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February 3, 1998

Article

A Thick Ada 95 Binding for
Unix Child Processes and Pipes

by Jim Rogers, Team Ada member
JimMaureenRogers@worldnet.att.net

This article presents a thick Ada 95 binding to the UNIX popen and pclose commands. These commands can be used to create a child process and obtain a pipe for reading or writing to that process.

Three files are provided: the specification and the implementation of the package Ada_Home.Unix.Pipe_Commands, and a test program.

The Unix Pipe Commands

The package Pipe_Commands defines the interface for a thick binding to the UNIX popen and pclose commands.

Popen initiates a child process and opens a pipe to that process. The pipe opened will either be opened with a read or a write mode. When opened for a write mode the utility allows the developer to write to the standard input of the child process. When opened for read mode the utility allows the developer to read the standard output of the child process.

Pclose closes the file stream associated with the pipe created by popen.

The Binding: Interface

The first file, ada_home-unix-pipe_commands.ads, contains the package specification Pipe_Commands. The thick binding offers four subprograms: two functions, execute and read_next, and two procedures, close and write_next.

• Function execute calls the popen command, starting the child process and returning a stream associated with the pipe to the new process.

• Function read_next reads the next line from a stream and returns an unbounded string. Access_Error will be raised if this function is called for a stream opened with write mode. End_Of_File will be raised if the child process has closed its output file. This normally happens when the child process terminates.

• Procedure write_next writes a string as a line of text to the stream. Access_Error is raised if the stream was opened with read mode.

• Procedure close closes an open stream.

The Binding: Implementation

The second file, ada_home-unix-pipe_commands.adb, contains the package body, with the interface for functions "popen" and "pclose" (a thin binding to C functions), which are used in the implementations of the four subprograms declared in the package specification.

Test Program

Finally, the third file, pipe_commands_test.adb, contains an example program of how this binding can be used. The program also demonstrates the use of the exceptions defined in the package specification.

Resources

• Source code:
  ada_home-unix-pipe_commands.ads, package specification
  ada_home-unix-pipe_commands.adb, package body
  ada_home.ads, root library package
  ada_home-unix.ads, parent package
  pipe_commands_test.adb, test program
  or
  pipe_commands.txt (all source code in one file)

  URL http://www.adahome.com/articles/1998-02/code/

  See also:

• Ada Programming FAQ
  URL http://www.adahome.com/FAQ/programming.html#title
• Ada Can Do It! Common false claims about things Ada won't let you do, and how to do them
  URL http://www.adahome.com/articles/1997-06/am_cando.html

Page last modified: 1998-02-04