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The Unix Module

This module contains the interfaces to the most important Unix library functions. Much of this module has been ported to Windows and can be used there. Whenever necessary we will indicate the restrictions in the use of the presented functions. A table resuming the restrictions is given by figure 18.1.

The Unix library belongs to the Objective CAML non-standard libraries which have to be bound by the -custom compiler command (see chapter 7, page ??). Depending on the desired form of the program, one of the following commands is used under Unix to produce bytecode, native code or an interaction loop:
$ ocamlc -custom unix.cma -cclib -lunix
$ ocamlopt unix.cma -cclib -lunix
$ ocamlmktop -custom -o unixtop unix.cma -cclib -lunix 
The purpose of constructing an interaction loop (of which the name will be unixtop) is to support an incremental development style. Each function can be compiled quickly from its type declaration. It is also possible to execute functional tests.

Depending on the version of Unix in use, the system library may not be located at the default place. If necessary the access path of the libraries may be indicated with the option -ccopt (see chapter 7).

Under Windows the commands to compile become:
$ ocamlc -custom unix.cma %CAMLLIB%\libunix.lib wsock32.lib
$ ocamlopt unix.cma %CAMLLIB%\libunix.lib wsock32.lib
$ ocamlmktop -custom -o unixtop.exe unix.cma %CAMLLIB%\libunix.lib wsock32.lib
The name of the obtained interaction loop is unixtop.exe.

Error Handling

Errors produced by system calls throw Unix_error exceptions, which can be handled by the Objective CAML program. Such errors contain three arguments: a value of type Unix.error which can be transformed into a character string by the function error_message, a string containing the name of the function producing the error and optionally, a string containing the argument of the function when the argument is of type string.

It is possible to define a generic calling function with error treatment:

# let wrap_unix funct arg =
try (funct arg) with
Unix.Unix_error (e,fm,argm) ->
Printf.printf "%s %s %s" (Unix.error_message e) fm argm ;;
val wrap_unix : ('a -> unit) -> 'a -> unit = <fun>
The function wrap_unix takes a function and its argument, and applies one to the other. If a Unix error occurs, an explaining message is printed. An equivalent function is defined in the Unix module:

# Unix.handle_unix_error ;;
- : ('a -> 'b) -> 'a -> 'b = <fun>

Portability of System Calls

Figure 18.1 indicates which of the communication and process handling functions presented in this chapter are accessible under Windows. The main shortcoming is the lack of the two functions fork and kill to create new processes and to send signals.

Fonction Unix Windows Comment
fork   use create_process
waitpid only for a given
      number of processes
open_process   use the interpretation of
      /bin/sh commands

Figure 18.1: Portability of the module Unix functions used in this chapter.

Furthermore, the function wait waiting for the end of a child process is not implemented, because fork is not.

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