Pipes

Process creation and pipes

Introduction

An external process can be launched and controlled by the Process type. It allows to read the process standard output as well as write on the process standard input.

Launching a process

A process can be launched and controlled by the limited type Process. First, let's declare the process instance:

with Util.Processes;
...
   Proc : Util.Processes.Process;

The process is launched by the Spawn procedure. The command and its arguments to execute is passed as a string to the procedure.

   Proc.Spawn ("gnatmake --version");

Once the process is spawned, the procedure returns: it does not wait for the process to terminate. We have to use the Wait procedure:

   Proc.Wait;

Then, the process exit status is obtained by calling Get_Exit_Status.

Pipe

If we want to read the process output, the Pipe_Stream simplifies this task. It is very close to the traditional popen operation of the C stdio library.

First, let's define the pipe stream as well as a buffer:

with Util.Streams.Pipes;
...
   Pipe    : aliased Util.Streams.Pipes.Pipe_Stream;
   Buffer  : Util.Streams.Buffered.Buffered_Stream;
   Content : Ada.Strings.Unbounded.Unbounded_String;

The process is launched easily by using the Open procedure. By default, the pipe will be configured to read the process output. It is possible to also redirect the process input.

   Pipe.Open ("gnatmake --version");

The buffer stream object helps in reading the process output. It is initialized as follows:

   Buffer.Initialize (null, Pipe'Unchecked_Access, 1024);

The process output can be read easily as follows:

   Buffer.Read (Content);

The Read procedure will read the complete output stream so it may contain multiple lines.

By closing the pipe, we wait for the process to terminate.

   Pipe.Close;

References

util-processes.ads

popen.adb

Process creation in Java and Ada