package monomorphic

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Module type
Class type

System interface.

Every function in this module raises Sys_error with an informative message when the underlying system call signal an error.

val argv : string array

The command line arguments given to the process. The first element is the command name used to invoke the program. The following elements are the command-line arguments given to the program.

val executable_name : string

The name of the file containing the executable currently running. This name may be absolute or relative to the current directory, depending on the platform and whether the program was compiled to bytecode or a native executable.

val file_exists : string -> bool

Test if a file with the given name exists.

val is_directory : string -> bool

Returns true if the given name refers to a directory, false if it refers to another kind of file.

  • raises Sys_error

    if no file exists with the given name.

  • since 3.10.0
val remove : string -> unit

Remove the given file name from the file system.

val rename : string -> string -> unit

Rename a file. rename oldpath newpath renames the file called oldpath, giving it newpath as its new name, moving it between directories if needed. If newpath already exists, its contents will be replaced with those of oldpath. Depending on the operating system, the metadata (permissions, owner, etc) of newpath can either be preserved or be replaced by those of oldpath.

  • since 4.06 concerning the "replace existing file" behavior
val getenv : string -> string

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

val getenv_opt : string -> string option

Return the value associated to a variable in the process environment or None if the variable is unbound.

  • since 4.05
val command : string -> int

Execute the given shell command and return its exit code.

The argument of Sys.command is generally the name of a command followed by zero, one or several arguments, separated by whitespace. The given argument is interpreted by a shell: either the Windows shell cmd.exe for the Win32 ports of OCaml, or the POSIX shell sh for other ports. It can contain shell builtin commands such as echo, and also special characters such as file redirections > and <, which will be honored by the shell.

Conversely, whitespace or special shell characters occurring in command names or in their arguments must be quoted or escaped so that the shell does not interpret them. The quoting rules vary between the POSIX shell and the Windows shell. The Filename.quote_command performs the appropriate quoting given a command name, a list of arguments, and optional file redirections.

val time : unit -> float

Return the processor time, in seconds, used by the program since the beginning of execution.

val chdir : string -> unit

Change the current working directory of the process.

val mkdir : string -> int -> unit

Create a directory with the given permissions.

  • since 4.12.0
val rmdir : string -> unit

Remove an empty directory.

  • since 4.12.0
val getcwd : unit -> string

Return the current working directory of the process.

val readdir : string -> string array

Return the names of all files present in the given directory. Names denoting the current directory and the parent directory ("." and ".." in Unix) are not returned. Each string in the result is a file name rather than a complete path. There is no guarantee that the name strings in the resulting array will appear in any specific order; they are not, in particular, guaranteed to appear in alphabetical order.

val interactive : bool ref

This reference is initially set to false in standalone programs and to true if the code is being executed under the interactive toplevel system ocaml.

val os_type : string

Operating system currently executing the OCaml program. One of

  • "Unix" (for all Unix versions, including Linux and Mac OS X),
  • "Win32" (for MS-Windows, OCaml compiled with MSVC++ or Mingw),
  • "Cygwin" (for MS-Windows, OCaml compiled with Cygwin).
type backend_type =
  1. | Native
  2. | Bytecode
  3. | Other of string

Currently, the official distribution only supports Native and Bytecode, but it can be other backends with alternative compilers, for example, javascript.

  • since 4.04.0
val backend_type : backend_type

Backend type currently executing the OCaml program.

  • since 4.04.0
val unix : bool

True if Sys.os_type = "Unix".

  • since 4.01.0
val win32 : bool

True if Sys.os_type = "Win32".

  • since 4.01.0
val cygwin : bool

True if Sys.os_type = "Cygwin".

  • since 4.01.0
val word_size : int

Size of one word on the machine currently executing the OCaml program, in bits: 32 or 64.

val int_size : int

Size of int, in bits. It is 31 (resp. 63) when using OCaml on a 32-bit (resp. 64-bit) platform. It may differ for other implementations, e.g. it can be 32 bits when compiling to JavaScript.

  • since 4.03.0
val big_endian : bool

Whether the machine currently executing the Caml program is big-endian.

  • since 4.00.0
val max_string_length : int

Maximum length of strings and byte sequences.

val max_array_length : int

Maximum length of a normal array (i.e. any array whose elements are not of type float). The maximum length of a float array is max_floatarray_length if OCaml was configured with --enable-flat-float-array and max_array_length if configured with --disable-flat-float-array.

val max_floatarray_length : int

Maximum length of a floatarray. This is also the maximum length of a float array when OCaml is configured with --enable-flat-float-array.

val runtime_variant : unit -> string

Return the name of the runtime variant the program is running on. This is normally the argument given to -runtime-variant at compile time, but for byte-code it can be changed after compilation.

  • since 4.03.0
val runtime_parameters : unit -> string

Return the value of the runtime parameters, in the same format as the contents of the OCAMLRUNPARAM environment variable.

  • since 4.03.0

Signal handling

type signal_behavior =
  1. | Signal_default
  2. | Signal_ignore
  3. | Signal_handle of int -> unit

What to do when receiving a signal:

  • Signal_default: take the default behavior (usually: abort the program)
  • Signal_ignore: ignore the signal
  • Signal_handle f: call function f, giving it the signal number as argument.
val signal : int -> signal_behavior -> signal_behavior

Set the behavior of the system on receipt of a given signal. The first argument is the signal number. Return the behavior previously associated with the signal. If the signal number is invalid (or not available on your system), an Invalid_argument exception is raised.

val set_signal : int -> signal_behavior -> unit

Same as Sys.signal but return value is ignored.

Signal numbers for the standard POSIX signals.

val sigabrt : int

Abnormal termination

val sigalrm : int


val sigfpe : int

Arithmetic exception

val sighup : int

Hangup on controlling terminal

val sigill : int

Invalid hardware instruction

val sigint : int

Interactive interrupt (ctrl-C)

val sigkill : int

Termination (cannot be ignored)

val sigpipe : int

Broken pipe

val sigquit : int

Interactive termination

val sigsegv : int

Invalid memory reference

val sigterm : int


val sigusr1 : int

Application-defined signal 1

val sigusr2 : int

Application-defined signal 2

val sigchld : int

Child process terminated

val sigcont : int


val sigstop : int


val sigtstp : int

Interactive stop

val sigttin : int

Terminal read from background process

val sigttou : int

Terminal write from background process

val sigvtalrm : int

Timeout in virtual time

val sigprof : int

Profiling interrupt

val sigbus : int

Bus error

  • since 4.03
val sigpoll : int

Pollable event

  • since 4.03
val sigsys : int

Bad argument to routine

  • since 4.03
val sigtrap : int

Trace/breakpoint trap

  • since 4.03
val sigurg : int

Urgent condition on socket

  • since 4.03
val sigxcpu : int

Timeout in cpu time

  • since 4.03
val sigxfsz : int

File size limit exceeded

  • since 4.03
exception Break

Exception raised on interactive interrupt if Sys.catch_break is on.

val catch_break : bool -> unit

catch_break governs whether interactive interrupt (ctrl-C) terminates the program or raises the Break exception. Call catch_break true to enable raising Break, and catch_break false to let the system terminate the program on user interrupt.

val ocaml_version : string

ocaml_version is the version of OCaml. It is a string of the form "major.minor[.patchlevel][(+|~)additional-info]", where major, minor, and patchlevel are integers, and additional-info is an arbitrary string. The [.patchlevel] part was absent before version 3.08.0 and became mandatory from 3.08.0 onwards. The [(+|~)additional-info] part may be absent.

val development_version : bool

true if this is a development version, false otherwise.

  • since 4.14.0
type extra_prefix =
  1. | Plus
  2. | Tilde
type extra_info = extra_prefix * string
type ocaml_release_info = {
  1. major : int;
  2. minor : int;
  3. patchlevel : int;
  4. extra : extra_info option;
val ocaml_release : ocaml_release_info
val enable_runtime_warnings : bool -> unit

Control whether the OCaml runtime system can emit warnings on stderr. Currently, the only supported warning is triggered when a channel created by open_* functions is finalized without being closed. Runtime warnings are disabled by default.

  • since 4.03.0
val runtime_warnings_enabled : unit -> bool

Return whether runtime warnings are currently enabled.

  • since 4.03.0


val opaque_identity : 'a -> 'a

For the purposes of optimization, opaque_identity behaves like an unknown (and thus possibly side-effecting) function.

At runtime, opaque_identity disappears altogether.

A typical use of this function is to prevent pure computations from being optimized away in benchmarking loops. For example:

for _round = 1 to 100_000 do
  ignore (Sys.opaque_identity (my_pure_computation ()))
  • since 4.03.0
module Immediate64 : sig ... end

This module allows to define a type t with the immediate64 attribute. This attribute means that the type is immediate on 64 bit architectures. On other architectures, it might or might not be immediate.


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