fork() without exec() is very, very dangerous, especially in multithreaded programs (roscpp spins up threads internally).  The signal handling is likely to be the least of your worries -- all the file handles, sockets, etc. will be shared between the processes.

You probably want to either:

 * Spin up a thread for each recording instance

 * Have another executable which does the recording (can probably just use rosbag), and fork()/exec() it.

Josh

On Tue, Aug 31, 2010 at 12:06 PM, Gautier Dumonteil <gautier.dumonteil@gmail.com> wrote:

Hi users,

In our reseach project, we would like to be able to record bags "automatically" (e.g recording when our UAV is flying).
So, I'm trying to write a simple node (recorder_interface) with two services "start_record" and "stop_record", where "start_record" callback launch the rosbag::recorder::run() in a child process using fork(). The "stop_record" callback just sends the SIGINT signal to recorder PID to stop recording ... but nothing happen. It seems that the ROS node forked doesn't handle SIGINT signals.

Also, when checking available nodes with "rosnode list", I've only the recorder_interface node. I should have also something like "record_1283279499007996727", which is the rosbag record node.

I'm wondering if a node can launch others nodes??

Here, the console output and code:

[ INFO] [1283279481.648585599]: Recorder_interface spinning: ready to record.
[ INFO] [1283279499.006083281]: Request to record topics: test
[ INFO] [1283279499.007996722]: Child: pid = 10497

[ INFO] [1283279499.008979878]: Subscribing to /fake_alti
[ INFO] [1283279499.009749320]: Recording to test_2010-08-31-14-31-39.bag.

#include <signal.h>
#include "ros/ros.h"
#include "recorder_interface/rec_interface.h"
#include "rosbag/recorder.h"
#include "rosbag/exceptions.h"

bool  recording;
int   recorder_exit_code;
pid_t rosbag_pid;

void rosbagRecord (rosbag::RecorderOptions opts)
{
    char ** argv = NULL;
    int argc=0;
    ros::init(argc, argv, "record", ros::init_options::AnonymousName);

    // **** Run the recorder
    rosbag::Recorder recorder(opts);
    recorder_exit_code = recorder.run();
    ROS_INFO("recorder_exit_code: %d\n",recorder_exit_code);
}

bool startRec(recorder_interface::rec_interface::Request &req,
                  recorder_interface::rec_interface::Response &res)
{
    if(!recording)
    {
        recording=true;
        ROS_INFO("Request to record topics: %s", req.topics.c_str ());
  
        // **** Fill rosbag options
        rosbag::RecorderOptions opts;
        opts.record_all = false;
        opts.quiet = true;
        opts.prefix = req.prefix;
        opts.append_date = true;
        opts.topics.push_back("/fake_alti");

        rosbag_pid = fork();
        if(rosbag_pid == -1)
        {
            // **** ouch, fork() failed
            perror("fork");
            exit(-1);
        }
        else if(rosbag_pid == 0)
        {
            // **** child
            ROS_INFO("Child: pid = %d\n", (int)getpid());
            rosbagRecord(opts);
            exit(-1);
        }
    }
    else
    {
        ROS_WARN("Already recording!");
    }
   return true;
}


bool stopRec(recorder_interface::rec_interface::Request &req,
                  recorder_interface::rec_interface::Response &res)
{ 
   if(recording)
    {
        int err = kill(rosbag_pid, SIGINT);
        ROS_INFO("err = %d\n",err);
        if(err==0)
        {
            res.ans=recorder_exit_code;
            recording=false;
        }
    }
    else
    {
        ROS_WARN("Not recording!");
    }
   return true;
}
 
int main(int argc, char **argv)
{
  recording=false;
 
  ros::init(argc, argv, "recorder_interface");
  ros::NodeHandle n;

  ros::ServiceServer serviceStart,serviceStop;
  serviceStart = n.advertiseService("start_record", startRec);
  serviceStop = n.advertiseService("stop_record", stopRec);
  ROS_INFO("Recorder_interface spinning: ready to record.");
  ros::spin();
  return 0;
}

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