beaglebone black gpio select is not working - linux

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

Answer 1

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;
}