# Module Netsys

    module Netsys: sig .. end

System calls missing in the `Unix` module

## Generic file descriptors

Not all OS provide generic read/write functions, or some emulation layer does not allow to use a descriptor with read/write. In the following functions, the style of the descriptor can be passed along with the descriptor to select the right I/O method. Effectively, the `fd_style` indicates which I/O function to call. Sometimes it is mandatory to call this function, sometimes it is only a good advice because the function provides the best interface for the kind of descriptor.

    type fd_style = [ `Read_write
           | `Recv_send of Unix.sockaddr * Unix.sockaddr
           | `Recv_send_implied
           | `Recvfrom_sendto
           | `TLS of Netsys_crypto_types.file_tls_endpoint
           | `W32_event
           | `W32_input_thread
           | `W32_output_thread
           | `W32_pipe
           | `W32_pipe_server
           | `W32_process ]

Some information what kind of operations are reasonable for descriptors:

- ``Read_write`: The descriptor is neither a socket not one of the other special cases, so only read/write is possible if read/write is possible at all. This style is also used if it is meaningless to use data I/O like read/write at all.
- ``Recv_send(sockaddr,peeraddr)`: The descriptor is a connected socket. recv/send are the preferred operations.
- ``Recvfrom_sendto`: The descriptor is an unconnected socket, and it is possible to ask for addresses when exchanging data, so recvfrom/sendto are the preferred operations.
- ``Recv_send_implied`: The descriptor is a socket with implied connection. There are no socket addresses. recv/send are the preferred operations. It is not possible to call `getsockname` or `getpeername`.
- ``W32_pipe`: The descriptor is a proxy descriptor for a Win32 named pipe as returned by `Netsys_win32.pipe_descr`.
- ``W32_pipe_server`: The descriptor is a proxy descriptor for a Win32 pipe server as returned by `Netsys_win32.pipe_server_descr`.
- ``W32_event`: The descriptor is a Win32 proxy descriptor for an event as returned by `Netsys_win32.create_event`. It is not possible to read/write with these descriptors.
- ``W32_process`: The descriptor is a proxy descriptor for a Win32 process as returned by `Netsys_win32.create_process`. It is not possible to read/write with these descriptors.
- ``W32_input_thread`: The descriptor is a proxy descriptor for a Win32-specific input thread as returned by `Netsys_win32.create_input_thread`.
- ``W32_output_thread`: The descriptor is a proxy descriptor for a Win32-specific output thread as returned by `Netsys_win32.create_output_thread`.
- ``TLS endpoint`: A TLS tunnel is running over the descriptor. The details of the tunnel can be found in `endpoint`. Note that it is allowed that the endpoint uses a second descriptor for either reading or writing (i.e. reading and writing go via different descriptors). In this case, it is sufficient that one of these descriptors is accompanied with ``TLS endpoint`.

Win32: For the exact meaning of proxy descriptors, please see `Netsys_win32`. In short, a proxy descriptor is an abstract handle for the I/O object. The handle itself cannot be used for I/O, however, but only some specialized function. The proxy descriptor can only be used to dereference the I/O object. Note that the following functions like `gread` and `gwrite` automatically look up the I/O object behind the proxy and call the right I/O function.

    val get_fd_style : Unix.file_descr -> fd_style

Get the file descriptor style.

The following styles cannot be determined automatically:

- ``TLS`

    val gread : fd_style -> Unix.file_descr -> Stdlib.Bytes.t -> int -> int -> int

`gread fd_style fd s pos len`: Reads up to `len` bytes from descriptor `fd` which is supposed to support the I/O style `fd_style`, i.e. the right system call (`read`, `recv`, `recvfrom`) is chosen to read from the descriptor. After `n <= len` bytes have been read these are put into string `s` at positions `pos` to `pos+n-1`, and `n` is returned. The function can fail with any I/O exception defined for the actually performed I/O operation. Whether the operation is blocking or non-blocking depends on the descriptor.

If `len>0` but `n=0` the end of the input data is reached.

    val gread_tbuf : fd_style ->
           Unix.file_descr -> Netsys_types.tbuffer -> int -> int -> int

Same for a tagged buffer

    val blocking_gread : fd_style -> Unix.file_descr -> Stdlib.Bytes.t -> int -> int -> int

`let p = blocking_gread fd_style fd s pos len`: Like `gread` up to `len` bytes are read from `fd` and stored in `s`. If the I/O operation is blocking but the descriptor is in non-blocking mode, this function blocks until the operation can be performed. If the operation is interrupted by a signal it is automatically restarted.

If `n < len` the end of the input data is reached (where `n` is the returned number).

See `wait_until_readable` below for further information which types of descriptors can be handled in non-blocking mode.

    val blocking_gread_tbuf : fd_style ->
           Unix.file_descr -> Netsys_types.tbuffer -> int -> int -> int

Same for a tagged buffer

    val really_gread : fd_style -> Unix.file_descr -> Stdlib.Bytes.t -> int -> int -> unit

`really_read fd_style fd s pos len`: Reads exactly `len` bytes from `fd` and stores them in `s` starting at `pos`. If the end of file condition is seen before `len` bytes are read, the exception `End_of_file` is raised, and it is unspecified how many bytes have been stored in `s`. Like `blocking_gread`, non-blocking descriptors are forced to block until the operation becomes possible, and interruptions by signals are handled.

See `wait_until_readable` below for further information which types of descriptors can be handled in non-blocking mode.

    val really_gread_tbuf : fd_style ->
           Unix.file_descr -> Netsys_types.tbuffer -> int -> int -> unit

Same for a tagged buffer

    val gwrite : fd_style -> Unix.file_descr -> Stdlib.Bytes.t -> int -> int -> int

`gwrite fd_style fd s pos len`: Writes up to `len` bytes to descriptor `fd` which is supposed to support the I/O style `fd_style`, i.e. the right system call (`write`, `send`, `sendto`) is chosen to write to the descriptor. . The `n <= len` written bytes are taken from string `s`, starting at position `pos` until `pos+n-1`. The number `n` is returned. The function can fail with any I/O exception defined for the actually performed I/O operation. Whether the operation is blocking or non-blocking depends on the descriptor.

    val gwrite_tstr : fd_style ->
           Unix.file_descr -> Netsys_types.tstring -> int -> int -> int

Same for a tagged string

    val gwrite_tbuf : fd_style ->
           Unix.file_descr -> Netsys_types.tbuffer -> int -> int -> int

Same for a tagged buffer

    val really_gwrite : fd_style -> Unix.file_descr -> Stdlib.Bytes.t -> int -> int -> unit

`really_write fd_style fd s pos len`: Writes exactly the `len` bytes from `s` to `fd` starting at `pos`. If the I/O operation is blocking but the descriptor is in non-blocking mode, this function blocks until the operation can be performed. If the operation is interrupted by a signal it is automatically restarted.

See `wait_until_writable` below for further information which types of descriptors can be handled in non-blocking mode.

    val really_gwrite_tstr : fd_style ->
           Unix.file_descr -> Netsys_types.tstring -> int -> int -> unit

Same for a tagged string

    val really_gwrite_tbuf : fd_style ->
           Unix.file_descr -> Netsys_types.tbuffer -> int -> int -> unit

Same for a tagged buffer

    exception Shutdown_not_supported

See `gshutdown`

    val gshutdown : fd_style -> Unix.file_descr -> Unix.shutdown_command -> unit

`gshutdown fd_style fd cmd`: If there is the possibility to shut down the connection for this kind of descriptor, the shutdown is tried. It is possible that the function raises the `EAGAIN` Unix error if the shutdown operation is non-blocking, and currently not possible. It is suggested to wait until the descriptor is writable, and to try again then.

If there is no shutdown operation for this kind of descriptor, the exception `Shutdown_not_supported` is raised. In this case it is usually sufficient to close the descriptor (`gclose`, see below), and when all descriptors to the resource are closed, the resource is shut down by the OS.

Details by `fd_style`:

- ``Recv_send` and ``Recv_send_implied`: The socket is shut down as requested by `Unix.shutdown`. This only triggers the shutdown, but does not wait until it is completed. Also, errors are usually not immediately reported.
- ``W32_pipe`: It is only possible to request `SHUTDOWN_ALL` for these descriptors. For other shutdown types, the error `EPERM` is reported. The shutdown is synchronous and completed when the function returns.
- ``W32_pipe_server`: It is only possible to request `SHUTDOWN_ALL` for these descriptors. For other shutdown types, the error `EPERM` is reported. A shutdown means here to stop accepting new connections. The shutdown is synchronous and completed when the function returns.
- ``W32_output_thread`: It is only possible to request `SHUTDOWN_SEND` for these descriptors. A `SHUTDOWN_ALL` is also interpreted as `SHUTDOWN_SEND`, and a `SHUTDOWN_RECEIVE` is ignored. A shutdown means here that the EOF is appended to the output buffer, and when the output thread has written the buffer contents, the underlying descriptor (not `fd`!) will be closed. The shutdown operation is non-blocking. If it is not possible at the moment of calling, the error `EAGAIN` is reported.
- ``TLS`: The shutdown only affects the tunnel as such, but not the underlying connection. `SHUTDOWN_SEND` and `SHUTDOWN_ALL` are supported. `SHUTDOWN_RECEIVE` is ignored.
- Other styles raise `Shutdown_not_supported`.

    val is_readable : fd_style -> Unix.file_descr -> bool

    val is_writable : fd_style -> Unix.file_descr -> bool

    val is_prird : fd_style -> Unix.file_descr -> bool

Test whether the descriptor would not block if one of the input, output, or priority input operations were done.

On POSIX systems the tests work for a wide variety of descriptor types (but not for regular files which are assumed to be always readable and writable). If the `poll` interface is available it is preferred over the `select` interface to conduct the test.

On Win32, the tests are limited to sockets, named pipes and event objects. (The latter two only in the form provided by `Netsys_win32`, see there.)

For ``TLS` fd styles, the functions are "best effort" only.

Generally, if the blocking status cannot be determined for a class of I/O operations, the functions return `true`, in the hope that it is better to block than to never conduct the operation.

    val wait_until_readable : fd_style -> Unix.file_descr -> float -> bool

    val wait_until_writable : fd_style -> Unix.file_descr -> float -> bool

    val wait_until_prird : fd_style -> Unix.file_descr -> float -> bool

Wait until an operation for a single descriptor becomes possible. The float argument is the timeout (negative value means no timeout). Returns whether the operation is possible (`true`). Otherwise, there was a timeout (`false`).

On POSIX systems this works for a wide variety of descriptor types (but not for regular files which are assumed to be always readable and writable). If the `poll` interface is available it is preferred over the `select` interface to wait for I/O. The functions also catch interruptions by signals.

On Win32, waiting is limited to sockets, named pipes and event objects. (The latter two only in the form provided by `Netsys_win32`, see there.)

For ``TLS` fd styles, the functions are "best effort" only.

Generally, if waiting is not supported for a class of I/O operations, the functions return immediately `true`, in the hope that it is better to block than to never conduct the operation.

    val gclose : fd_style -> Unix.file_descr -> unit

Shuts down the system object referenced by the descriptor so far possible, and closes the descriptor.

Errors are logged to `Netlog` as ``Crit` events, and do not generate exceptions.

The exact semantics of the close operation varies from descriptor style to descriptor style. Generally, `gclose` assumes that all I/O is done, and all buffers are flushed, and that one can tear down the underlying communication circuits. `gclose` is always the right choice when the I/O channel needs to be aborted after a fatal error, and it does not matter whether errors occur or not. If a data connection needs to be orderly closed (i.e. without data loss), one should first try to finish the communication, either by protocol means (e.g. wait for ACK messages), or by calling `gshutdown` first (see above).

## Functions for sockets

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

After a non-blocking connect has been initiated, this function can be used to wait until (1) the connect is successful, or (2) the connect fails, or (3) the operation times out. The `float` argument is the timeout value (negative value means no timeout). The function returns `true` for the cases (1) and (2), and `false` for case (3). The cases (1) and (2) can be further analyzed by calling `connect_check` (see below).

On POSIX, this function is identical to `wait_until_writable`. On Win32 the wait condition is slightly different.

On Win32, this function also tolerates client proxy descriptors for Win32 named pipes. However, there is no waiting - the function immediately returns.

    val connect_check : Unix.file_descr -> unit

Tests whether the socket is connected with the peer after calling `Unix.connect`. If the socket is connected, the function returns normally. Otherwise, the current socket error is raised as a `Unix.Unix_error` exception. This function is intended to be called after a non-blocking connect has been initiated, and the success or error is indicated (e.g. after `wait_until_connected` returns).

Side effect: The per-socket error code may be reset.

On Win32, this function also tolerates client proxy descriptors for Win32 named pipes. However, there is no real check - the function immediately returns.

    val domain_of_inet_addr : Unix.inet_addr -> Unix.socket_domain

Returns the socket domain of Internet addresses, i.e. whether the address is IPv4 or IPv6

    val protostring_of_inet_addr : Unix.inet_addr -> string

    val inet_addr_of_protostring : string -> Unix.inet_addr

Converts an IP address to the 4 bytes (IPv4) or 16 bytes (IPv6) representation in network byte order, and vice-versa

    val getpeername : Unix.file_descr -> Unix.sockaddr

like `Unix.getpeername`, but errors are fixed up. `ENOTCONN` is ensured when the socked is unconnected or shut down.

## Helper functions

    val is_absolute : string -> bool

Whether this file path is absolute. Works for Unix and Win32.

    val abspath : string -> string

Return an absolute path for this file. When `Netsys_posix.realpath` is available, this function is called, and the canonical path is returned. On Win32, first an arbitrary absolute path is created, and then the path is tried to be simplified by resolving "." and "..". If neither method works, the function raises `Invalid_argument`.

Note that the file needs to exist in general.

    val restart : ('a -> 'b) -> 'a -> 'b

`restart f arg` calls `f arg`, and restarts this call if the exception `Unix_error(EINTR,_,_)` is caught.

Note that there are some cases where this handling of `EINTR` is not sufficient:

- Functions that have a timeout argument like `Unix.select`: When `EINTR` is caught the timeout should be adjusted.
- `Unix.connect` with a blocking descriptor because this is not well-enough specified by POSIX

    val restart_tmo : (float -> 'b) -> float -> 'b

`restart_tmo f tmo` calls `f` with a timeout argument `tmo`, and restarted the call if the exception `Unix_error(EINTR,_,_)` is caught. In the restart case, the timeout argument is reduced by the already elapsed time.

Negative timeout arguments are interpreted as "no timeout".

    val restarting_select : Unix.file_descr list ->
           Unix.file_descr list ->
           Unix.file_descr list ->
           float -> Unix.file_descr list * Unix.file_descr list * Unix.file_descr list

A wrapper around `Unix.select` that handles the `EINTR` condition.

Note: This function calls `Unix.select` and shares all pros and cons with `Unix.select`. In particular, the OS often sets a limit on the number (and/or the numeric value) of the descriptors (e.g. for Linux it is 1024, for Windows it is 64). On Ocaml 3.11 the Windows version of `Unix.select` includes some support for other types of descriptors than sockets.

    val restart_wait : [ `R | `W ] -> fd_style -> Unix.file_descr -> ('a -> 'b) -> 'a -> 'b

`restart_wait mode fd_style fd f arg`: Calls `f arg`, and handles the following exceptions:

- `Unix_error(EINTR,_,_)`: Just calls `f` again
- `Unix_error(EAGAIN,_,_)`: waits until `fd` is readable or writable as designated by `mode`, and calls `f` again
- `Unix_error(EWOUDLBLOCK,_,_)`: same
- `Netsys_types.EAGAIN_RD`: waits until `fd` is readable, and calls `f` again
- `Netsys_types.EAGAIN_WR`: waits until `fd` is writable, and calls `f` again

    val sleep : float -> unit

    val restarting_sleep : float -> unit

Sleep for the passed time. `restarting_sleep` additionally handles `EINTR`.

    val unix_error_of_code : int -> Unix.error

Converts an integer error into the corresponding variant

    val int64_of_file_descr : Unix.file_descr -> int64

Returns the file descriptor as int64 number. Works for all OS.

    val string_of_fd : Unix.file_descr -> string

Return a string describing the descriptor (for debugging)

    val string_of_sockaddr : ?norm:bool -> Unix.sockaddr -> string

Returns a human-readable string describing the address (for debug messages). If `norm`, IPv4 addresses mapped to the IPv6 address space are returned in the normal dotted quad format (i.e. x.y.z.u instead of ::ffff:x.y.z.u).

Note that the reverse (parsing such a string) can be accomplished with `Netsockaddr.socksymbol_of_string` and `Uq_resolver.sockaddr_of_socksymbol`.

    val string_of_fd_style : fd_style -> string

Returns a string describing the fd style (debugging)

    val is_stdin : Unix.file_descr -> bool

    val is_stdout : Unix.file_descr -> bool

    val is_stderr : Unix.file_descr -> bool

Returns whether the descriptors are stdin/stdout/stderr

    val set_close_on_exec : Unix.file_descr -> unit

    val clear_close_on_exec : Unix.file_descr -> unit

Working versions of the functions with the same name in `Unix`

    val _exit : int -> unit

Exit the program immediately without running the atexit handlers. The argument is the exit code, just as for `exit`.

## IPv6

    val is_ipv6_system : unit -> bool

Whether IPv6 is available and usable. At the moment this tests for the presence of a global IPv6 address on any interface. The test also requires that the getifaddrs() call is available. The test can be overridden with `set_ipv6_system`.

    val set_ipv6_system : bool -> unit

Sets whether IPv6 is usable

## IP addresses

    val logand_inet_addr : Unix.inet_addr -> Unix.inet_addr -> Unix.inet_addr

Returns the bitwise AND of the two argument addresses

    val logor_inet_addr : Unix.inet_addr -> Unix.inet_addr -> Unix.inet_addr

Returns the bitwise OR of the two argument addresses

    val logxor_inet_addr : Unix.inet_addr -> Unix.inet_addr -> Unix.inet_addr

Returns the bitwise XOR of the two argument addresses

    val lognot_inet_addr : Unix.inet_addr -> Unix.inet_addr

Returns the bitwise NOT of the argument address

    val norm_inet_addr : Unix.inet_addr -> Unix.inet_addr

Normalization: If the input address is an IPv4 address mapped into the IPv6 address space, the IPv4 address is extracted. Otherwise, the input address is returned unchanged.

    val ipv6_inet_addr : Unix.inet_addr -> Unix.inet_addr

IPv6-ification: If the input address is for IPv4, it is mapped to the IPv6 address space (so an IPv6 socket can be bound)

    val is_ipv4_inet_addr : Unix.inet_addr -> bool

Whether the address is an IPv4 address (including IPv4 addresses mapped into the IPv6 adress space)

    val is_ipv6_inet_addr : Unix.inet_addr -> bool

Whether the address is an IPv6 address (excluding IPv4 addresses mapped into the IPv6 adress space)

    val is_multicast_inet_addr : Unix.inet_addr -> bool

Whether the address is a multicast address (either IPv4 or IPv6)

## Multicast Functions

    val mcast_set_loop : Unix.file_descr -> bool -> unit

Whether sent multicast messages are received by the sending host

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

Set TTL/hops value

    val mcast_add_membership : Unix.file_descr -> Unix.inet_addr -> Unix.inet_addr -> unit

Join a multicast group.

First inet addr is the group to join. Second inet addr selects the network interface (or `Unix.inet_addr_any`).

    val mcast_drop_membership : Unix.file_descr -> Unix.inet_addr -> Unix.inet_addr -> unit

Leave a multicast group.

First inet addr is the group to leave. Second inet addr selects the network interface (or `Unix.inet_addr_any`).

## Profiling

    val moncontrol : bool -> unit

Interface to the `moncontrol` routine of the GPROF profiler. `moncontrol false` stops profiling; `moncontrol true` starts profiling again.

This is a no-op if the program is not compiler for profiling.

## Further Documentation

### How to orderly close I/O channels

After reading from uni-directional descriptors, and seeing the EOF, it is usually sufficient to call `gclose` to free OS resources.

After writing to uni-directional descriptors one should call `gshutdown` to send an EOF (`SHUTDOWN_SEND`). For some descriptors one will get the exception `Shutdown_not_supported` which can be ignored in this context The `gshutdown` function cannot, however, report in all cases whether the operation was successful. As a rule of thumb, error reporting works for local data connections, but not always for remote connections, and there is no way to fix this. After writing EOF, call `gclose` to free OS resources.

For bidirectional connections, it is even more complicated. If the connection is local, a bidirectional connection behaves much like a pair of unidirectional connections. However, in the network case, we have to go down to the protocol level.

For TCP the golden rule is that the client initiates the connection, and the client finishes the connection. The case that the server finishes the connection is not well-specified - or better, the server needs the ACK from the client after triggering the connection termination. In practice we have the cases:

- Client sends EOF, and server replies with EOF: This is the normal case for which TCP is designed. Client code can invoke `gshutdown` with `SHUTDOWN_SEND` and then waits until the EOF from the server arrives, and then `gclose`s the descriptor. It may happen that the client gets an error if some problem occurs, so this is reliable from the perspective of the client. The server first sees the EOF from the client, and then responds with another `gshutdown`, followed by `gclose`. From the server's perspective it does not matter whether the operation results in an error or not - the client has lost interest anyway.
- Client sends EOF, and server replies with data, and then EOF. Here, the client has to read the final data, and then wait for the server's EOF after sending its own EOF. On the server's side, some data is written before the final EOF. The question is how the server can be sure that the data really arrived. Unfortunately, there is no way to do so. The server may not get all errors because these may arrive at the server computer after `gshutdown`. There is no way to fix this. (One should better fix the application protocol. Note that even prominent researchers trapped into this problem. For example, the first version of HTTP had this problem.)
- Server sends EOF, and client replies with EOF: This is the difficult case. Here, the server wants to be sure that the data sent immediately before its EOF really arrives at the client. After `gshutdown` it is forbidden to immediately `gclose`, because this may result in a connection reset. Instead, the server has to wait for the client's EOF. (This is called "lingering".) If the client's EOF is seen one can `gclose`.
- Server sends EOF, and client replies with data, followed by EOF: I admit I don't know whether TCP can handle this somehow.

## Debugging

    module Debug: sig .. end