[ros-users] ROS nodes & SIGINT handler

[Gautier Dumonteil]

  I saw that rosbag is multithreaded but I was not too aware that fork() 
without exec() is dangerous in that case.
Anyway, I try fork()/exec() combination and it works fine.

Thank you guys for your help!

Gautier

Le 31/08/2010 18:10, Josh Faust a écrit :
> 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 at gmail.com <mailto:gautier.dumonteil at 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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