Raspberry Pi web server using PiFace, getting "Bad file descriptor"

Question

Hi

Using a Raspberry Pi Model B with PiFace. I have downloaded, built and installed the PiFace libraries and enabled the spi devices. I wrote a C program and can turn the relays on and off. That program also sends HTML to the stdout. Indeed when executed I see the HTML on stdout.

I have downloaded and installed Apache2. A simple 'Hello World' app runs when I hit the cgi-bin directory with a web-browser. When, I try to hit my c-program using the cgi-bin in my web browser, I get the HTML presented to my browser. I print the time to the webpage so that I know it is working. However, the PiFace IO does not work. If I just run the c-program from the CLI in the cgi-bin directory, it generates the HTML with the time-stamp and actually executes the IO operations.

I browse to the Apache2 error log and see this error message "ERROR: Can not send SPI messageBad file descriptor".

My c-program has execute permissions.

The spi_bcm2708 module is loaded as confirmed by lsmod.

I get crw-rw---T on both /dev/spidev0.0 and /dev/spidev0.1 when using ls -l /dev/spidev*

Any ideas for what else I need to do to make the IO operate under CGI?

Thanks

mconners · Accepted Answer

Also, the httpd.conf should give you the username that apache it running as, or you can do

ps auxww | grep apache

to see who the owner is, in /etc/apache2/envvars, I have

export APACHE_RUN_USER=www-data

export APACHE_RUN_GROUP=www-data

defined on one of my machines, so you may want to edit that file.

Mike

mconners · Answer

It may be that the web server doesn't have permission to open the SPI device. Apache servers are often setup to run as "nobody". Are you running as root when you run the program from the command line? You may have to perform some permission gymnastics to make it accessible from a cgi program.

Mike