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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_IFCHR of int
| S_IFBLK of int
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 = 

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 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.


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

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 waits 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 waited 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.

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().


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)


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.

Optional POSIX functions

val have_fadvise : unit -> bool

Returns whether the OS supports the fadvise POSIX option

type advice = 

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 = 
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


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))


type sem_kind = [ `Anonymous | `Named ] 
type 'sem_kind semaphore 
type named_semaphore = [ `Named ] semaphore 
type anon_semaphore = [ `Anonymous ] semaphore 
type sem_open_flag = 

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.


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 = 
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.


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.


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 Stdlib.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 = 

A clock measuring wallclock time


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_EVENT of not_event
| 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.

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.


module Debug: sig .. end
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