> Unix.file_descr * string

`let (fd,name) = shm_create prefix size`: Creates an shm object with a unique name. The name has the passed `prefix`. The `prefix` must start with "/" but must not contain any further "/". The object has a length of `size` bytes. The object has a permissions 0o600 (independent of umask).

# POSIX semaphores

    val have_named_posix_semaphores : unit -> bool

Returns `true` if named POSIX semaphores are supported on this system

    val have_anon_posix_semaphores : unit -> bool

Returns `true` if anonymous POSIX semaphores are supported on this system

    val have_posix_semaphores : unit -> bool

Returns `true` if both kinds of semaphores are supported on this system
Constants.

    val sem_value_max : int

The maximum value of a semaphore, but at most `max_int`

    val sem_size : int

The size of an anonymous semaphore in bytes (`sizeof(sem_t)`)
Types.

    type sem_kind = [ `Anonymous | `Named ]

    type 'sem_kind semaphore

    type named_semaphore = [ `Named ] semaphore

    type anon_semaphore = [ `Anonymous ] semaphore

    type sem_open_flag =

`|` | `SEM_O_CREAT`
`|` | `SEM_O_EXCL`

Named semaphores.

    val sem_open : string ->
           sem_open_flag list -> int -> int -> named_semaphore

`sem_open name flags mode init_value`: Opens a named semaphore which is optionally created. Sempahore names usually begin with a slash followed by a single name component (not containing a further slash).

Interpretation of `flags`:

- `SEM_O_CREAT`: The semaphore is created if not yet existing. The `mode` and `init_value` are interpreted if the creation actually occurs. `mode` is the permission of the semaphore. `init_value` is the (non-negative) initial value, up to `sem_value_max`.
- `SEM_O_EXCL`: The semaphore is only opened if the semaphore does not exist yet. Othwerwise an `EEXIST` error is returned

    val sem_close : named_semaphore -> unit

Closes a named semaphore. Semaphores are also automatically closed when the GC finds that the semaphore is unreachable.

    val sem_unlink : string -> unit

Unlinks the semaphore name

    val sem_create : string -> int -> named_semaphore * string

`let (sem,name) = sem_create prefix init_value`: Creates a new semaphore with a unique name. The name has the passed `prefix`. The `prefix` must start with "/" but must not contain any further "/". The semaphore is initialized with `init_value`. The object has permissions 0o600 (modulo umask).
Anonymous semaphores.

    val sem_init : Netsys_types.memory -> int -> bool -> int -> anon_semaphore

`sem_init 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 : anon_semaphore -> unit

Destroys the anonymous semaphore

    val as_sem : Netsys_types.memory -> int -> anon_semaphore

`as_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.
Operations.

    val sem_getvalue : 'kind semaphore -> int

Returns 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 : 'kind semaphore -> unit

Unlocks the semaphore (increases the value by 1)

    type sem_wait_behavior =

`|` | `SEM_WAIT_BLOCK`
`|` | `SEM_WAIT_NONBLOCK`

    val sem_wait : 'kind semaphore -> sem_wait_behavior -> unit

Locks 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.

## Semaphores and notification

Not yet implemented.

## Semaphores and notification

Not yet implemented.

# Locales

    type langinfo = {

`` | `nl_CODESET : `string`;` | `(*` | from `LC_CTYPE`: codeset name | `*)`
`` | `nl_D_T_FMT : `string`;` | `(*` | from `LC_TIME`: string for formatting date and time | `*)`
`` | `nl_D_FMT : `string`;` | `(*` | from `LC_TIME`: date format string | `*)`
`` | `nl_T_FMT : `string`;` | `(*` | from `LC_TIME`: time format string | `*)`
`` | `nl_T_FMT_AMPM : `string`;` | `(*` | from `LC_TIME`: a.m. or p.m. time format string | `*)`
`` | `nl_AM_STR : `string`;` | `(*` | from `LC_TIME`: Ante Meridian affix | `*)`
`` | `nl_PM_STR : `string`;` | `(*` | from `LC_TIME`: Post Meridian affix | `*)`
`` | `nl_DAY_1 : `string`;` | `(*` | from `LC_TIME`: name of the first day of the week (for example, Sunday) | `*)`
`` | `nl_DAY_2 : `string`;` | `(*` | from `LC_TIME`: name of the second day of the week (for example, Monday) | `*)`
`` | `nl_DAY_3 : `string`;` | `(*` | from `LC_TIME`: name of the third day of the week (for example, Tuesday) | `*)`
`` | `nl_DAY_4 : `string`;` | `(*` | from `LC_TIME`: name of the fourth day of the week (for example, Wednesday) | `*)`
`` | `nl_DAY_5 : `string`;` | `(*` | from `LC_TIME`: name of the fifth day of the week (for example, Thursday) | `*)`
`` | `nl_DAY_6 : `string`;` | `(*` | from `LC_TIME`: name of the sixth day of the week (for example, Friday) | `*)`
`` | `nl_DAY_7 : `string`;` | `(*` | from `LC_TIME`: name of the seventh day of the week (for example, Saturday) | `*)`
`` | `nl_ABDAY_1 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the first day of the week | `*)`
`` | `nl_ABDAY_2 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the second day of the week | `*)`
`` | `nl_ABDAY_3 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the third day of the week | `*)`
`` | `nl_ABDAY_4 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the fourth day of the week | `*)`
`` | `nl_ABDAY_5 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the fifth day of the week | `*)`
`` | `nl_ABDAY_6 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the sixth day of the week | `*)`
`` | `nl_ABDAY_7 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the seventh day of the week | `*)`
`` | `nl_MON_1 : `string`;` | `(*` | from `LC_TIME`: name of the first month of the year | `*)`
`` | `nl_MON_2 : `string`;` | `(*` | from `LC_TIME`: name of the second month | `*)`
`` | `nl_MON_3 : `string`;` | `(*` | from `LC_TIME`: name of the third month | `*)`
`` | `nl_MON_4 : `string`;` | `(*` | from `LC_TIME`: name of the fourth month | `*)`
`` | `nl_MON_5 : `string`;` | `(*` | from `LC_TIME`: name of the fifth month | `*)`
`` | `nl_MON_6 : `string`;` | `(*` | from `LC_TIME`: name of the sixth month | `*)`
`` | `nl_MON_7 : `string`;` | `(*` | from `LC_TIME`: name of the seventh month | `*)`
`` | `nl_MON_8 : `string`;` | `(*` | from `LC_TIME`: name of the eighth month | `*)`
`` | `nl_MON_9 : `string`;` | `(*` | from `LC_TIME`: name of the ninth month | `*)`
`` | `nl_MON_10 : `string`;` | `(*` | from `LC_TIME`: name of the tenth month | `*)`
`` | `nl_MON_11 : `string`;` | `(*` | from `LC_TIME`: name of the eleventh month | `*)`
`` | `nl_MON_12 : `string`;` | `(*` | from `LC_TIME`: name of the twelfth month | `*)`
`` | `nl_ABMON_1 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the first month | `*)`
`` | `nl_ABMON_2 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the second month | `*)`
`` | `nl_ABMON_3 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the third month | `*)`
`` | `nl_ABMON_4 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the fourth month | `*)`
`` | `nl_ABMON_5 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the fifth month | `*)`
`` | `nl_ABMON_6 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the sixth month | `*)`
`` | `nl_ABMON_7 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the seventh month | `*)`
`` | `nl_ABMON_8 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the eighth month | `*)`
`` | `nl_ABMON_9 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the ninth month | `*)`
`` | `nl_ABMON_10 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the tenth month | `*)`
`` | `nl_ABMON_11 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the eleventh month | `*)`
`` | `nl_ABMON_12 : `string`;` | `(*` | from `LC_TIME`: abbreviated name of the twelfth month | `*)`
`` | `nl_ERA : `string`;` | `(*` | from `LC_TIME`: era description segments | `*)`
`` | `nl_ERA_D_FMT : `string`;` | `(*` | from `LC_TIME`: era date format string | `*)`
`` | `nl_ERA_D_T_FMT : `string`;` | `(*` | from `LC_TIME`: era date and time format string | `*)`
`` | `nl_ERA_T_FMT : `string`;` | `(*` | from `LC_TIME`: era time format string | `*)`
`` | `nl_ALT_DIGITS : `string`;` | `(*` | from `LC_TIME`: alternative symbols for digits | `*)`
`` | `nl_RADIXCHAR : `string`;` | `(*` | from `LC_NUMERIC`: radix character | `*)`
`` | `nl_THOUSEP : `string`;` | `(*` | from `LC_NUMERIC`: separator for thousands | `*)`
`` | `nl_YESEXPR : `string`;` | `(*` | from `LC_MESSAGES`: affirmative response expression | `*)`
`` | `nl_NOEXPR : `string`;` | `(*` | from `LC_MESSAGES`: negative response expression | `*)`
`` | `nl_CRNCYSTR : `string`;` | `(*` | from `LC_MONETARY`: currency | `*)`

}

    val query_langinfo : string -> langinfo

`query_langinfo locale`: Temporarily sets the passed `locale` and determines the language attributes. After that the orignal locale is restored. Pass "" as `locale` to get the locale requested in the environment.

The value for "" is cached.

# Clocks

Support for clocks can be assumed to exist on all current POSIX systems.

    type timespec = float * int

`(t,t_nanos)`: Specifies a time by a base time `t` to which the nanoseconds `t_nanos` are added.

If this pair is returned by a function `t` will always be integral. If a pair is passed to a function, it does not matter whether this is the case or not, but using integral values for `t` ensure maximum precision.

    val nanosleep : timespec -> timespec Pervasives.ref -> unit

`nanosleep t t_rem`: Sleeps for `t` seconds. The sleep can either be finished, or the sleep can be interrupted by a signal. In the latter case, the function will raise `EÍNTR`, and write to `t_rem` the remaining seconds.

    type clock_id

    type clock =

`|` | `CLOCK_REALTIME` | `(*` | A clock measuring wallclock time | `*)`
`|` | `CLOCK_MONOTONIC` | `(*` | A clock measuring kernel time (non-settable). Optional, i.e. not supported by all OS | `*)`
`|` | `CLOCK_ID of `clock_id`` | `(*` | A clock ID | `*)`

    val clock_gettime : clock -> timespec

Get the time of this clock

    val clock_settime : clock -> timespec -> unit

Set the time of this clock

    val clock_getres : clock -> timespec

Get the resolution of this clock

    val clock_getcpuclockid : int -> clock_id

Return the ID of a clock that counts the CPU seconds of the given process. Pass the PID or 0 for the current process.

This function is not supported on all OS.

# POSIX timers

    type posix_timer

    type timer_expiration =

`|` | `TEXP_NONE`
`|` | `TEXP_EVENT of `not_event``
`|` | `TEXP_EVENT_CREATE`
`|` | `TEXP_SIGNAL of `int``

    val have_posix_timer : unit -> bool

    val timer_create : clock ->
           timer_expiration -> posix_timer

Create a new timer that will report expiration as given by the arg:

- `TEXP_NONE`: no notification
- `TEXP_EVENT e`: the `not_event` `e` is signalled
- `TEXP_EVENT_CREATE`: a special `not_event` is created for the timer. (Get the event via `timer_event`, see below.)
- `TEXP_SIGNAL n`: the signal `n` is sent to the process

Note that `TEXP_EVENT_CREATE` is much faster on Linux than `TEXP_EVENT`, because it can be avoided to start a new thread whenever the timer expires. Instead, the timerfd machinery is used.

`TEXP_EVENT` and `TEXP_EVENT_CREATE` are only supported on systems with pthreads.

    val timer_settime : posix_timer ->
           bool -> timespec -> timespec -> unit

`timer_settime tm abstime interval value`:

If `value=(0.0,0)`, the timer is stopped.

If `value` is a positive time, the timer is started (or the timeout is changed if it is already started). If `abstime`, `value` is interpreted as the absolute point in time of the expiration. Otherwise `value` sets the number of seconds until the expiration.

If `interval` is positive, the started timer will repeat to expire after this many seconds once it has expired for the first time. If `interval=(0.0,0)`, the timer is a one-shot timer.

    val timer_gettime : posix_timer -> timespec

Returns the number of seconds until the expiration, or `(0.0,0)` if the timer is off

    val timer_delete : posix_timer -> unit

Deletes the timer

    val timer_event : posix_timer -> not_event

Returns the notification event for the timer styles `TEXP_EVENT` and `TEXP_EVENT_CREATE`.

Note that the latter type of event does not allow to call `set_event`.

Intentionally there is no wrapper for `timer_getoverrun`. Additional overruns can occur because of the further processing of the notifications: The OCaml runtime can merge signals, which would not be noticed by the kernel overrun counter, and events can also be merged. The workaround is to use one-shot timers with absolute expiration timestamps, and to check for overruns manually. Once we have `TEXP_NQ` the issue is solved.
Intentionally there is no wrapper for `timer_getoverrun`. Additional overruns can occur because of the further processing of the notifications: The OCaml runtime can merge signals, which would not be noticed by the kernel overrun counter, and events can also be merged. The workaround is to use one-shot timers with absolute expiration timestamps, and to check for overruns manually. Once we have `TEXP_NQ` the issue is solved.

# Linux I/O Priorities

Intentionally there is no wrapper for `timer_getoverrun`. Additional overruns can occur because of the further processing of the notifications: The OCaml runtime can merge signals, which would not be noticed by the kernel overrun counter, and events can also be merged. The workaround is to use one-shot timers with absolute expiration timestamps, and to check for overruns manually. Once we have `TEXP_NQ` the issue is solved.

# Linux I/O Priorities

These system calls are only available on Linux since kernel 2.6.13, and not even on every architecture. i386, x86_64, ia64, and PPC are known to work.

Per-process I/O priorities are currently only supported by the CFQ I/O scheduler.

    val have_ioprio : unit -> bool

Returns `true` if the system call `ioprio_get` is supported

    type ioprio_target =

`|` | `Ioprio_process of `int`` | `(*` | A single process | `*)`
`|` | `Ioprio_pgrp of `int`` | `(*` | A process group | `*)`
`|` | `Ioprio_user of `int`` | `(*` | All processes owned by this user | `*)`

    type ioprio =

`|` | `Noprio` | `(*` | I/O prioritization is unsupported by block layer | `*)`
`|` | `Real_time of `int`` | `(*` | 0..7 (higest..lowest prio) | `*)`
`|` | `Best_effort of `int`` | `(*` | 0..7 (higest..lowest prio) | `*)`
`|` | `Idle`

    val ioprio_get : ioprio_target -> ioprio

Retrieve the priority of the target. If several processes match the target, the highest priority is returned. If no process matches, the unix error `ESRCH` will be raised.

    val ioprio_set : ioprio_target -> ioprio -> unit

Sets the priority of the target processes.

# Debugging

    module Debug: sig .. end