beaglebone black gpio select is not working

Question

I'm trying to detect when a gpio pin goes from low to high and am having trouble. From what I've read I should be able to configure the pin as input this way:

# echo in > /sys/class/gpio/gpio51/direction
# echo rising > /sys/class/gpio/gpio51/edge

Next I try running a c program that waits for the rising edge using select. The code looks like this (notice I commented out an attempt to just read the file, since reading is supposed to block if you don't set O_NONBLOCK):

#include<stdio.h>                                                                                                                                                                
#include<fcntl.h>                                                                                                                                                                
#include <sys/select.h>                                                                                                                                                          


int main(void) {                                                                                                                                                                 
  int fd = open("/sys/class/gpio/gpio51/value", O_RDONLY & ~O_NONBLOCK);                                                                                                         
  //int fd = open("/sys/class/gpio/gpio51/value", O_RDONLY | O_NONBLOCK);                                                                                                        
  //unsigned char buf[2];                                                                                                                                                        
  //int x = read(fd, &buf, 2);                                                                                                                                                   
  //printf("%d %d: %s\n", fd, x, buf);                                                                                                                                           
  fd_set exceptfds;                                                                                                                                                              
  int    res;                                                                                                                                                                    

  FD_ZERO(&exceptfds);                                                                                                                                                           
  FD_SET(fd, &exceptfds);                                                                                                                                                        
  //printf("waiting for %d: %s\n", exceptfds);                                                                                                                                   
  res = select(fd+1,                                                                                                                                                             
               NULL,               // readfds - not needed                                                                                                                       
               NULL,               // writefds - not needed                                                                                                                      
               &exceptfds,                                                                                                                                                       
               NULL);              // timeout (never)                                                                                                                            

  if (res > 0 && FD_ISSET(fd, &exceptfds)) {                                                                                                                                     
    printf("finished\n");                                                                                                                                                        
  }                                                                                                                                                                              
  return 0;                                                                                                                                                                      
}

The program exits immediately no matter what the state of the pin (high or low). Can anyone see something wrong with the way I'm doing this?

PS. I have a python library that uses poll() to do just this, and the python works as expected. I pull the pin low, call the python, it blocks, pull the pin high and the code continues. So I don't think it is a problem with the linux gpio driver.

https://bitbucket.org/cswank/gadgets/src/590504d4a30b8a83143e06c44b1c32207339c097/gadgets/io/poller.py?at=master

Craig Swank Craig Swank · Accepted Answer · 2013-12-18T14:14:46

I figured it out. You must read from the file descriptor before the select call returns. Here is an example that works:

#include<stdio.h>                                                                                                                                                                                                                  
#include<fcntl.h>                                                                                                                                                                                                                  
#include <sys/select.h>                                                                                                                                                                                                            

#define MAX_BUF 64                                                                                                                                                                                                                 

int main(void) {                                                                                                                                                                                                                   
  int len;                                                                                                                                                                                                                         
  char *buf[MAX_BUF];                                                                                                                                                                                                              
  int fd = open("/sys/class/gpio/gpio51/value", O_RDONLY);                                                                                                                                                                         
  fd_set exceptfds;                                                                                                                                                                                                                
  int    res;                                                                                                                                                                                                                      

  FD_ZERO(&exceptfds);                                                                                                                                                                                                             
  FD_SET(fd, &exceptfds);                                                                                                                                                                                                          
  len = read(fd, buf, MAX_BUF);  //won't work without this read.                                                                                                                                                                                                  
  res = select(fd+1,                                                                                                                                                                                                               
               NULL,               // readfds - not needed                                                                                                                                                                         
               NULL,               // writefds - not needed                                                                                                                                                                        
               &exceptfds,                                                                                                                                                                                                         
               NULL);              // timeout (never)                                                                                                                                                                              

  if (res > 0 && FD_ISSET(fd, &exceptfds)) {                                                                                                                                                                                       
    printf("finished\n");                                                                                                                                                                                                          
  }                                                                                                                                                                                                                                
  return 0;                                                                                                                                                                                                                        
}

beaglebone black gpio select is not working

1 Answers