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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
       | `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.
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
val gread : fd_style -> Unix.file_descr -> string -> 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 blocking_gread : fd_style -> Unix.file_descr -> string -> 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 really_gread : fd_style -> Unix.file_descr -> string -> 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 gwrite : fd_style -> Unix.file_descr -> string -> 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 really_gwrite : fd_style -> Unix.file_descr -> string -> 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.

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.
  • 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.)

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.)

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 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 -> 'a) -> float -> 'a
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 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 : Unix.sockaddr -> string
Returns a human-readable string describing the address (for debug messages)

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.

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 is_ipv4_inet_addr : Unix.inet_addr -> bool
Whether the address is an IPv4 address
val is_ipv6_inet_addr : Unix.inet_addr -> bool
Whether the address is an IPv6 address
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.


Deprecated



The following interfaces have been replaced by more generic implementations that work on more platforms.
val blocking_read : Unix.file_descr -> string -> int -> int -> int
Same as blocking_gread `Read_write
val really_read : Unix.file_descr -> string -> int -> int -> unit
Same as really_gread `Read_write
val really_write : Unix.file_descr -> string -> int -> int -> unit
Same as really_gwrite `Read_write

The following interfaces have been moved to Netsys_posix.
type shm_open_flag = Netsys_posix.shm_open_flag =
| SHM_O_RDONLY
| SHM_O_RDWR
| SHM_O_CREAT
| SHM_O_EXCL
| SHM_O_TRUNC 
val have_posix_shm : unit -> bool
val shm_open : string -> shm_open_flag list -> int -> Unix.file_descr
val shm_unlink : string -> unit

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 gcloses 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
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