reading a port (GPIO) value in C (beagleboard-xm)

Question

I have an embedded board (beagleboard-xm) that runs ubuntu 12.04. I need to read a GPIO continuously to see if the value of the port changes. My code is herebelow:

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

FILE *fp;

int main(void)
{
    //linux equivalent code "echo 139 > export" to export the port
    if ((fp = fopen("/sys/class/gpio/export", "w")) == NULL){
            printf("Cannot open export file.\n");
            exit(1);
        }
        fprintf( fp, "%d", 139 );
        fclose(fp);
      //  linux equivalent code "echo low > direction" to set the port as an input 
        if ((fp = fopen("/sys/class/gpio/gpio139/direction", "rb+")) == NULL){
            printf("Cannot open direction file.\n");
            exit(1);
        }
        fprintf(fp, "low");
        fclose(fp);

       // **here comes where I have the problem, reading the value**
        int value2;
        while(1){
        value2= system("cat /sys/class/gpio/gpio139/value");
        printf("value is: %d\n", value2);
        }
    return 0;
}

the code above reads the port continuously (0 by default), however, when I change the port as 1, system call output the correct value, however printf still prints 0 as an output. What is the problem with value2 that does not store the value that system() outputs.

If I use the code below instead of while loop above, I get an error about opening the value file (Cannot open value file.), if I put the fopen line outside of while loop, it does not show the changes in the value file.

char buffer[10];
while(1){
    if ((fp = fopen("/sys/class/gpio/gpio139/value", "rb")) == NULL){
            printf("Cannot open value file.\n");
            exit(1);
        }
    fread(buffer, sizeof(char), sizeof(buffer)-1, fp);
    int value = atoi(buffer);
    printf("value: %d\n", value);

    }

My question: how do I need to fix the code? or how should I read the value file? As an additional info that I wonder: what is the difference to e.g. export the port by system("echo 139 > /sys/class/gpio/export") and fp = fopen("/sys/class/gpio/export","w"); fprintf(fp,"%d",139); which method do you suggest me to use? Why?

Thank you in advance.

Answer 1

The system() function returns the return value of cat, which is 0. It does not return the standard output from cat, which is what you were expecting.

I think the problem with your second bit of code is that you're not calling fclose(). Since you're running in a tight loop, you almost immediately exceed the number of open files allowed.

So, call fclose(), and think about putting a sleep() in there too.