Communication between Erlang and Unix proceses

[mwilligs@REDACTED]

Hi there,

              I have been trying the communication between Unix proceses
and Erlang processes through the usage of the open_port
function. But, I have been having some problems to receive
the messages from the Unix process.
I can’t discriminate when I received a message and when no message is
received. Please, take a look at the source files: con2.c and con1.erl
The erlang process receives 11 messages “Hello” through the port, and then
I tried to close the port, but it did not work. How should I do that?

And, if you take a look at the programs con3.c and con2.erl, you will see
that I tried to receive messages that were suppose to be written from the
Unix Console through fgets(buf, 1000, stderr) call. But, instead of that I
am not being able to write the message and the Erlang process reads
garbage from the port.
How can I discriminate when a real message is received?

Thanks again,
                      Miguel Willigs


Files attached:
===============


-module(con1).
-export([read/0]).


%write (Path, Data) ->
%Port = open_port({spawn, Path}, [{packet, 2}]),
%Port ! {self(), {command, Data}}.

read () ->
Port = open_port({spawn, "/home/ljara/erlang/con2"}, [{packet, 2}]),
recibir().

recibir() ->

receive
{Port, Data} ->
io:format('erlang received from port:~p~n', [Data]),
recibir(),

receive
{Port, "close"} ->
Port ! {self(), close}

end
end.


*****************************************************************************************************************************************************


/*********************************************************************/
/*  con2.c - Programa que recibe mensajes desde un programa Erlang   */
/*  Fecha: 24/10/23                                                  */
/*  Autor: IYDDEPTO                                                  */
/*********************************************************************/

#include <stdio.h>
#include <string.h>


/****************************************/

typedef unsigned char byte;

//Funciones
void put_int16(int i, byte *s);


/****************************************/

int main()
{
int len, flag=0, i=0;

byte buf[1000];

char ini_message[50];

strcpy(ini_message, "com2 in C Starting \n");

strcpy(buf, "Hello");

fprintf(stderr, ini_message);

	while (flag!=1 && i<10)
	{
	//gets(buf);
//	fgets(buf, 1000, stderr);

		//Si ya no se quiere enviar mensajes
		if(strcmp(buf, "fin")==0)
		{
		flag=1;
		//Se cierra el puerto
		fprintf(stderr, "I have touched the flag\n");
		}

		if((write_cmd(buf, strlen(buf)))<0)
		{
		perror("Message Sending Failed");
		exit(1);
		}
	i++;
	fprintf(stderr, "Valor de i:%d\n", i);
	}

	if((write_cmd("close", strlen("close")))<0)
	{
	perror("Message Sending Failed");
	exit(1);
	}

fprintf(stderr, "C Program finished\n");
return(0);
}

/***************************************************/

/* Pack the 2 bytes length (MSB first) and send it */
int write_cmd(byte *buf, int len)
{
byte str[2];

put_int16(len, str);

	if (write_exact(str, 2) != 2)
	return(-1);

return write_exact(buf, len);

}

/***************************************************/

int write_exact(byte *buf, int len)
{
int i, wrote = 0;

i = write(1, buf+wrote, len-wrote);
return (i);


}

/***************************************************/

void put_int16(int i, byte *s)
{
*s = (i >> 8) & 0xff;
s[1] = i & 0xff;
}


******************************************************************************************************************************************************

-module(con2).
-export([read/0]).


%write (Path, Data) ->
%Port = open_port({spawn, Path}, [{packet, 2}]),
%Port ! {self(), {command, Data}}.

read () ->
Port = open_port({spawn, "/home/ljara/erlang/con3"}, [{packet, 2}]),
recibir().

recibir() ->

receive
{Port, Data} ->
io:format('erlang received from port:~p~n', [Data]),
recibir(),

receive
{Port, "close"} ->
Port ! {self(), close}

end
end.


******************************************************************************************************************************************************

/*********************************************************************/
/*  con3.c - Programa que recibe mensajes desde un programa Erlang   */
/*  Fecha: 24/10/23                                                  */
/*  Autor: IYDDEPTO                                                  */
/*********************************************************************/

#include <stdio.h>
#include <string.h>


/****************************************/

typedef unsigned char byte;

//Funciones
void put_int16(int i, byte *s);


/****************************************/

int main()
{
int len, flag=0, i=0;

byte buf[1000];

char ini_message[50];

strcpy(ini_message, "com2 in C Starting \n");

//strcpy(buf, "Hello");

fprintf(stderr, ini_message);

	while (flag!=1 && i<10)
	{
	//gets(buf);
	fgets(buf, 1000, stderr);

		//Si ya no se quiere enviar mensajes
		if(strcmp(buf, "fin")==0)
		{
		flag=1;
		//Se cierra el puerto
		fprintf(stderr, "I have touched the flag\n");
		}

		if((write_cmd(buf, strlen(buf)))<0)
		{
		perror("Message Sending Failed");
		exit(1);
		}
	i++;
	fprintf(stderr, "Valor de i:%d\n", i);
	}

	if((write_cmd("close", strlen("close")))<0)
	{
	perror("Message Sending Failed");
	exit(1);
	}

fprintf(stderr, "C Program finished\n");
return(0);
}

/***************************************************/

/* Pack the 2 bytes length (MSB first) and send it */
int write_cmd(byte *buf, int len)
{
byte str[2];

put_int16(len, str);

	if (write_exact(str, 2) != 2)
	return(-1);

return write_exact(buf, len);

}

/***************************************************/

int write_exact(byte *buf, int len)
{
int i, wrote = 0;

i = write(1, buf+wrote, len-wrote);
return (i);


}

/***************************************************/

void put_int16(int i, byte *s)
{
*s = (i >> 8) & 0xff;
s[1] = i & 0xff;
}