SCPI on a Linux Board - Letter of Intent

Some more progress: first contact from LabVIEW

I don't have the reply working in the LabVIEW flow yet - it works different than serial/usb. But things look good.

Yay!

It works. Above is the capture of a LabVIEW back and forth communication - with the Raspberry Pi as a VISA TCP/IP device.

*IDN? command sent, and the reply from the Pi arrived at LabVIEW.

Hi Jan!

I can't help on the LabView portion, but if you've got the Pi portion of the code somewhere, I'd be happy to examine it in case something sticks out. In terms of general ideas for debugging, you're already using gdb so that's great, it might be worth compiling on the Pi directly once, just to 100% confirm you're linking to the same versions of libraries. Also, 'strace' is sometimes useful too, although it chucks out a lot of debug.

The code as it stands now. It's a server side socket program that listens and serves info to a tcp/ip client knocking on its port:

/*
 ============================================================================
 Name        : main.c
 Author      : Jan Cumps
 Version     :
 Copyright   : free for all
 Description :
 ============================================================================
 */


/* A simple server in the internet domain using TCP
   The port number is passed as an argument */
#include 
#include 
#include 
#include 


#include "scpi/scpi.h"
#include "scpi-def.h"


int newsockfd;


/** SCPI utilities
 *
 */


size_t SCPI_Write(scpi_t * context, const char * data, size_t len) {
    (void) context;
    int n = write(newsockfd, data, len);
    if (n < 0) error("ERROR writing to socket");
    return n;
}


scpi_result_t SCPI_Flush(scpi_t * context) {
    (void) context;


    return SCPI_RES_OK;
}


int SCPI_Error(scpi_t * context, int_fast16_t err) {
    (void) context;


    fprintf(stderr, "**ERROR: %d, \"%s\"\r\n", (int16_t) err, SCPI_ErrorTranslate(err));
    return 0;
}


scpi_result_t SCPI_Control(scpi_t * context, scpi_ctrl_name_t ctrl, scpi_reg_val_t val) {
    (void) context;


    if (SCPI_CTRL_SRQ == ctrl) {
        fprintf(stderr, "**SRQ: 0x%X (%d)\r\n", val, val);
    } else {
        fprintf(stderr, "**CTRL %02x: 0x%X (%d)\r\n", ctrl, val, val);
    }
    return SCPI_RES_OK;
}


scpi_result_t SCPI_Reset(scpi_t * context) {
    (void) context;


    fprintf(stderr, "**Reset\r\n");
    return SCPI_RES_OK;
}


scpi_result_t SCPI_SystemCommTcpipControlQ(scpi_t * context) {
    (void) context;


    return SCPI_RES_ERR;
}






void error(char *msg)
{
    perror(msg);
    exit(1);
}


int main(int argc, char *argv[])
{
     int sockfd, portno, clilen;
     char buffer[256];
     struct sockaddr_in serv_addr, cli_addr;
     int n;


     SCPI_Init(&scpi_context,
             scpi_commands,
             &scpi_interface,
             scpi_units_def,
             SCPI_IDN1, SCPI_IDN2, SCPI_IDN3, SCPI_IDN4,
             scpi_input_buffer, SCPI_INPUT_BUFFER_LENGTH,
             scpi_error_queue_data, SCPI_ERROR_QUEUE_SIZE);
     if (argc < 2) {
         fprintf(stderr,"ERROR, no port provided\n");
         exit(1);
     }
     sockfd = socket(AF_INET, SOCK_STREAM, 0);
     if (sockfd < 0)
        error("ERROR opening socket");
     bzero((char *) &serv_addr, sizeof(serv_addr));
     portno = atoi(argv[1]);
     serv_addr.sin_family = AF_INET;
     serv_addr.sin_addr.s_addr = INADDR_ANY;
     serv_addr.sin_port = htons(portno);
     if (bind(sockfd, (struct sockaddr *) &serv_addr,
              sizeof(serv_addr)) < 0)
              error("ERROR on binding");
     listen(sockfd,5);
     clilen = sizeof(cli_addr);
     newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
     if (newsockfd < 0)
          error("ERROR on accept");
     while (1) {
         bzero(buffer,256);
         n = read(newsockfd,buffer,255);
         if (n < 0) {
        error("ERROR reading from socket");
         } else if (n > 0) {
             printf("Here is the message: %s\n",buffer);
             scpi_instrument_input(buffer, n);


         }


     }


     return 0;
}

My issue is in this small part:

     while (1) {
         bzero(buffer,256);
         n = read(newsockfd,buffer,255);
         if (n < 0) {
        error("ERROR reading from socket");
         } else if (n > 0) {
             printf("Here is the message: %s\n",buffer);
             scpi_instrument_input(buffer, n);
         }
     }

As long as the client is connected, n = 0 when no data received from the client. All good

When the client (example unix telnet session) closes, the read() function always returns the value 32.

Before I define a fix for that, I want to try and understand why that is.

Before anyone thinks that I'm a server side network socket expert: I borrowed this from http://www.cs.rpi.edu/~moorthy/Courses/os98/Pgms/server.c

Because that program doesn't loop (it waits for traffic, notifies and dies), it doesn't have the issue.

I need to do some more studying ...

Hi Jan,

It's a while since I looked at the POSIX socket API, so I'm not sure either, but it might be worth also testing that newsockfd does not equal 0 before trying the read operation, e.g.

if (newsockfd) {
  n=read(newsockfd, buffer, 255);
}

From memory I think using 'select' API is a good method to handle acting as a TCP server, but I need to read up on it. I'll try to dig up some useful links. There are some good patterns to handling socket reads, I just need to find them.

You may be on to something. I only call newsockfd() once at the start and assign it to a global variable (blush). So it will never be 0, but it may very well be invalid.

I'll check if I have to reallocate each time, or if I have to check that handle's validity ...

Hi Jan,

This is using select, so not the same, but nevertheless they are handling the read slightly differently.

The code tests for zero, and if it is zero, then the socket needs to be closed.

Unfortunately that means you mean need a loop to open a new socket after that, i.e. to re-execute from your line 117 in your code I think.

The photo is from 'Unix Network Programming' and it is old, so some things could have changed.

Hi Jan,

This is using select, so not the same, but nevertheless they are handling the read slightly differently.

The code tests for zero, and if it is zero, then the socket needs to be closed.

Unfortunately that means you mean need a loop to open a new socket after that, i.e. to re-execute from your line 117 in your code I think.

The photo is from 'Unix Network Programming' and it is old, so some things could have changed.

I'm slowly getting there.

This example keeps alive even if I close on the LabVIEW site. That's a step forward.

#include<stdio.h>
#include<string.h>    //strlen
#include<sys/socket.h>
#include<arpa/inet.h> //inet_addr
#include<unistd.h>    //write


int main(int argc , char *argv[])
{
int socket_desc , client_sock , c , read_size;
struct sockaddr_in server , client;
char client_message[2000];


//Create socket
socket_desc = socket(AF_INET , SOCK_STREAM , 0);
if (socket_desc == -1)
{
printf("Could not create socket");
}
puts("Socket created");


//Prepare the sockaddr_in structure
server.sin_family = AF_INET;
server.sin_addr.s_addr = INADDR_ANY;
server.sin_port = htons( 2222 );


//Bind
if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
{
//print the error message
perror("bind failed. Error");
return 1;
}
puts("bind done");
while (1) {


//Listen
listen(socket_desc , 3);


//Accept and incoming connection
puts("Waiting for incoming connections...");
c = sizeof(struct sockaddr_in);


//accept connection from an incoming client
client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c);
if (client_sock < 0)
{
perror("accept failed");
return 1;
}
puts("Connection accepted");


//Receive a message from client
while( (read_size = recv(client_sock , client_message , 2000 , 0)) > 0 )
{
//Send the message back to client
write(client_sock , client_message , strlen(client_message));
}


if(read_size == 0)
{
puts("Client disconnected");
fflush(stdout);
}
else if(read_size == -1)
{
perror("recv failed");
}
}


return 0;
}

I had a read on the timeout. It happens when the expected numbers of characters haven't arrived after the given timeout.

Because TCP/IP in VISA doesn't support querying the number of characters waiting to be read (I've tried and I get Not Supported by this instrument), I have made this decision:

After reading from the TCP/IP pipe, throw away that particular timeout error, because it is expected. Comment f you would do this differently.