motor poll example from @rhempel

sysfs_notify_monitor.c

/* ------------------------------------------------------------------------------
 * This sample is based on code from:
 *
 * http://stackoverflow.com/questions/16367623/using-the-linux-sysfs-notify-call
 */

#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h> 
#include <sys/stat.h> 
#include <poll.h>

#include <sysexits.h>
#include <argp.h>

/* ------------------------------------------------------------------------------
 */

    const char *argp_program_version = "1.0";

     const char *argp_program_bug_address = "<[email protected]>";
     
     /* Program documentation. */
     static char doc[] = "sysfs_notify_monitor -- monitor sysfs attribute files for sysfs_notify calls";
     
     /* A description of the arguments we accept. */
     static char args_doc[] = "filename";
     
     /* The options we understand. */
     static struct argp_option options[] = {
       {"timeout",  't', "msec", 0, "Wait at most n milliseconds\n"
				    "Negative n means no timeout\n"
				    "If not specified, default timeout is 10 seconds" },
       { 0 }
     };
     
     /* Used by main to communicate with parse_opt. */
     struct arguments
     {
       int timeout;
       char *sysfs_file;
     };
     
     /* Parse a single option. */
     static error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       /* Get the input argument from argp_parse, which we
          know is a pointer to our arguments structure. */
       struct arguments *arguments = state->input;
     
       switch (key)
         {
         case 't':
           arguments->timeout = arg ? atoi (arg) : 10000;
           break;
     
         case ARGP_KEY_ARG:
           if (state->arg_num >= 1)
		/* Too many arguments. */
		argp_usage (state);
           else if (state->arg_num == 0)
           	arguments->sysfs_file = arg;

           state->next = state->argc;
     
           break;
     
         case ARGP_KEY_END:
           if (state->arg_num < 1)
             /* Not enough arguments. */
             argp_usage (state);
           break;
     
         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }
     
     /* Our argp parser. */
     static struct argp argp = { options, parse_opt, args_doc, doc };


int main(int argc, char **argv)
{
    int cnt, notifyFd, rv;
    char attrData[32];
    struct pollfd ufds[1];

       struct arguments arguments;
     
       /* Default values. */
       arguments.timeout = 10000;
       arguments.sysfs_file = "";

       /* Parse our arguments; every option seen by parse_opt will
          be reflected in arguments. */
       argp_parse (&argp, argc, argv, 0, 0, &arguments);

    
    // Open a connection to the attribute file.
    if ((notifyFd = open(arguments.sysfs_file, O_RDONLY)) < 0)
    {
        perror("Unable to open");
        exit(EX_OSFILE);
    }

    ufds[0].fd = notifyFd;
    ufds[0].events = POLLPRI|POLLERR;

    // Someone suggested dummy reads before the poll() call
    cnt = read( notifyFd, attrData, 32 );
    ufds[0].revents = 0;
    if (( rv = poll( ufds, 1, arguments.timeout)) < 0 )
    {
        perror("Poll error");
        exit(EX_OSERR);
    }
    else if (rv == 0)
    {
        printf("Poll timeout on %s\n", arguments.sysfs_file);
    }
    else if (ufds[0].revents & POLLPRI|POLLERR)
    {
        cnt = read( notifyFd, attrData, 32 );
        printf("sysfs_notify on %s\n", arguments.sysfs_file);
        printf( "%s\n", attrData );
    }
    printf( "revents[0]: %08X\n", ufds[0].revents );

    close( notifyFd );
     
    exit (0);
}