Chapter 20 | The unix library: Unix system calls

The `unix` library makes many Unix system calls and system-related library functions available to Objective Caml programs. This chapter describes briefly the functions provided. Refer to sections 2 and 3 of the Unix manual for more details on the behavior of these functions.
Not all functions are provided by all Unix variants. If some functions are not available, they will raise `Invalid``_``arg` when called.
Programs that use the `unix` library must be linked as follows:

            ocamlc other options unix.cma other files
            ocamlopt other options unix.cmxa other files

For interactive use of the `unix` library, do:

            ocamlmktop -o mytop unix.cma
            ./mytop

MacOS:

> A fairly complete emulation of the Unix system calls is provided in the MacOS version of Objective Caml. The end of this chapter gives more information on the functions that are not supported under MacOS.

Windows:

> A fairly complete emulation of the Unix system calls is provided in the Windows version of Objective Caml. The end of this chapter gives more information on the functions that are not supported under Windows.

20.1 | Module `Unix`: interface to the Unix system

Error report

    type error =

> Errors defined in the POSIX standard

        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 *)

> Additional errors, mostly BSD

      | 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 *)

> All other errors are mapped to EUNKNOWNERR

      | EUNKNOWNERR of int  (* Unknown error *)

> The type of error codes.

    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.

    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_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''.

    val getenv: string -> string

> Return the value associated to a variable in the process environment. Raise `Not_found` if the variable is unbound. (This function is identical to `Sys.getenv`.)

    val putenv: string -> string -> unit

> `Unix.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
      | WSIGNALED of int
      | WSTOPPED of int

> The termination status of a process. `WEXITED` means that the process terminated normally by `exit`; the argument is the return code. `WSIGNALED` means that the process was killed by a signal; the argument is the signal number. `WSTOPPED` means that the process was stopped by a signal; the argument is the signal number.

    type wait_flag =
        WNOHANG
      | WUNTRACED

> Flags for `waitpid`. `WNOHANG` means do not block if no child has died yet, but immediately return with a pid equal to 0. `WUNTRACED` means report also the children that receive stop signals.

    val execv : prog:string -> args:string array -> unit

> `execv prog args` execute the program in file `prog`, with the arguments `args`, and the current process environment.

    val execve : prog:string -> args:string array -> env:string array -> unit

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

    val execvp : prog:string -> args:string array -> unit
    val execvpe : prog:string -> args:string array -> env:string array -> unit

> Same as `execv` and `execvp` respectively, 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.

    val wait : unit -> int * process_status

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

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

> Same as `wait`, but waits for the 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, or also report stopped children.

    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` and therefore can contain redirections, quotes, variables, etc. The result `WEXITED 127` indicates that the shell couldn't be executed.

    val getpid : unit -> int

> Return the pid of the process.

    val getppid : unit -> int

> Return the pid of the parent process.

    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.

Basic file input/output

    type file_descr

> The abstract type of file descriptors.

    val stdin : file_descr
    val stdout : file_descr
    val stderr : file_descr

> File descriptors for standard input, standard output and 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 *)

> The flags to `open`.

    type file_perm = int

> The type of file access rights.

    val openfile : string -> mode:open_flag list -> perm: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. Return a file descriptor on the named file.

    val close : file_descr -> unit

> Close a file descriptor.

    val read : file_descr -> buf:string -> pos:int -> len:int -> int

> `read fd buff ofs len` reads `len` characters from descriptor `fd`, storing them in string `buff`, starting at position `ofs` in string `buff`. Return the number of characters actually read.

    val write : file_descr -> buf:string -> pos:int -> len:int -> int

> `write fd buff ofs len` writes `len` characters to descriptor `fd`, taking them from string `buff`, starting at position `ofs` in string `buff`. Return the number of characters actually written.

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.

    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.

    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
      | SEEK_CUR
      | SEEK_END

> Positioning modes for `lseek`. `SEEK_SET` indicates positions relative to the beginning of the file, `SEEK_CUR` relative to the current position, `SEEK_END` relative to the end of the file.

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

> Set the current position for a file descriptor

    val truncate : string -> len:int -> unit

> Truncates the named file to the given size.

    val ftruncate : file_descr -> len:int -> unit

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

File statistics

    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 minor number *)
        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 informations returned by the `stat` calls.

    val stat : string -> stats

> Return the information for the named file.

    val lstat : string -> stats

> Same as `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.

Operations on file names

    val unlink : string -> unit

> Removes the named file

    val rename : src:string -> dst:string -> unit

> `rename old new` changes the name of a file from `old` to `new`.

    val link : src:string -> dst:string -> unit

> `link source dest` creates a hard link named `dest` to the file named `new`.

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 `access` call.

    val chmod : string -> perm:file_perm -> unit

> Change the permissions of the named file.

    val fchmod : file_descr -> perm:file_perm -> unit

> Change the permissions of an opened file.

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

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

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

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

    val umask : int -> int

> Set the process creation mask, and return the previous mask.

    val access : string -> perm:access_permission list -> unit

> Check that the process has the given permissions over the named file. Raise `Unix_error` otherwise.

Operations on file descriptors

    val dup : file_descr -> file_descr

> Return a new file descriptor referencing the same file as the given descriptor.

    val dup2 : src:file_descr -> dst:file_descr -> unit

> `dup2 fd1 fd2` duplicates `fd1` to `fd2`, closing `fd2` if already opened.

    val set_nonblock : file_descr -> unit
    val clear_nonblock : file_descr -> unit

> Set or clear 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 set_close_on_exec : file_descr -> unit
    val clear_close_on_exec : file_descr -> unit

> Set or clear 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` functions.

Directories

    val mkdir : string -> perm:file_perm -> unit

> Create a directory with the given permissions.

    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.

    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. Raise `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 : 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.

    val mkfifo : string -> perm:file_perm -> unit

> Create a named pipe with the given permissions.

High-level process and redirection management

    val create_process :
      prog:string -> args:string array ->
      stdin:file_descr -> stdout:file_descr -> stderr:file_descr -> int

> `create_process prog args new_stdin new_stdout new_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 `new_stdin`, `new_stdout` and `new_stderr`. Passing e.g. `stdout` for `new_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. All file descriptors of the current process are closed in the new process, except those redirected to standard input and outputs.

    val create_process_env :
      prog:string -> args:string array -> env:string array ->
      stdin:file_descr -> stdout:file_descr -> stderr:file_descr -> int

> `create_process_env prog args env new_stdin new_stdout new_stderr` works as `create_process`, except that the extra argument `env` specifies the environment passed to the program.

    val open_process_in: string -> in_channel
    val open_process_out: string -> out_channel
    val open_process: string -> in_channel * out_channel

> High-level pipe and process management. These functions run the given command in parallel with the program, and return channels connected to the standard input and/or the standard output of the command. The command is interpreted by the shell `/bin/sh` (cf. `system`). Warning: writes on channels are buffered, hence be careful to call `flush` at the right times to ensure correct synchronization.

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

> Similar to `open_process`, but the second argument specifies the environment passed to the command. The result is a triple of channels connected to the standard output, standard input, and standard error of the command.

    val close_process_in: in_channel -> process_status
    val close_process_out: out_channel -> process_status
    val close_process: in_channel * out_channel -> process_status
    val close_process_full: in_channel * out_channel * in_channel -> process_status

> Close channels opened by `open_process_in`, `open_process_out`, `open_process` and `open_process_full`, respectively, wait for the associated command to terminate, and return its termination status.

Symbolic links

    val symlink : src:string -> dst:string -> unit

> `symlink source dest` creates the file `dest` as a symbolic link to the file `source`.

    val readlink : string -> string

> Read the contents of a link.

Polling

    val select :
      read:file_descr list -> write:file_descr list -> except:file_descr list ->
      timeout: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 `lockf`.

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

> `lockf fd cmd size` 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 `lseek`), and extends `size` bytes forward if `size` is positive, `size` bytes backwards if `size` is negative, or to the end of the file if `size` is zero. A write lock (set with `F_LOCK` or `F_TLOCK`) prevents any other process from acquiring a read or write lock on the region. A read lock (set with `F_RLOCK` or `F_TRLOCK`) prevents any other process from acquiring a write lock on the region, but lets other processes acquire read locks on it.

Signals

    val kill : pid:int -> signal:int -> unit

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

    type sigprocmask_command = SIG_SETMASK | SIG_BLOCK | SIG_UNBLOCK
    val sigprocmask: mode:sigprocmask_command -> int list -> int list

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

    val sigpending: unit -> int list

> Return the set of blocked signals that are currently pending.

    val sigsuspend: int list -> unit

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

    val pause : unit -> unit

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

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..59 *)
        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 `time`, but with resolution better than 1 second.

    val gmtime : float -> tm

> Convert a time in seconds, as returned by `time`, into a date and a time. Assumes Greenwich meridian time zone, also known as UTC.

    val localtime : float -> tm

> Convert a time in seconds, as returned by `time`, into a date and a time. Assumes the local time zone.

    val mktime : tm -> float * tm

> Convert a date and time, specified by the `tm` argument, into a time in seconds, as returned by `time`. 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. The `tm` argument is interpreted in the local time zone.

    val alarm : int -> int

> Schedule a `SIGALRM` signals after the given number of seconds.

    val sleep : int -> unit

> Stop execution for the given number of seconds.

    val times : unit -> process_times

> Return the execution times of the process.

    val utimes : string -> access:float -> modif: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.

    type interval_timer =
        ITIMER_REAL
      | ITIMER_VIRTUAL
      | ITIMER_PROF

> The three kinds of interval timers. `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.

    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.

    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 disable the timer. Setting `s.it_interval` to zero causes the timer to be disabled after its next expiration.

User id, group id

    val getuid : unit -> int

> Return the user id of the user executing the process.

    val geteuid : unit -> int

> Return the effective user id under which the process runs.

    val setuid : int -> unit

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

    val getgid : unit -> int

> Return the group id of the user executing the process.

    val getegid : unit -> int

> Return the effective group id under which the process runs.

    val setgid : int -> unit

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

    val getgroups : unit -> int array

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

    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, or raise `Not_found`.

    val getgrnam : string -> group_entry

> Find an entry in `group` with the given name, or raise `Not_found`.

    val getpwuid : int -> passwd_entry

> Find an entry in `passwd` with the given user id, or raise `Not_found`.

    val getgrgid : int -> group_entry

> Find an entry in `group` with the given group id, or raise `Not_found`.

Internet addresses

    type inet_addr

> The abstract type of Internet addresses.

    val inet_addr_of_string : string -> inet_addr
    val string_of_inet_addr : inet_addr -> string

> Conversions between string with the format `XXX.YYY.ZZZ.TTT` and Internet addresses. `inet_addr_of_string` raises `Failure` when given a string that does not match this format.

    val inet_addr_any : inet_addr

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

Sockets

    type socket_domain =
        PF_UNIX                             (* Unix domain *)
      | PF_INET                             (* Internet domain *)

> The type of socket domains.

    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.

    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 :
      domain:socket_domain -> kind:socket_type -> protocol: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.

    val socketpair :
      domain:socket_domain -> kind:socket_type -> protocol:int ->
        file_descr * file_descr

> Create a pair of unnamed sockets, connected together.

    val accept : 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.

    val bind : file_descr -> addr:sockaddr -> unit

> Bind a socket to an address.

    val connect : file_descr -> addr:sockaddr -> unit

> Connect a socket to an address.

    val listen : file_descr -> max: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 -> mode: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 `recv`, `recvfrom`, `send` and `sendto`.

    val recv :
            file_descr -> buf:string -> pos:int -> len:int
              -> mode:msg_flag list -> int
    val recvfrom :
            file_descr -> buf:string -> pos:int -> len:int
              -> mode:msg_flag list -> int * sockaddr

> Receive data from an unconnected socket.

    val send : file_descr -> buf:string -> pos:int -> len:int
              -> mode:msg_flag list -> int
    val sendto :
            file_descr -> buf:string -> pos:int -> len:int
              -> mode:msg_flag list -> addr:sockaddr -> int

> Send data over an unconnected socket.

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 *)

> The socket options that can be consulted with `getsockopt` and modified with `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               (* Report the error status and clear it *)
      | 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 `getsockopt_int` and modified with `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 `getsockopt_optint` and modified with `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 `getsockopt_float` and modified with `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
    val setsockopt_int : file_descr -> socket_int_option -> int -> unit

> Same, for an integer-valued socket option.

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

> Same, for a socket option whose value is an `int option`.

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

> Same, for a socket option whose value is a floating-point number.

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.

    val shutdown_connection : in_channel -> unit

> ``Shut down'' a connection established with `open_connection`; that is, transmit an end-of-file condition to the server reading on the other side of the connection.

    val establish_server : (in_channel -> out_channel -> unit) ->
                           addr: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 `establish_server` never returns normally.

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, or raise `Not_found`.

    val gethostbyaddr : inet_addr -> host_entry

> Find an entry in `hosts` with the given address, or raise `Not_found`.

    val getprotobyname : string -> protocol_entry

> Find an entry in `protocols` with the given name, or raise `Not_found`.

    val getprotobynumber : int -> protocol_entry

> Find an entry in `protocols` with the given protocol number, or raise `Not_found`.

    val getservbyname : string -> protocol:string -> service_entry

> Find an entry in `services` with the given name, or raise `Not_found`.

    val getservbyport : int -> protocol:string -> service_entry

> Find an entry in `services` with the given service number, or raise `Not_found`.

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 = {

> Input modes:

        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. *)

> Output modes:

        mutable c_opost: bool;   (* Enable output processing. *)

> Control modes:

        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. *)

> Local modes:

        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. *)

> Control characters:

        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.

    type setattr_when = TCSANOW | TCSADRAIN | TCSAFLUSH
    val tcsetattr: file_descr -> mode: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.

    val tcsendbreak: file_descr -> duration: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).

    val tcdrain: file_descr -> unit

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

    type flush_queue = TCIFLUSH | TCOFLUSH | TCIOFLUSH
    val tcflush: file_descr -> mode: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.

    type flow_action = TCOOFF | TCOON | TCIOFF | TCION
    val tcflow: file_descr -> mode: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.

    val setsid : unit -> int

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

MacOS:

> Under MacOS, the Unix library is only available in the toplevel application, not in MPW tools. Below is a list of the functions that are not implemented, or only partially implemented, under MacOS. Functions not mentioned are fully implemented and behave as described previously in this chapter.

> Functions | Comment
> `chown`, `fchown` | not implemented
> `chroot` | not implemented
> `environment`, `putenv` | not implemented
> `execv`, `execve`, `execvp`, `execvpe` | not implemented
> `fork` | not implemented, use threads
> `getegid`, `geteuid`, `getgid`, `getuid` | always return 1
> `getgrnam`, `getgrgid` | not implemented
> `getlogin` | returns the user name as set in the Internet control panel
> `getpid` | returns the low-order 31 bits of the PSN
> `getppid` | not implemented
> `getpwnam`, `getpwuid` | not implemented
> `kill` | not implemented
> `link` | not implemented
> `mkfifo` | not implemented
> `nice` | not implemented
> `setgid`, `setuid` | not implemented
> `times` | only the process user time is returned
> `umask` | not implemented
> `wait` | not implemented
> `waitpid` | not implemented
> `establish``_``server` | not implemented; use threads
> terminal functions (`tc*`) | not implemented

Windows:

> The Cygwin port of Objective Caml fully implements all functions from the Unix module. The native Win32 port implements a subset of them. Below is a list of the functions that are not implemented, or only partially implemented, by the Win32 port. Functions not mentioned are fully implemented and behave as described previously in this chapter.

> Functions | Comment
> `fork` | not implemented, use `create``_``process` or threads
> `wait` | not implemented, use `waitpid`
> `waitpid` | can only wait for a given PID, not any child process
> `getppid` | not implemented (meaningless under Windows)
> `nice` | not implemented
> `in``_``channel``_``of``_``descr` | does not work on sockets under Windows 95, 98, ME; works fine under NT and 2000
> `out``_``channel``_``of``_``descr` | ditto
> `truncate`, `ftruncate` | not implemented
> `lstat`, `fstat` | not implemented
> `link`, `symlink`, `readlink` | not implemented (no links under Windows)
> `fchmod` | not implemented
> `chown`, `fchown` | not implemented (make no sense on a DOS file system)
> `umask` | not implemented
> `set``_``nonblock`, `clear``_``nonblock` | implemented as dummy functions; use threads instead of non-blocking I/O
> `rewinddir` | not implemented; re-open the directory instead
> `mkfifo` | not implemented
> `select` | implemented, but works only for sockets; use threads if you need to wait on other kinds of file descriptors
> `lockf` | not implemented
> `kill`, `pause` | not implemented (no inter-process signals in Windows)
> `alarm`, `times` | not implemented
> `getitimer`, `setitimer` | not implemented
> `getuid`, `getgid` | always return 1
> `getgid`, `getegid`, `getgroups` | not implemented
> `setuid`, `setgid` | not implemented
> `getpwnam`, `getpwuid` | always raise `Not``_``found`
> `getgrnam`, `getgrgid` | always raise `Not``_``found`
> type `socket``_``domain` | the domain `PF``_``UNIX` is not supported; `PF``_``INET` is fully supported
> `establish``_``server` | not implemented; use threads
> terminal functions (`tc*`) | not implemented