gnuradio source only outputting zeros - c++

Question

I made a custom source block that is reading switch values on a zedboard. It is accessing them via a proc driver that I wrote. The /var/log/kern.log is reporting proper output. The debug printf in the source block is reporting proper output.
However pushing the data to a filesink as well as a GUI number sink is only reading zeros. Did I not set up the block properly?
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif
#include <gnuradio/io_signature.h>
#include "switches_impl.h"
#include <stdio.h>
#include <stdlib.h>
#include <uinstd.h>
namespace gr {
namespace zedboard {
switches::sptr
switches::make()
{
return gnuradio::get_initial_sptr
(new switches_impl());
}
/*
* The private constructor
*/
switches_impl::switches_impl()
: gr::block("switches",
gr::io_signature::make(0,0,0),
gr::io_signature::make(1, 1, sizeof(unsigned int *)))
{}
/*
* Our virtual destructor.
*/
switches_impl::~switches_impl()
{
}
void
switches_impl::forecast (int noutput_items, gr_vector_int &ninput_items_required)
{
/* <+forecast+> e.g. ninput_items_required[0] = noutput_items */
}
int
switches_impl::general_work (int noutput_items,
gr_vector_int &ninput_items,
gr_vector_const_void_star &input_items,
gr_vector_void_star &output_items)
{
//const <+ITYPE+> *in = (const <+ITYPE+> *) input_items[0];
unsigned int *out = (unsigned int *) output_items[0];
// Do <+signal processing+>
// Tell runtime system how many input items we consumed on
// each input stream.
char buffer[5];
size_t size = 1;
size_t nitems = 5;
FILE* fp;
fp = fopen("/proc/zedSwitches","r");
if (fp == NULL)
{
printf("Cannot open for read\n");
return -1;
}
/*
Expect return format:
0x00
*/
fread(buffer, size, nitems, fp);
fclose(fp);
out=(unsigned int *)strtoul(buffer,NULL,0);
printf("read: 0x%02x",out);
consume_each (noutput_items);
// Tell runtime system how many output items we produced.
return noutput_items;
}
} /* namespace zedboard */
} /* namespace gr */

Answer 1

A pointer is a pointer to data, usually:
unsigned int *out = (unsigned int *) output_items[0];
out refers to the buffer for your output.
But you overwrite that pointer with another pointer:
out=(unsigned int *)strtoul(buffer,NULL,0);
which just bends around your copy of that pointer, and doesn't affect the content of that buffer at all. Basic C!
You probably meant to say something like:
out[0]= strtoul(buffer,NULL,0);
That will put your value into the first element of the buffer.
However, you tell GNU Radio that you not only produced a single item (the line above), but noutput_items:
return noutput_items;
That must read
return 1;
when you're only producing a single item, or you must actually produce as many items as you return.
Your consume_each call is nonsensical – GNU Radio Sources are typically instances of gr::sync_block, which means that you'd write a work() instead of a general_work() method as you did.
From the fact alone that this is a general_work and not a work I'd say you haven't used gr_modtool (with block type set to source!) to generate the stub for this block – you really should. Again, I'd like to point you to the Guided Tutorials which should really quickly explain usage of gr_modtool as well as the underlying GNU Radio concepts.