# Module Unix

    module Unix: sig .. end

Interface to the Unix system.

To use the labeled version of this module, add `module Unix``=` `UnixLabels` in your implementation.

Note: all the functions of this module (except `Unix.error_message` and `Unix.handle_unix_error`) are liable to raise the `Unix.Unix_error` exception whenever the underlying system call signals an error.

## Error report

    type error =

`|` | `E2BIG` | `(*` | Argument list too long | `*)`
`|` | `EACCES` | `(*` | Permission denied | `*)`
`|` | `EAGAIN` | `(*` | Resource temporarily unavailable; try again | `*)`
`|` | `EBADF` | `(*` | Bad file descriptor | `*)`
`|` | `EBUSY` | `(*` | Resource unavailable | `*)`
`|` | `ECHILD` | `(*` | No child process | `*)`
`|` | `EDEADLK` | `(*` | Resource deadlock would occur | `*)`
`|` | `EDOM` | `(*` | Domain error for math functions, etc. | `*)`
`|` | `EEXIST` | `(*` | File exists | `*)`
`|` | `EFAULT` | `(*` | Bad address | `*)`
`|` | `EFBIG` | `(*` | File too large | `*)`
`|` | `EINTR` | `(*` | Function interrupted by signal | `*)`
`|` | `EINVAL` | `(*` | Invalid argument | `*)`
`|` | `EIO` | `(*` | Hardware I/O error | `*)`
`|` | `EISDIR` | `(*` | Is a directory | `*)`
`|` | `EMFILE` | `(*` | Too many open files by the process | `*)`
`|` | `EMLINK` | `(*` | Too many links | `*)`
`|` | `ENAMETOOLONG` | `(*` | Filename too long | `*)`
`|` | `ENFILE` | `(*` | Too many open files in the system | `*)`
`|` | `ENODEV` | `(*` | No such device | `*)`
`|` | `ENOENT` | `(*` | No such file or directory | `*)`
`|` | `ENOEXEC` | `(*` | Not an executable file | `*)`
`|` | `ENOLCK` | `(*` | No locks available | `*)`
`|` | `ENOMEM` | `(*` | Not enough memory | `*)`
`|` | `ENOSPC` | `(*` | No space left on device | `*)`
`|` | `ENOSYS` | `(*` | Function not supported | `*)`
`|` | `ENOTDIR` | `(*` | Not a directory | `*)`
`|` | `ENOTEMPTY` | `(*` | Directory not empty | `*)`
`|` | `ENOTTY` | `(*` | Inappropriate I/O control operation | `*)`
`|` | `ENXIO` | `(*` | No such device or address | `*)`
`|` | `EPERM` | `(*` | Operation not permitted | `*)`
`|` | `EPIPE` | `(*` | Broken pipe | `*)`
`|` | `ERANGE` | `(*` | Result too large | `*)`
`|` | `EROFS` | `(*` | Read-only file system | `*)`
`|` | `ESPIPE` | `(*` | Invalid seek e.g. on a pipe | `*)`
`|` | `ESRCH` | `(*` | No such process | `*)`
`|` | `EXDEV` | `(*` | Invalid link | `*)`
`|` | `EWOULDBLOCK` | `(*` | Operation would block | `*)`
`|` | `EINPROGRESS` | `(*` | Operation now in progress | `*)`
`|` | `EALREADY` | `(*` | Operation already in progress | `*)`
`|` | `ENOTSOCK` | `(*` | Socket operation on non-socket | `*)`
`|` | `EDESTADDRREQ` | `(*` | Destination address required | `*)`
`|` | `EMSGSIZE` | `(*` | Message too long | `*)`
`|` | `EPROTOTYPE` | `(*` | Protocol wrong type for socket | `*)`
`|` | `ENOPROTOOPT` | `(*` | Protocol not available | `*)`
`|` | `EPROTONOSUPPORT` | `(*` | Protocol not supported | `*)`
`|` | `ESOCKTNOSUPPORT` | `(*` | Socket type not supported | `*)`
`|` | `EOPNOTSUPP` | `(*` | Operation not supported on socket | `*)`
`|` | `EPFNOSUPPORT` | `(*` | Protocol family not supported | `*)`
`|` | `EAFNOSUPPORT` | `(*` | Address family not supported by protocol family | `*)`
`|` | `EADDRINUSE` | `(*` | Address already in use | `*)`
`|` | `EADDRNOTAVAIL` | `(*` | Can't assign requested address | `*)`
`|` | `ENETDOWN` | `(*` | Network is down | `*)`
`|` | `ENETUNREACH` | `(*` | Network is unreachable | `*)`
`|` | `ENETRESET` | `(*` | Network dropped connection on reset | `*)`
`|` | `ECONNABORTED` | `(*` | Software caused connection abort | `*)`
`|` | `ECONNRESET` | `(*` | Connection reset by peer | `*)`
`|` | `ENOBUFS` | `(*` | No buffer space available | `*)`
`|` | `EISCONN` | `(*` | Socket is already connected | `*)`
`|` | `ENOTCONN` | `(*` | Socket is not connected | `*)`
`|` | `ESHUTDOWN` | `(*` | Can't send after socket shutdown | `*)`
`|` | `ETOOMANYREFS` | `(*` | Too many references: can't splice | `*)`
`|` | `ETIMEDOUT` | `(*` | Connection timed out | `*)`
`|` | `ECONNREFUSED` | `(*` | Connection refused | `*)`
`|` | `EHOSTDOWN` | `(*` | Host is down | `*)`
`|` | `EHOSTUNREACH` | `(*` | No route to host | `*)`
`|` | `ELOOP` | `(*` | Too many levels of symbolic links | `*)`
`|` | `EOVERFLOW` | `(*` | File size or position not representable | `*)`
`|` | `EUNKNOWNERR of `int`` | `(*` | Unknown error | `*)`

The type of error codes. Errors defined in the POSIX standard and additional errors from UNIX98 and BSD. All other errors are mapped to EUNKNOWNERR.

    exception Unix_error of error * string * string

Raised by the system calls below when an error is encountered. The first component is the error code; the second component is the function name; the third component is the string parameter to the function, if it has one, or the empty string otherwise.

`UnixLabels.Unix_error` and `Unix.Unix_error` are the same, and catching one will catch the other.

    val error_message : error -> string

Return a string describing the given error code.

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

`handle_unix_error f x` applies `f` to `x` and returns the result. If the exception `Unix.Unix_error` is raised, it prints a message describing the error and exits with code 2.

## Access to the process environment

    val environment : unit -> string array

Return the process environment, as an array of strings with the format ``variable=value''. The returned array is empty if the process has special privileges.

    val unsafe_environment : unit -> string array

Return the process environment, as an array of strings with the format ``variable=value''. Unlike `Unix.environment`, this function returns a populated array even if the process has special privileges. See the documentation for `Unix.unsafe_getenv` for more details.

- Since 4.06.0 (4.12.0 in UnixLabels)

    val getenv : string -> string

Return the value associated to a variable in the process environment, unless the process has special privileges.

- Raises `Not_found` if the variable is unbound or the process has special privileges. This function is identical to `Sys.getenv`.

    val unsafe_getenv : string -> string

Return the value associated to a variable in the process environment.

Unlike `Unix.getenv`, this function returns the value even if the process has special privileges. It is considered unsafe because the programmer of a setuid or setgid program must be careful to avoid using maliciously crafted environment variables in the search path for executables, the locations for temporary files or logs, and the like.

- Since 4.06.0
- Raises `Not_found` if the variable is unbound.

    val putenv : string -> string -> unit

`putenv name value` sets the value associated to a variable in the process environment. `name` is the name of the environment variable, and `value` its new associated value.

## Process handling

    type process_status =

`|` | `WEXITED of `int`` | `(*` | The process terminated normally by `exit`; the argument is the return code. | `*)`
`|` | `WSIGNALED of `int`` | `(*` | The process was killed by a signal; the argument is the signal number. | `*)`
`|` | `WSTOPPED of `int`` | `(*` | The process was stopped by a signal; the argument is the signal number. | `*)`

The termination status of a process. See module `Sys` for the definitions of the standard signal numbers. Note that they are not the numbers used by the OS.

    type wait_flag =

`|` | `WNOHANG` | `(*` | Do not block if no child has died yet, but immediately return with a pid equal to 0. | `*)`
`|` | `WUNTRACED` | `(*` | Report also the children that receive stop signals. | `*)`

Flags for `Unix.waitpid`.

    val execv : string -> string array -> 'a

`execv prog args` execute the program in file `prog`, with the arguments `args`, and the current process environment. These `execv*` functions never return: on success, the current program is replaced by the new one.

- Raises `Unix_error` on failure

    val execve : string -> string array -> string array -> 'a

Same as `Unix.execv`, except that the third argument provides the environment to the program executed.

    val execvp : string -> string array -> 'a

Same as `Unix.execv`, except that the program is searched in the path.

    val execvpe : string -> string array -> string array -> 'a

Same as `Unix.execve`, except that the program is searched in the path.

    val fork : unit -> int

Fork a new process. The returned integer is 0 for the child process, the pid of the child process for the parent process.

- Raises `Invalid_argument` on Windows. Use `Unix.create_process` or threads instead.

    val wait : unit -> int * process_status

Wait until one of the children processes die, and return its pid and termination status.

- Raises `Invalid_argument` on Windows. Use `Unix.waitpid` instead.

    val waitpid : wait_flag list -> int -> int * process_status

Same as `Unix.wait`, but waits for the child process whose pid is given. A pid of `-1` means wait for any child. A pid of `0` means wait for any child in the same process group as the current process. Negative pid arguments represent process groups. The list of options indicates whether `waitpid` should return immediately without waiting, and whether it should report stopped children.

On Windows: can only wait for a given PID, not any child process.

    val system : string -> process_status

Execute the given command, wait until it terminates, and return its termination status. The string is interpreted by the shell `/bin/sh` (or the command interpreter `cmd.exe` on Windows) and therefore can contain redirections, quotes, variables, etc. To properly quote whitespace and shell special characters occurring in file names or command arguments, the use of `Filename.quote_command` is recommended. The result `WEXITED 127` indicates that the shell couldn't be executed.

    val _exit : int -> 'a

Terminate the calling process immediately, returning the given status code to the operating system: usually 0 to indicate no errors, and a small positive integer to indicate failure. Unlike `exit`, `Unix._exit` performs no finalization whatsoever: functions registered with `at_exit` are not called, input/output channels are not flushed, and the C run-time system is not finalized either.

The typical use of `Unix._exit` is after a `Unix.fork` operation, when the child process runs into a fatal error and must exit. In this case, it is preferable to not perform any finalization action in the child process, as these actions could interfere with similar actions performed by the parent process. For example, output channels should not be flushed by the child process, as the parent process may flush them again later, resulting in duplicate output.

- Since 4.12.0

    val getpid : unit -> int

Return the pid of the process.

    val getppid : unit -> int

Return the pid of the parent process.

- Raises `Invalid_argument` on Windows (because it is meaningless)

    val nice : int -> int

Change the process priority. The integer argument is added to the ``nice'' value. (Higher values of the ``nice'' value mean lower priorities.) Return the new nice value.

- Raises `Invalid_argument` on Windows

## Basic file input/output

    type file_descr

The abstract type of file descriptors.

    val stdin : file_descr

File descriptor for standard input.

    val stdout : file_descr

File descriptor for standard output.

    val stderr : file_descr

File descriptor for standard error.

    type open_flag =

`|` | `O_RDONLY` | `(*` | Open for reading | `*)`
`|` | `O_WRONLY` | `(*` | Open for writing | `*)`
`|` | `O_RDWR` | `(*` | Open for reading and writing | `*)`
`|` | `O_NONBLOCK` | `(*` | Open in non-blocking mode | `*)`
`|` | `O_APPEND` | `(*` | Open for append | `*)`
`|` | `O_CREAT` | `(*` | Create if nonexistent | `*)`
`|` | `O_TRUNC` | `(*` | Truncate to 0 length if existing | `*)`
`|` | `O_EXCL` | `(*` | Fail if existing | `*)`
`|` | `O_NOCTTY` | `(*` | Don't make this dev a controlling tty | `*)`
`|` | `O_DSYNC` | `(*` | Writes complete as `Synchronised I/O data integrity completion' | `*)`
`|` | `O_SYNC` | `(*` | Writes complete as `Synchronised I/O file integrity completion' | `*)`
`|` | `O_RSYNC` | `(*` | Reads complete as writes (depending on O_SYNC/O_DSYNC) | `*)`
`|` | `O_SHARE_DELETE` | `(*` | Windows only: allow the file to be deleted while still open | `*)`
`|` | `O_CLOEXEC` | `(*` | Set the close-on-exec flag on the descriptor returned by `Unix.openfile`. See `Unix.set_close_on_exec` for more information. | `*)`
`|` | `O_KEEPEXEC` | `(*` | Clear the close-on-exec flag. This is currently the default. | `*)`

The flags to `Unix.openfile`.

    type file_perm = int

The type of file access rights, e.g. `0o640` is read and write for user, read for group, none for others

    val openfile : string -> open_flag list -> file_perm -> file_descr

Open the named file with the given flags. Third argument is the permissions to give to the file if it is created (see `Unix.umask`). Return a file descriptor on the named file.

    val close : file_descr -> unit

Close a file descriptor.

    val fsync : file_descr -> unit

Flush file buffers to disk.

- Since 4.08.0 (4.12.0 in UnixLabels)

    val read : file_descr -> bytes -> int -> int -> int

`read fd buf pos len` reads `len` bytes from descriptor `fd`, storing them in byte sequence `buf`, starting at position `pos` in `buf`. Return the number of bytes actually read.

    val write : file_descr -> bytes -> int -> int -> int

`write fd buf pos len` writes `len` bytes to descriptor `fd`, taking them from byte sequence `buf`, starting at position `pos` in `buff`. Return the number of bytes actually written. `write` repeats the writing operation until all bytes have been written or an error occurs.

    val single_write : file_descr -> bytes -> int -> int -> int

Same as `Unix.write`, but attempts to write only once. Thus, if an error occurs, `single_write` guarantees that no data has been written.

    val write_substring : file_descr -> string -> int -> int -> int

Same as `Unix.write`, but take the data from a string instead of a byte sequence.

- Since 4.02.0

    val single_write_substring : file_descr -> string -> int -> int -> int

Same as `Unix.single_write`, but take the data from a string instead of a byte sequence.

- Since 4.02.0

## Interfacing with the standard input/output library

    val in_channel_of_descr : file_descr -> in_channel

Create an input channel reading from the given descriptor. The channel is initially in binary mode; use `set_binary_mode_in ic false` if text mode is desired. Text mode is supported only if the descriptor refers to a file or pipe, but is not supported if it refers to a socket.

On Windows: `set_binary_mode_in` always fails on channels created with this function.

Beware that input channels are buffered, so more characters may have been read from the descriptor than those accessed using channel functions. Channels also keep a copy of the current position in the file.

Closing the channel `ic` returned by `in_channel_of_descr fd` using `close_in ic` also closes the underlying descriptor `fd`. It is incorrect to close both the channel `ic` and the descriptor `fd`.

If several channels are created on the same descriptor, one of the channels must be closed, but not the others. Consider for example a descriptor `s` connected to a socket and two channels `ic = in_channel_of_descr s` and `oc = out_channel_of_descr s`. The recommended closing protocol is to perform `close_out oc`, which flushes buffered output to the socket then closes the socket. The `ic` channel must not be closed and will be collected by the GC eventually.

    val out_channel_of_descr : file_descr -> out_channel

Create an output channel writing on the given descriptor. The channel is initially in binary mode; use `set_binary_mode_out oc false` if text mode is desired. Text mode is supported only if the descriptor refers to a file or pipe, but is not supported if it refers to a socket.

On Windows: `set_binary_mode_out` always fails on channels created with this function.

Beware that output channels are buffered, so you may have to call `flush` to ensure that all data has been sent to the descriptor. Channels also keep a copy of the current position in the file.

Closing the channel `oc` returned by `out_channel_of_descr fd` using `close_out oc` also closes the underlying descriptor `fd`. It is incorrect to close both the channel `ic` and the descriptor `fd`.

See `Unix.in_channel_of_descr` for a discussion of the closing protocol when several channels are created on the same descriptor.

    val descr_of_in_channel : in_channel -> file_descr

Return the descriptor corresponding to an input channel.

    val descr_of_out_channel : out_channel -> file_descr

Return the descriptor corresponding to an output channel.

## Seeking and truncating

    type seek_command =

`|` | `SEEK_SET` | `(*` | indicates positions relative to the beginning of the file | `*)`
`|` | `SEEK_CUR` | `(*` | indicates positions relative to the current position | `*)`
`|` | `SEEK_END` | `(*` | indicates positions relative to the end of the file | `*)`

Positioning modes for `Unix.lseek`.

    val lseek : file_descr -> int -> seek_command -> int

Set the current position for a file descriptor, and return the resulting offset (from the beginning of the file).

    val truncate : string -> int -> unit

Truncates the named file to the given size.

    val ftruncate : file_descr -> int -> unit

Truncates the file corresponding to the given descriptor to the given size.

## File status

    type file_kind =

`|` | `S_REG` | `(*` | Regular file | `*)`
`|` | `S_DIR` | `(*` | Directory | `*)`
`|` | `S_CHR` | `(*` | Character device | `*)`
`|` | `S_BLK` | `(*` | Block device | `*)`
`|` | `S_LNK` | `(*` | Symbolic link | `*)`
`|` | `S_FIFO` | `(*` | Named pipe | `*)`
`|` | `S_SOCK` | `(*` | Socket | `*)`

    type stats = {

`` | `st_dev : `int`;` | `(*` | Device number | `*)`
`` | `st_ino : `int`;` | `(*` | Inode number | `*)`
`` | `st_kind : `file_kind`;` | `(*` | Kind of the file | `*)`
`` | `st_perm : `file_perm`;` | `(*` | Access rights | `*)`
`` | `st_nlink : `int`;` | `(*` | Number of links | `*)`
`` | `st_uid : `int`;` | `(*` | User id of the owner | `*)`
`` | `st_gid : `int`;` | `(*` | Group ID of the file's group | `*)`
`` | `st_rdev : `int`;` | `(*` | Device ID (if special file) | `*)`
`` | `st_size : `int`;` | `(*` | Size in bytes | `*)`
`` | `st_atime : `float`;` | `(*` | Last access time | `*)`
`` | `st_mtime : `float`;` | `(*` | Last modification time | `*)`
`` | `st_ctime : `float`;` | `(*` | Last status change time | `*)`

`}`

The information returned by the `Unix.stat` calls.

    val stat : string -> stats

Return the information for the named file.

    val lstat : string -> stats

Same as `Unix.stat`, but in case the file is a symbolic link, return the information for the link itself.

    val fstat : file_descr -> stats

Return the information for the file associated with the given descriptor.

    val isatty : file_descr -> bool

Return `true` if the given file descriptor refers to a terminal or console window, `false` otherwise.

## File operations on large files

    module LargeFile: sig .. end

File operations on large files.

## Mapping files into memory

    val map_file : file_descr ->
           ?pos:int64 ->
           ('a, 'b) Bigarray.kind ->
           'c Bigarray.layout ->
           bool -> int array -> ('a, 'b, 'c) Bigarray.Genarray.t

Memory mapping of a file as a Bigarray. `map_file fd kind layout shared dims` returns a Bigarray of kind `kind`, layout `layout`, and dimensions as specified in `dims`. The data contained in this Bigarray are the contents of the file referred to by the file descriptor `fd` (as opened previously with `Unix.openfile`, for example). The optional `pos` parameter is the byte offset in the file of the data being mapped; it defaults to 0 (map from the beginning of the file).

If `shared` is `true`, all modifications performed on the array are reflected in the file. This requires that `fd` be opened with write permissions. If `shared` is `false`, modifications performed on the array are done in memory only, using copy-on-write of the modified pages; the underlying file is not affected.

`Unix.map_file` is much more efficient than reading the whole file in a Bigarray, modifying that Bigarray, and writing it afterwards.

To adjust automatically the dimensions of the Bigarray to the actual size of the file, the major dimension (that is, the first dimension for an array with C layout, and the last dimension for an array with Fortran layout) can be given as `-1`. `Unix.map_file` then determines the major dimension from the size of the file. The file must contain an integral number of sub-arrays as determined by the non-major dimensions, otherwise `Failure` is raised.

If all dimensions of the Bigarray are given, the file size is matched against the size of the Bigarray. If the file is larger than the Bigarray, only the initial portion of the file is mapped to the Bigarray. If the file is smaller than the big array, the file is automatically grown to the size of the Bigarray. This requires write permissions on `fd`.

Array accesses are bounds-checked, but the bounds are determined by the initial call to `map_file`. Therefore, you should make sure no other process modifies the mapped file while you're accessing it, or a SIGBUS signal may be raised. This happens, for instance, if the file is shrunk.

`Invalid_argument` or `Failure` may be raised in cases where argument validation fails.

- Since 4.06.0

## Operations on file names

    val unlink : string -> unit

Removes the named file.

If the named file is a directory, raises:

- `EPERM` on POSIX compliant system
- `EISDIR` on Linux >= 2.1.132
- `EACCESS` on Windows

    val rename : string -> string -> unit

`rename src dst` changes the name of a file from `src` to `dst`, moving it between directories if needed. If `dst` already exists, its contents will be replaced with those of `src`. Depending on the operating system, the metadata (permissions, owner, etc) of `dst` can either be preserved or be replaced by those of `src`.

    val link : ?follow:bool -> string -> string -> unit

`link ?follow src dst` creates a hard link named `dst` to the file named `src`.

- Raises
  - `ENOSYS` On Unix if `~follow:_` is requested, but linkat is unavailable.
  - `ENOSYS` On Windows if `~follow:false` is requested.

`follow` : indicates whether a `src` symlink is followed or a hardlink to `src` itself will be created. On Unix systems this is done using the `linkat(2)` function. If `?follow` is not provided, then the `link(2)` function is used whose behaviour is OS-dependent, but more widely available.

    val realpath : string -> string

`realpath p` is an absolute pathname for `p` obtained by resolving all extra `/` characters, relative path segments and symbolic links.

- Since 4.13.0

## File permissions and ownership

    type access_permission =

`|` | `R_OK` | `(*` | Read permission | `*)`
`|` | `W_OK` | `(*` | Write permission | `*)`
`|` | `X_OK` | `(*` | Execution permission | `*)`
`|` | `F_OK` | `(*` | File exists | `*)`

Flags for the `Unix.access` call.

    val chmod : string -> file_perm -> unit

Change the permissions of the named file.

    val fchmod : file_descr -> file_perm -> unit

Change the permissions of an opened file.

- Raises `Invalid_argument` on Windows

    val chown : string -> int -> int -> unit

Change the owner uid and owner gid of the named file.

- Raises `Invalid_argument` on Windows

    val fchown : file_descr -> int -> int -> unit

Change the owner uid and owner gid of an opened file.

- Raises `Invalid_argument` on Windows

    val umask : int -> int

Set the process's file mode creation mask, and return the previous mask.

- Raises `Invalid_argument` on Windows

    val access : string -> access_permission list -> unit

Check that the process has the given permissions over the named file.

On Windows: execute permission `X_OK` cannot be tested, just tests for read permission instead.

- Raises `Unix_error` otherwise.

## Operations on file descriptors

    val dup : ?cloexec:bool -> file_descr -> file_descr

Return a new file descriptor referencing the same file as the given descriptor. See `Unix.set_close_on_exec` for documentation on the `cloexec` optional argument.

    val dup2 : ?cloexec:bool -> file_descr -> file_descr -> unit

`dup2 src dst` duplicates `src` to `dst`, closing `dst` if already opened. See `Unix.set_close_on_exec` for documentation on the `cloexec` optional argument.

    val set_nonblock : file_descr -> unit

Set the ``non-blocking'' flag on the given descriptor. When the non-blocking flag is set, reading on a descriptor on which there is temporarily no data available raises the `EAGAIN` or `EWOULDBLOCK` error instead of blocking; writing on a descriptor on which there is temporarily no room for writing also raises `EAGAIN` or `EWOULDBLOCK`.

    val clear_nonblock : file_descr -> unit

Clear the ``non-blocking'' flag on the given descriptor. See `Unix.set_nonblock`.

    val set_close_on_exec : file_descr -> unit

Set the ``close-on-exec'' flag on the given descriptor. A descriptor with the close-on-exec flag is automatically closed when the current process starts another program with one of the `exec`, `create_process` and `open_process` functions.

It is often a security hole to leak file descriptors opened on, say, a private file to an external program: the program, then, gets access to the private file and can do bad things with it. Hence, it is highly recommended to set all file descriptors ``close-on-exec'', except in the very few cases where a file descriptor actually needs to be transmitted to another program.

The best way to set a file descriptor ``close-on-exec'' is to create it in this state. To this end, the `openfile` function has `O_CLOEXEC` and `O_KEEPEXEC` flags to enforce ``close-on-exec'' mode or ``keep-on-exec'' mode, respectively. All other operations in the Unix module that create file descriptors have an optional argument `?cloexec:bool` to indicate whether the file descriptor should be created in ``close-on-exec'' mode (by writing `~cloexec:true`) or in ``keep-on-exec'' mode (by writing `~cloexec:false`). For historical reasons, the default file descriptor creation mode is ``keep-on-exec'', if no `cloexec` optional argument is given. This is not a safe default, hence it is highly recommended to pass explicit `cloexec` arguments to operations that create file descriptors.

The `cloexec` optional arguments and the `O_KEEPEXEC` flag were introduced in OCaml 4.05. Earlier, the common practice was to create file descriptors in the default, ``keep-on-exec'' mode, then call `set_close_on_exec` on those freshly-created file descriptors. This is not as safe as creating the file descriptor in ``close-on-exec'' mode because, in multithreaded programs, a window of vulnerability exists between the time when the file descriptor is created and the time `set_close_on_exec` completes. If another thread spawns another program during this window, the descriptor will leak, as it is still in the ``keep-on-exec'' mode.

Regarding the atomicity guarantees given by `~cloexec:true` or by the use of the `O_CLOEXEC` flag: on all platforms it is guaranteed that a concurrently-executing Caml thread cannot leak the descriptor by starting a new process. On Linux, this guarantee extends to concurrently-executing C threads. As of Feb 2017, other operating systems lack the necessary system calls and still expose a window of vulnerability during which a C thread can see the newly-created file descriptor in ``keep-on-exec'' mode.

    val clear_close_on_exec : file_descr -> unit

Clear the ``close-on-exec'' flag on the given descriptor. See `Unix.set_close_on_exec`.

## Directories

    val mkdir : string -> file_perm -> unit

Create a directory with the given permissions (see `Unix.umask`).

    val rmdir : string -> unit

Remove an empty directory.

    val chdir : string -> unit

Change the process working directory.

    val getcwd : unit -> string

Return the name of the current working directory.

    val chroot : string -> unit

Change the process root directory.

- Raises `Invalid_argument` on Windows

    type dir_handle

The type of descriptors over opened directories.

    val opendir : string -> dir_handle

Open a descriptor on a directory

    val readdir : dir_handle -> string

Return the next entry in a directory.

- Raises `End_of_file` when the end of the directory has been reached.

    val rewinddir : dir_handle -> unit

Reposition the descriptor to the beginning of the directory

    val closedir : dir_handle -> unit

Close a directory descriptor.

## Pipes and redirections

    val pipe : ?cloexec:bool -> unit -> file_descr * file_descr

Create a pipe. The first component of the result is opened for reading, that's the exit to the pipe. The second component is opened for writing, that's the entrance to the pipe. See `Unix.set_close_on_exec` for documentation on the `cloexec` optional argument.

    val mkfifo : string -> file_perm -> unit

Create a named pipe with the given permissions (see `Unix.umask`).

- Raises `Invalid_argument` on Windows

## High-level process and redirection management

    val create_process : string ->
           string array -> file_descr -> file_descr -> file_descr -> int

`create_process prog args stdin stdout stderr` forks a new process that executes the program in file `prog`, with arguments `args`. The pid of the new process is returned immediately; the new process executes concurrently with the current process. The standard input and outputs of the new process are connected to the descriptors `stdin`, `stdout` and `stderr`. Passing e.g. `Unix.stdout` for `stdout` prevents the redirection and causes the new process to have the same standard output as the current process. The executable file `prog` is searched in the path. The new process has the same environment as the current process.

    val create_process_env : string ->
           string array ->
           string array -> file_descr -> file_descr -> file_descr -> int

`create_process_env prog args env stdin stdout stderr` works as `Unix.create_process`, except that the extra argument `env` specifies the environment passed to the program.

    val open_process_in : string -> in_channel

High-level pipe and process management. This function runs the given command in parallel with the program. The standard output of the command is redirected to a pipe, which can be read via the returned input channel. The command is interpreted by the shell `/bin/sh` (or `cmd.exe` on Windows), cf. `Unix.system`. The `Filename.quote_command` function can be used to quote the command and its arguments as appropriate for the shell being used. If the command does not need to be run through the shell, `Unix.open_process_args_in` can be used as a more robust and more efficient alternative to `Unix.open_process_in`.

    val open_process_out : string -> out_channel

Same as `Unix.open_process_in`, but redirect the standard input of the command to a pipe. Data written to the returned output channel is sent to the standard input of the command. Warning: writes on output channels are buffered, hence be careful to call `flush` at the right times to ensure correct synchronization. If the command does not need to be run through the shell, `Unix.open_process_args_out` can be used instead of `Unix.open_process_out`.

    val open_process : string -> in_channel * out_channel

Same as `Unix.open_process_out`, but redirects both the standard input and standard output of the command to pipes connected to the two returned channels. The input channel is connected to the output of the command, and the output channel to the input of the command. If the command does not need to be run through the shell, `Unix.open_process_args` can be used instead of `Unix.open_process`.

    val open_process_full : string ->
           string array -> in_channel * out_channel * in_channel

Similar to `Unix.open_process`, but the second argument specifies the environment passed to the command. The result is a triple of channels connected respectively to the standard output, standard input, and standard error of the command. If the command does not need to be run through the shell, `Unix.open_process_args_full` can be used instead of `Unix.open_process_full`.

    val open_process_args_in : string -> string array -> in_channel

`open_process_args_in prog args` runs the program `prog` with arguments `args`. The new process executes concurrently with the current process. The standard output of the new process is redirected to a pipe, which can be read via the returned input channel.

The executable file `prog` is searched in the path. This behaviour changed in 4.12; previously `prog` was looked up only in the current directory.

The new process has the same environment as the current process.

- Since 4.08.0

    val open_process_args_out : string -> string array -> out_channel

Same as `Unix.open_process_args_in`, but redirect the standard input of the new process to a pipe. Data written to the returned output channel is sent to the standard input of the program. Warning: writes on output channels are buffered, hence be careful to call `flush` at the right times to ensure correct synchronization.

- Since 4.08.0

    val open_process_args : string -> string array -> in_channel * out_channel

Same as `Unix.open_process_args_out`, but redirects both the standard input and standard output of the new process to pipes connected to the two returned channels. The input channel is connected to the output of the program, and the output channel to the input of the program.

- Since 4.08.0

    val open_process_args_full : string ->
           string array ->
           string array -> in_channel * out_channel * in_channel

Similar to `Unix.open_process_args`, but the third argument specifies the environment passed to the new process. The result is a triple of channels connected respectively to the standard output, standard input, and standard error of the program.

- Since 4.08.0

    val process_in_pid : in_channel -> int

Return the pid of a process opened via `Unix.open_process_in` or `Unix.open_process_args_in`.

- Since 4.08.0 (4.12.0 in UnixLabels)

    val process_out_pid : out_channel -> int

Return the pid of a process opened via `Unix.open_process_out` or `Unix.open_process_args_out`.

- Since 4.08.0 (4.12.0 in UnixLabels)

    val process_pid : in_channel * out_channel -> int

Return the pid of a process opened via `Unix.open_process` or `Unix.open_process_args`.

- Since 4.08.0 (4.12.0 in UnixLabels)

    val process_full_pid : in_channel * out_channel * in_channel -> int

Return the pid of a process opened via `Unix.open_process_full` or `Unix.open_process_args_full`.

- Since 4.08.0 (4.12.0 in UnixLabels)

    val close_process_in : in_channel -> process_status

Close channels opened by `Unix.open_process_in`, wait for the associated command to terminate, and return its termination status.

    val close_process_out : out_channel -> process_status

Close channels opened by `Unix.open_process_out`, wait for the associated command to terminate, and return its termination status.

    val close_process : in_channel * out_channel -> process_status

Close channels opened by `Unix.open_process`, wait for the associated command to terminate, and return its termination status.

    val close_process_full : in_channel * out_channel * in_channel ->
           process_status

Close channels opened by `Unix.open_process_full`, wait for the associated command to terminate, and return its termination status.

## Symbolic links

    val symlink : ?to_dir:bool -> string -> string -> unit

`symlink ?to_dir src dst` creates the file `dst` as a symbolic link to the file `src`. On Windows, `~to_dir` indicates if the symbolic link points to a directory or a file; if omitted, `symlink` examines `src` using `stat` and picks appropriately, if `src` does not exist then `false` is assumed (for this reason, it is recommended that the `~to_dir` parameter be specified in new code). On Unix, `~to_dir` is ignored.

Windows symbolic links are available in Windows Vista onwards. There are some important differences between Windows symlinks and their POSIX counterparts.

Windows symbolic links come in two flavours: directory and regular, which designate whether the symbolic link points to a directory or a file. The type must be correct - a directory symlink which actually points to a file cannot be selected with chdir and a file symlink which actually points to a directory cannot be read or written (note that Cygwin's emulation layer ignores this distinction).

When symbolic links are created to existing targets, this distinction doesn't matter and `symlink` will automatically create the correct kind of symbolic link. The distinction matters when a symbolic link is created to a non-existent target.

The other caveat is that by default symbolic links are a privileged operation. Administrators will always need to be running elevated (or with UAC disabled) and by default normal user accounts need to be granted the SeCreateSymbolicLinkPrivilege via Local Security Policy (secpol.msc) or via Active Directory.

`Unix.has_symlink` can be used to check that a process is able to create symbolic links.

    val has_symlink : unit -> bool

Returns `true` if the user is able to create symbolic links. On Windows, this indicates that the user not only has the SeCreateSymbolicLinkPrivilege but is also running elevated, if necessary. On other platforms, this is simply indicates that the symlink system call is available.

- Since 4.03.0

    val readlink : string -> string

Read the contents of a symbolic link.

## Polling

    val select : file_descr list ->
           file_descr list ->
           file_descr list ->
           float -> file_descr list * file_descr list * file_descr list

Wait until some input/output operations become possible on some channels. The three list arguments are, respectively, a set of descriptors to check for reading (first argument), for writing (second argument), or for exceptional conditions (third argument). The fourth argument is the maximal timeout, in seconds; a negative fourth argument means no timeout (unbounded wait). The result is composed of three sets of descriptors: those ready for reading (first component), ready for writing (second component), and over which an exceptional condition is pending (third component).

## Locking

    type lock_command =

`|` | `F_ULOCK` | `(*` | Unlock a region | `*)`
`|` | `F_LOCK` | `(*` | Lock a region for writing, and block if already locked | `*)`
`|` | `F_TLOCK` | `(*` | Lock a region for writing, or fail if already locked | `*)`
`|` | `F_TEST` | `(*` | Test a region for other process locks | `*)`
`|` | `F_RLOCK` | `(*` | Lock a region for reading, and block if already locked | `*)`
`|` | `F_TRLOCK` | `(*` | Lock a region for reading, or fail if already locked | `*)`

Commands for `Unix.lockf`.

    val lockf : file_descr -> lock_command -> int -> unit

`lockf fd mode len` puts a lock on a region of the file opened as `fd`. The region starts at the current read/write position for `fd` (as set by `Unix.lseek`), and extends `len` bytes forward if `len` is positive, `len` bytes backwards if `len` is negative, or to the end of the file if `len` is zero. A write lock prevents any other process from acquiring a read or write lock on the region. A read lock prevents any other process from acquiring a write lock on the region, but lets other processes acquire read locks on it.

The `F_LOCK` and `F_TLOCK` commands attempts to put a write lock on the specified region. The `F_RLOCK` and `F_TRLOCK` commands attempts to put a read lock on the specified region. If one or several locks put by another process prevent the current process from acquiring the lock, `F_LOCK` and `F_RLOCK` block until these locks are removed, while `F_TLOCK` and `F_TRLOCK` fail immediately with an exception. The `F_ULOCK` removes whatever locks the current process has on the specified region. Finally, the `F_TEST` command tests whether a write lock can be acquired on the specified region, without actually putting a lock. It returns immediately if successful, or fails otherwise.

What happens when a process tries to lock a region of a file that is already locked by the same process depends on the OS. On POSIX-compliant systems, the second lock operation succeeds and may "promote" the older lock from read lock to write lock. On Windows, the second lock operation will block or fail.

## Signals

Note: installation of signal handlers is performed via the functions `Sys.signal` and `Sys.set_signal`.

    val kill : int -> int -> unit

`kill pid signal` sends signal number `signal` to the process with id `pid`.

On Windows: only the `Sys.sigkill` signal is emulated.

    type sigprocmask_command =

`|` | `SIG_SETMASK`
`|` | `SIG_BLOCK`
`|` | `SIG_UNBLOCK`

    val sigprocmask : sigprocmask_command -> int list -> int list

`sigprocmask mode sigs` changes the set of blocked signals. If `mode` is `SIG_SETMASK`, blocked signals are set to those in the list `sigs`. If `mode` is `SIG_BLOCK`, the signals in `sigs` are added to the set of blocked signals. If `mode` is `SIG_UNBLOCK`, the signals in `sigs` are removed from the set of blocked signals. `sigprocmask` returns the set of previously blocked signals.

When the systhreads version of the `Thread` module is loaded, this function redirects to `Thread.sigmask`. I.e., `sigprocmask` only changes the mask of the current thread.

- Raises `Invalid_argument` on Windows (no inter-process signals on Windows)

    val sigpending : unit -> int list

Return the set of blocked signals that are currently pending.

- Raises `Invalid_argument` on Windows (no inter-process signals on Windows)

    val sigsuspend : int list -> unit

`sigsuspend sigs` atomically sets the blocked signals to `sigs` and waits for a non-ignored, non-blocked signal to be delivered. On return, the blocked signals are reset to their initial value.

- Raises `Invalid_argument` on Windows (no inter-process signals on Windows)

    val pause : unit -> unit

Wait until a non-ignored, non-blocked signal is delivered.

- Raises `Invalid_argument` on Windows (no inter-process signals on Windows)

## Time functions

    type process_times = {

`` | `tms_utime : `float`;` | `(*` | User time for the process | `*)`
`` | `tms_stime : `float`;` | `(*` | System time for the process | `*)`
`` | `tms_cutime : `float`;` | `(*` | User time for the children processes | `*)`
`` | `tms_cstime : `float`;` | `(*` | System time for the children processes | `*)`

`}`

The execution times (CPU times) of a process.

    type tm = {

`` | `tm_sec : `int`;` | `(*` | Seconds 0..60 | `*)`
`` | `tm_min : `int`;` | `(*` | Minutes 0..59 | `*)`
`` | `tm_hour : `int`;` | `(*` | Hours 0..23 | `*)`
`` | `tm_mday : `int`;` | `(*` | Day of month 1..31 | `*)`
`` | `tm_mon : `int`;` | `(*` | Month of year 0..11 | `*)`
`` | `tm_year : `int`;` | `(*` | Year - 1900 | `*)`
`` | `tm_wday : `int`;` | `(*` | Day of week (Sunday is 0) | `*)`
`` | `tm_yday : `int`;` | `(*` | Day of year 0..365 | `*)`
`` | `tm_isdst : `bool`;` | `(*` | Daylight time savings in effect | `*)`

`}`

The type representing wallclock time and calendar date.

    val time : unit -> float

Return the current time since 00:00:00 GMT, Jan. 1, 1970, in seconds.

    val gettimeofday : unit -> float

Same as `Unix.time`, but with resolution better than 1 second.

    val gmtime : float -> tm

Convert a time in seconds, as returned by `Unix.time`, into a date and a time. Assumes UTC (Coordinated Universal Time), also known as GMT. To perform the inverse conversion, set the TZ environment variable to "UTC", use `Unix.mktime`, and then restore the original value of TZ.

    val localtime : float -> tm

Convert a time in seconds, as returned by `Unix.time`, into a date and a time. Assumes the local time zone. The function performing the inverse conversion is `Unix.mktime`.

    val mktime : tm -> float * tm

Convert a date and time, specified by the `tm` argument, into a time in seconds, as returned by `Unix.time`. The `tm_isdst`, `tm_wday` and `tm_yday` fields of `tm` are ignored. Also return a normalized copy of the given `tm` record, with the `tm_wday`, `tm_yday`, and `tm_isdst` fields recomputed from the other fields, and the other fields normalized (so that, e.g., 40 October is changed into 9 November). The `tm` argument is interpreted in the local time zone.

    val alarm : int -> int

Schedule a `SIGALRM` signal after the given number of seconds.

- Raises `Invalid_argument` on Windows

    val sleep : int -> unit

Stop execution for the given number of seconds.

    val sleepf : float -> unit

Stop execution for the given number of seconds. Like `sleep`, but fractions of seconds are supported.

- Since 4.03.0 (4.12.0 in UnixLabels)

    val times : unit -> process_times

Return the execution times of the process.

On Windows: partially implemented, will not report timings for child processes.

    val utimes : string -> float -> float -> unit

Set the last access time (second arg) and last modification time (third arg) for a file. Times are expressed in seconds from 00:00:00 GMT, Jan. 1, 1970. If both times are `0.0`, the access and last modification times are both set to the current time.

    type interval_timer =

`|` | `ITIMER_REAL` | `(*` | decrements in real time, and sends the signal `SIGALRM` when expired. | `*)`
`|` | `ITIMER_VIRTUAL` | `(*` | decrements in process virtual time, and sends `SIGVTALRM` when expired. | `*)`
`|` | `ITIMER_PROF` | `(*` | (for profiling) decrements both when the process is running and when the system is running on behalf of the process; it sends `SIGPROF` when expired. | `*)`

The three kinds of interval timers.

    type interval_timer_status = {

`` | `it_interval : `float`;` | `(*` | Period | `*)`
`` | `it_value : `float`;` | `(*` | Current value of the timer | `*)`

`}`

The type describing the status of an interval timer

    val getitimer : interval_timer -> interval_timer_status

Return the current status of the given interval timer.

- Raises `Invalid_argument` on Windows

    val setitimer : interval_timer ->
           interval_timer_status -> interval_timer_status

`setitimer t s` sets the interval timer `t` and returns its previous status. The `s` argument is interpreted as follows: `s.it_value`, if nonzero, is the time to the next timer expiration; `s.it_interval`, if nonzero, specifies a value to be used in reloading `it_value` when the timer expires. Setting `s.it_value` to zero disables the timer. Setting `s.it_interval` to zero causes the timer to be disabled after its next expiration.

- Raises `Invalid_argument` on Windows

## User id, group id

    val getuid : unit -> int

Return the user id of the user executing the process.

On Windows: always returns `1`.

    val geteuid : unit -> int

Return the effective user id under which the process runs.

On Windows: always returns `1`.

    val setuid : int -> unit

Set the real user id and effective user id for the process.

- Raises `Invalid_argument` on Windows

    val getgid : unit -> int

Return the group id of the user executing the process.

On Windows: always returns `1`.

    val getegid : unit -> int

Return the effective group id under which the process runs.

On Windows: always returns `1`.

    val setgid : int -> unit

Set the real group id and effective group id for the process.

- Raises `Invalid_argument` on Windows

    val getgroups : unit -> int array

Return the list of groups to which the user executing the process belongs.

On Windows: always returns `[|1|]`.

    val setgroups : int array -> unit

`setgroups groups` sets the supplementary group IDs for the calling process. Appropriate privileges are required.

- Raises `Invalid_argument` on Windows

    val initgroups : string -> int -> unit

`initgroups user group` initializes the group access list by reading the group database /etc/group and using all groups of which `user` is a member. The additional group `group` is also added to the list.

- Raises `Invalid_argument` on Windows

    type passwd_entry = {

`` | `pw_name : `string`;`
`` | `pw_passwd : `string`;`
`` | `pw_uid : `int`;`
`` | `pw_gid : `int`;`
`` | `pw_gecos : `string`;`
`` | `pw_dir : `string`;`
`` | `pw_shell : `string`;`

`}`

Structure of entries in the `passwd` database.

    type group_entry = {

`` | `gr_name : `string`;`
`` | `gr_passwd : `string`;`
`` | `gr_gid : `int`;`
`` | `gr_mem : `string array`;`

`}`

Structure of entries in the `groups` database.

    val getlogin : unit -> string

Return the login name of the user executing the process.

    val getpwnam : string -> passwd_entry

Find an entry in `passwd` with the given name.

- Raises `Not_found` if no such entry exists, or always on Windows.

    val getgrnam : string -> group_entry

Find an entry in `group` with the given name.

- Raises `Not_found` if no such entry exists, or always on Windows.

    val getpwuid : int -> passwd_entry

Find an entry in `passwd` with the given user id.

- Raises `Not_found` if no such entry exists, or always on Windows.

    val getgrgid : int -> group_entry

Find an entry in `group` with the given group id.

- Raises `Not_found` if no such entry exists, or always on Windows.

## Internet addresses

    type inet_addr

The abstract type of Internet addresses.

    val inet_addr_of_string : string -> inet_addr

Conversion from the printable representation of an Internet address to its internal representation. The argument string consists of 4 numbers separated by periods (`XXX.YYY.ZZZ.TTT`) for IPv4 addresses, and up to 8 numbers separated by colons for IPv6 addresses.

- Raises `Failure` when given a string that does not match these formats.

    val string_of_inet_addr : inet_addr -> string

Return the printable representation of the given Internet address. See `Unix.inet_addr_of_string` for a description of the printable representation.

    val inet_addr_any : inet_addr

A special IPv4 address, for use only with `bind`, representing all the Internet addresses that the host machine possesses.

    val inet_addr_loopback : inet_addr

A special IPv4 address representing the host machine (`127.0.0.1`).

    val inet6_addr_any : inet_addr

A special IPv6 address, for use only with `bind`, representing all the Internet addresses that the host machine possesses.

    val inet6_addr_loopback : inet_addr

A special IPv6 address representing the host machine (`::1`).

    val is_inet6_addr : inet_addr -> bool

Whether the given `inet_addr` is an IPv6 address.

- Since 4.12.0

## Sockets

    type socket_domain =

`|` | `PF_UNIX` | `(*` | Unix domain | `*)`
`|` | `PF_INET` | `(*` | Internet domain (IPv4) | `*)`
`|` | `PF_INET6` | `(*` | Internet domain (IPv6) | `*)`

The type of socket domains. Not all platforms support IPv6 sockets (type `PF_INET6`).

On Windows: `PF_UNIX` supported since 4.14.0 on Windows 10 1803 and later.

    type socket_type =

`|` | `SOCK_STREAM` | `(*` | Stream socket | `*)`
`|` | `SOCK_DGRAM` | `(*` | Datagram socket | `*)`
`|` | `SOCK_RAW` | `(*` | Raw socket | `*)`
`|` | `SOCK_SEQPACKET` | `(*` | Sequenced packets socket | `*)`

The type of socket kinds, specifying the semantics of communications. `SOCK_SEQPACKET` is included for completeness, but is rarely supported by the OS, and needs system calls that are not available in this library.

    type sockaddr =

`|` | `ADDR_UNIX of `string``
`|` | `ADDR_INET of `inet_addr * int``

The type of socket addresses. `ADDR_UNIX name` is a socket address in the Unix domain; `name` is a file name in the file system. `ADDR_INET(addr,port)` is a socket address in the Internet domain; `addr` is the Internet address of the machine, and `port` is the port number.

    val socket : ?cloexec:bool ->
           socket_domain -> socket_type -> int -> file_descr

Create a new socket in the given domain, and with the given kind. The third argument is the protocol type; 0 selects the default protocol for that kind of sockets. See `Unix.set_close_on_exec` for documentation on the `cloexec` optional argument.

    val domain_of_sockaddr : sockaddr -> socket_domain

Return the socket domain adequate for the given socket address.

    val socketpair : ?cloexec:bool ->
           socket_domain ->
           socket_type -> int -> file_descr * file_descr

Create a pair of unnamed sockets, connected together. See `Unix.set_close_on_exec` for documentation on the `cloexec` optional argument.

    val accept : ?cloexec:bool -> file_descr -> file_descr * sockaddr

Accept connections on the given socket. The returned descriptor is a socket connected to the client; the returned address is the address of the connecting client. See `Unix.set_close_on_exec` for documentation on the `cloexec` optional argument.

    val bind : file_descr -> sockaddr -> unit

Bind a socket to an address.

    val connect : file_descr -> sockaddr -> unit

Connect a socket to an address.

    val listen : file_descr -> int -> unit

Set up a socket for receiving connection requests. The integer argument is the maximal number of pending requests.

    type shutdown_command =

`|` | `SHUTDOWN_RECEIVE` | `(*` | Close for receiving | `*)`
`|` | `SHUTDOWN_SEND` | `(*` | Close for sending | `*)`
`|` | `SHUTDOWN_ALL` | `(*` | Close both | `*)`

The type of commands for `shutdown`.

    val shutdown : file_descr -> shutdown_command -> unit

Shutdown a socket connection. `SHUTDOWN_SEND` as second argument causes reads on the other end of the connection to return an end-of-file condition. `SHUTDOWN_RECEIVE` causes writes on the other end of the connection to return a closed pipe condition (`SIGPIPE` signal).

    val getsockname : file_descr -> sockaddr

Return the address of the given socket.

    val getpeername : file_descr -> sockaddr

Return the address of the host connected to the given socket.

    type msg_flag =

`|` | `MSG_OOB`
`|` | `MSG_DONTROUTE`
`|` | `MSG_PEEK`

The flags for `Unix.recv`, `Unix.recvfrom`, `Unix.send` and `Unix.sendto`.

    val recv : file_descr -> bytes -> int -> int -> msg_flag list -> int

Receive data from a connected socket.

    val recvfrom : file_descr ->
           bytes -> int -> int -> msg_flag list -> int * sockaddr

Receive data from an unconnected socket.

    val send : file_descr -> bytes -> int -> int -> msg_flag list -> int

Send data over a connected socket.

    val send_substring : file_descr -> string -> int -> int -> msg_flag list -> int

Same as `send`, but take the data from a string instead of a byte sequence.

- Since 4.02.0

    val sendto : file_descr ->
           bytes -> int -> int -> msg_flag list -> sockaddr -> int

Send data over an unconnected socket.

    val sendto_substring : file_descr ->
           string -> int -> int -> msg_flag list -> sockaddr -> int

Same as `sendto`, but take the data from a string instead of a byte sequence.

- Since 4.02.0

## Socket options

    type socket_bool_option =

`|` | `SO_DEBUG` | `(*` | Record debugging information | `*)`
`|` | `SO_BROADCAST` | `(*` | Permit sending of broadcast messages | `*)`
`|` | `SO_REUSEADDR` | `(*` | Allow reuse of local addresses for bind | `*)`
`|` | `SO_KEEPALIVE` | `(*` | Keep connection active | `*)`
`|` | `SO_DONTROUTE` | `(*` | Bypass the standard routing algorithms | `*)`
`|` | `SO_OOBINLINE` | `(*` | Leave out-of-band data in line | `*)`
`|` | `SO_ACCEPTCONN` | `(*` | Report whether socket listening is enabled | `*)`
`|` | `TCP_NODELAY` | `(*` | Control the Nagle algorithm for TCP sockets | `*)`
`|` | `IPV6_ONLY` | `(*` | Forbid binding an IPv6 socket to an IPv4 address | `*)`
`|` | `SO_REUSEPORT` | `(*` | Allow reuse of address and port bindings | `*)`

The socket options that can be consulted with `Unix.getsockopt` and modified with `Unix.setsockopt`. These options have a boolean (`true`/`false`) value.

    type socket_int_option =

`|` | `SO_SNDBUF` | `(*` | Size of send buffer | `*)`
`|` | `SO_RCVBUF` | `(*` | Size of received buffer | `*)`
`|` | `SO_ERROR` | `(*` | Deprecated. Use Unix.getsockopt_error instead. Deprecated. Use `Unix.getsockopt_error` instead. | `*)`
`|` | `SO_TYPE` | `(*` | Report the socket type | `*)`
`|` | `SO_RCVLOWAT` | `(*` | Minimum number of bytes to process for input operations | `*)`
`|` | `SO_SNDLOWAT` | `(*` | Minimum number of bytes to process for output operations | `*)`

The socket options that can be consulted with `Unix.getsockopt_int` and modified with `Unix.setsockopt_int`. These options have an integer value.

    type socket_optint_option =

`|` | `SO_LINGER` | `(*` | Whether to linger on closed connections that have data present, and for how long (in seconds) | `*)`

The socket options that can be consulted with `Unix.getsockopt_optint` and modified with `Unix.setsockopt_optint`. These options have a value of type `int option`, with `None` meaning ``disabled''.

    type socket_float_option =

`|` | `SO_RCVTIMEO` | `(*` | Timeout for input operations | `*)`
`|` | `SO_SNDTIMEO` | `(*` | Timeout for output operations | `*)`

The socket options that can be consulted with `Unix.getsockopt_float` and modified with `Unix.setsockopt_float`. These options have a floating-point value representing a time in seconds. The value 0 means infinite timeout.

    val getsockopt : file_descr -> socket_bool_option -> bool

Return the current status of a boolean-valued option in the given socket.

    val setsockopt : file_descr -> socket_bool_option -> bool -> unit

Set or clear a boolean-valued option in the given socket.

    val getsockopt_int : file_descr -> socket_int_option -> int

Same as `Unix.getsockopt` for an integer-valued socket option.

    val setsockopt_int : file_descr -> socket_int_option -> int -> unit

Same as `Unix.setsockopt` for an integer-valued socket option.

    val getsockopt_optint : file_descr -> socket_optint_option -> int option

Same as `Unix.getsockopt` for a socket option whose value is an `int option`.

    val setsockopt_optint : file_descr -> socket_optint_option -> int option -> unit

Same as `Unix.setsockopt` for a socket option whose value is an `int option`.

    val getsockopt_float : file_descr -> socket_float_option -> float

Same as `Unix.getsockopt` for a socket option whose value is a floating-point number.

    val setsockopt_float : file_descr -> socket_float_option -> float -> unit

Same as `Unix.setsockopt` for a socket option whose value is a floating-point number.

    val getsockopt_error : file_descr -> error option

Return the error condition associated with the given socket, and clear it.

## High-level network connection functions

    val open_connection : sockaddr -> in_channel * out_channel

Connect to a server at the given address. Return a pair of buffered channels connected to the server. Remember to call `flush` on the output channel at the right times to ensure correct synchronization.

The two channels returned by `open_connection` share a descriptor to a socket. Therefore, when the connection is over, you should call `close_out` on the output channel, which will also close the underlying socket. Do not call `close_in` on the input channel; it will be collected by the GC eventually.

    val shutdown_connection : in_channel -> unit

``Shut down'' a connection established with `Unix.open_connection`; that is, transmit an end-of-file condition to the server reading on the other side of the connection. This does not close the socket and the channels used by the connection. See `Unix.open_connection` for how to close them once the connection is over.

    val establish_server : (in_channel -> out_channel -> unit) -> sockaddr -> unit

Establish a server on the given address. The function given as first argument is called for each connection with two buffered channels connected to the client. A new process is created for each connection. The function `Unix.establish_server` never returns normally.

The two channels given to the function share a descriptor to a socket. The function does not need to close the channels, since this occurs automatically when the function returns. If the function prefers explicit closing, it should close the output channel using `close_out` and leave the input channel unclosed, for reasons explained in `Unix.in_channel_of_descr`.

- Raises `Invalid_argument` on Windows. Use threads instead.

## Host and protocol databases

    type host_entry = {

`` | `h_name : `string`;`
`` | `h_aliases : `string array`;`
`` | `h_addrtype : `socket_domain`;`
`` | `h_addr_list : `inet_addr array`;`

`}`

Structure of entries in the `hosts` database.

    type protocol_entry = {

`` | `p_name : `string`;`
`` | `p_aliases : `string array`;`
`` | `p_proto : `int`;`

`}`

Structure of entries in the `protocols` database.

    type service_entry = {

`` | `s_name : `string`;`
`` | `s_aliases : `string array`;`
`` | `s_port : `int`;`
`` | `s_proto : `string`;`

`}`

Structure of entries in the `services` database.

    val gethostname : unit -> string

Return the name of the local host.

    val gethostbyname : string -> host_entry

Find an entry in `hosts` with the given name.

- Raises `Not_found` if no such entry exists.

    val gethostbyaddr : inet_addr -> host_entry

Find an entry in `hosts` with the given address.

- Raises `Not_found` if no such entry exists.

    val getprotobyname : string -> protocol_entry

Find an entry in `protocols` with the given name.

- Raises `Not_found` if no such entry exists.

    val getprotobynumber : int -> protocol_entry

Find an entry in `protocols` with the given protocol number.

- Raises `Not_found` if no such entry exists.

    val getservbyname : string -> string -> service_entry

Find an entry in `services` with the given name.

- Raises `Not_found` if no such entry exists.

    val getservbyport : int -> string -> service_entry

Find an entry in `services` with the given service number.

- Raises `Not_found` if no such entry exists.

    type addr_info = {

`` | `ai_family : `socket_domain`;` | `(*` | Socket domain | `*)`
`` | `ai_socktype : `socket_type`;` | `(*` | Socket type | `*)`
`` | `ai_protocol : `int`;` | `(*` | Socket protocol number | `*)`
`` | `ai_addr : `sockaddr`;` | `(*` | Address | `*)`
`` | `ai_canonname : `string`;` | `(*` | Canonical host name | `*)`

`}`

Address information returned by `Unix.getaddrinfo`.

    type getaddrinfo_option =

`|` | `AI_FAMILY of `socket_domain`` | `(*` | Impose the given socket domain | `*)`
`|` | `AI_SOCKTYPE of `socket_type`` | `(*` | Impose the given socket type | `*)`
`|` | `AI_PROTOCOL of `int`` | `(*` | Impose the given protocol | `*)`
`|` | `AI_NUMERICHOST` | `(*` | Do not call name resolver, expect numeric IP address | `*)`
`|` | `AI_CANONNAME` | `(*` | Fill the `ai_canonname` field of the result | `*)`
`|` | `AI_PASSIVE` | `(*` | Set address to ``any'' address for use with `Unix.bind` | `*)`

Options to `Unix.getaddrinfo`.

    val getaddrinfo : string -> string -> getaddrinfo_option list -> addr_info list

`getaddrinfo host service opts` returns a list of `Unix.addr_info` records describing socket parameters and addresses suitable for communicating with the given host and service. The empty list is returned if the host or service names are unknown, or the constraints expressed in `opts` cannot be satisfied.

`host` is either a host name or the string representation of an IP address. `host` can be given as the empty string; in this case, the ``any'' address or the ``loopback'' address are used, depending whether `opts` contains `AI_PASSIVE`. `service` is either a service name or the string representation of a port number. `service` can be given as the empty string; in this case, the port field of the returned addresses is set to 0. `opts` is a possibly empty list of options that allows the caller to force a particular socket domain (e.g. IPv6 only or IPv4 only) or a particular socket type (e.g. TCP only or UDP only).

    type name_info = {

`` | `ni_hostname : `string`;` | `(*` | Name or IP address of host | `*)`
`` | `ni_service : `string`;` | `(*` | Name of service or port number | `*)`

`}`

Host and service information returned by `Unix.getnameinfo`.

    type getnameinfo_option =

`|` | `NI_NOFQDN` | `(*` | Do not qualify local host names | `*)`
`|` | `NI_NUMERICHOST` | `(*` | Always return host as IP address | `*)`
`|` | `NI_NAMEREQD` | `(*` | Fail if host name cannot be determined | `*)`
`|` | `NI_NUMERICSERV` | `(*` | Always return service as port number | `*)`
`|` | `NI_DGRAM` | `(*` | Consider the service as UDP-based instead of the default TCP | `*)`

Options to `Unix.getnameinfo`.

    val getnameinfo : sockaddr -> getnameinfo_option list -> name_info

`getnameinfo addr opts` returns the host name and service name corresponding to the socket address `addr`. `opts` is a possibly empty list of options that governs how these names are obtained.

- Raises `Not_found` if an error occurs.

## Terminal interface

The following functions implement the POSIX standard terminal interface. They provide control over asynchronous communication ports and pseudo-terminals. Refer to the `termios` man page for a complete description.

    type terminal_io = {

`` | `mutable c_ignbrk : `bool`;` | `(*` | Ignore the break condition. | `*)`
`` | `mutable c_brkint : `bool`;` | `(*` | Signal interrupt on break condition. | `*)`
`` | `mutable c_ignpar : `bool`;` | `(*` | Ignore characters with parity errors. | `*)`
`` | `mutable c_parmrk : `bool`;` | `(*` | Mark parity errors. | `*)`
`` | `mutable c_inpck : `bool`;` | `(*` | Enable parity check on input. | `*)`
`` | `mutable c_istrip : `bool`;` | `(*` | Strip 8th bit on input characters. | `*)`
`` | `mutable c_inlcr : `bool`;` | `(*` | Map NL to CR on input. | `*)`
`` | `mutable c_igncr : `bool`;` | `(*` | Ignore CR on input. | `*)`
`` | `mutable c_icrnl : `bool`;` | `(*` | Map CR to NL on input. | `*)`
`` | `mutable c_ixon : `bool`;` | `(*` | Recognize XON/XOFF characters on input. | `*)`
`` | `mutable c_ixoff : `bool`;` | `(*` | Emit XON/XOFF chars to control input flow. | `*)`
`` | `mutable c_opost : `bool`;` | `(*` | Enable output processing. | `*)`
`` | `mutable c_obaud : `int`;` | `(*` | Output baud rate (0 means close connection). | `*)`
`` | `mutable c_ibaud : `int`;` | `(*` | Input baud rate. | `*)`
`` | `mutable c_csize : `int`;` | `(*` | Number of bits per character (5-8). | `*)`
`` | `mutable c_cstopb : `int`;` | `(*` | Number of stop bits (1-2). | `*)`
`` | `mutable c_cread : `bool`;` | `(*` | Reception is enabled. | `*)`
`` | `mutable c_parenb : `bool`;` | `(*` | Enable parity generation and detection. | `*)`
`` | `mutable c_parodd : `bool`;` | `(*` | Specify odd parity instead of even. | `*)`
`` | `mutable c_hupcl : `bool`;` | `(*` | Hang up on last close. | `*)`
`` | `mutable c_clocal : `bool`;` | `(*` | Ignore modem status lines. | `*)`
`` | `mutable c_isig : `bool`;` | `(*` | Generate signal on INTR, QUIT, SUSP. | `*)`
`` | `mutable c_icanon : `bool`;` | `(*` | Enable canonical processing (line buffering and editing) | `*)`
`` | `mutable c_noflsh : `bool`;` | `(*` | Disable flush after INTR, QUIT, SUSP. | `*)`
`` | `mutable c_echo : `bool`;` | `(*` | Echo input characters. | `*)`
`` | `mutable c_echoe : `bool`;` | `(*` | Echo ERASE (to erase previous character). | `*)`
`` | `mutable c_echok : `bool`;` | `(*` | Echo KILL (to erase the current line). | `*)`
`` | `mutable c_echonl : `bool`;` | `(*` | Echo NL even if c_echo is not set. | `*)`
`` | `mutable c_vintr : `char`;` | `(*` | Interrupt character (usually ctrl-C). | `*)`
`` | `mutable c_vquit : `char`;` | `(*` | Quit character (usually ctrl-\). | `*)`
`` | `mutable c_verase : `char`;` | `(*` | Erase character (usually DEL or ctrl-H). | `*)`
`` | `mutable c_vkill : `char`;` | `(*` | Kill line character (usually ctrl-U). | `*)`
`` | `mutable c_veof : `char`;` | `(*` | End-of-file character (usually ctrl-D). | `*)`
`` | `mutable c_veol : `char`;` | `(*` | Alternate end-of-line char. (usually none). | `*)`
`` | `mutable c_vmin : `int`;` | `(*` | Minimum number of characters to read before the read request is satisfied. | `*)`
`` | `mutable c_vtime : `int`;` | `(*` | Maximum read wait (in 0.1s units). | `*)`
`` | `mutable c_vstart : `char`;` | `(*` | Start character (usually ctrl-Q). | `*)`
`` | `mutable c_vstop : `char`;` | `(*` | Stop character (usually ctrl-S). | `*)`

`}`

    val tcgetattr : file_descr -> terminal_io

Return the status of the terminal referred to by the given file descriptor.

- Raises `Invalid_argument` on Windows

    type setattr_when =

`|` | `TCSANOW`
`|` | `TCSADRAIN`
`|` | `TCSAFLUSH`

    val tcsetattr : file_descr -> setattr_when -> terminal_io -> unit

Set the status of the terminal referred to by the given file descriptor. The second argument indicates when the status change takes place: immediately (`TCSANOW`), when all pending output has been transmitted (`TCSADRAIN`), or after flushing all input that has been received but not read (`TCSAFLUSH`). `TCSADRAIN` is recommended when changing the output parameters; `TCSAFLUSH`, when changing the input parameters.

- Raises `Invalid_argument` on Windows

    val tcsendbreak : file_descr -> int -> unit

Send a break condition on the given file descriptor. The second argument is the duration of the break, in 0.1s units; 0 means standard duration (0.25s).

- Raises `Invalid_argument` on Windows

    val tcdrain : file_descr -> unit

Waits until all output written on the given file descriptor has been transmitted.

- Raises `Invalid_argument` on Windows

    type flush_queue =

`|` | `TCIFLUSH`
`|` | `TCOFLUSH`
`|` | `TCIOFLUSH`

    val tcflush : file_descr -> flush_queue -> unit

Discard data written on the given file descriptor but not yet transmitted, or data received but not yet read, depending on the second argument: `TCIFLUSH` flushes data received but not read, `TCOFLUSH` flushes data written but not transmitted, and `TCIOFLUSH` flushes both.

- Raises `Invalid_argument` on Windows

    type flow_action =

`|` | `TCOOFF`
`|` | `TCOON`
`|` | `TCIOFF`
`|` | `TCION`

    val tcflow : file_descr -> flow_action -> unit

Suspend or restart reception or transmission of data on the given file descriptor, depending on the second argument: `TCOOFF` suspends output, `TCOON` restarts output, `TCIOFF` transmits a STOP character to suspend input, and `TCION` transmits a START character to restart input.

- Raises `Invalid_argument` on Windows

    val setsid : unit -> int

Put the calling process in a new session and detach it from its controlling terminal.

- Raises `Invalid_argument` on Windows