Multiple producer single consumer with Circular Buffer - c

Question

Need help in getting the following to work.
I have a multiple producer threads (each writing say 100 bytes of data) to ringbuffer.
And one single reader(consumer) thread ,reads 100 bytes at a time and writes to stdout.(Finally i want to write to files based on the data)
With this implementation ,I get the data read from ring buffer wrong sometimes. see below
Since the ringbuffer size is small it becomes full and some part of data is loss.This is not my current problem.
** Questions:
On printing the data thats read from ringbuffer ,some data gets
interchanged !!I'm unable to find the bug.
Is the logic/approach correct ? (or) Is there a
better way to do this
ringbuffer.h
#define RING_BUFFER_SIZE 500
struct ringbuffer
{
char *buffer;
int wr_pointer;
int rd_pointer;
int size;
int fill_count;
};
ringbuffer.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "ringbuffer.h"
int init_ringbuffer(char *rbuffer, struct ringbuffer *rb, size_t size)
{
rb->buffer = rbuffer;
rb->size = size;
rb->rd_pointer = 0;
rb->wr_pointer = 0;
rb->fill_count = 0;
return 0;
}
int rb_get_free_space (struct ringbuffer *rb)
{
return (rb->size - rb->fill_count);
}
int rb_write (struct ringbuffer *rb, unsigned char * buf, int len)
{
int availableSpace;
int i;
availableSpace = rb_get_free_space(rb);
printf("In Write AVAIL SPC=%d\n",availableSpace);
/* Check if Ring Buffer is FULL */
if(len > availableSpace)
{
printf("NO SPACE TO WRITE - RETURN\n");
return -1;
}
i = rb->wr_pointer;
if(i == rb->size) //At the end of Buffer
{
i = 0;
}
else if (i + len > rb->size)
{
memcpy(rb->buffer + i, buf, rb->size - i);
buf += rb->size - i;
len = len - (rb->size - i);
rb->fill_count += len;
i = 0;
}
memcpy(rb->buffer + i, buf, len);
rb->wr_pointer = i + len;
rb->fill_count += len;
printf("w...rb->write=%tx\n", rb->wr_pointer );
printf("w...rb->read=%tx\n", rb->rd_pointer );
printf("w...rb->fill_count=%d\n", rb->fill_count );
return 0;
}
int rb_read (struct ringbuffer *rb, unsigned char * buf, int max)
{
int i;
printf("In Read,Current DATA size in RB=%d\n",rb->fill_count);
/* Check if Ring Buffer is EMPTY */
if(max > rb->fill_count)
{
printf("In Read, RB EMPTY - RETURN\n");
return -1;
}
i = rb->rd_pointer;
if (i == rb->size)
{
i = 0;
}
else if(i + max > rb->size)
{
memcpy(buf, rb->buffer + i, rb->size - i);
buf += rb->size - i;
max = max - (rb->size - i);
rb->fill_count -= max;
i = 0;
}
memcpy(buf, rb->buffer + i, max);
rb->rd_pointer = i + max;
rb->fill_count -= max;
printf("r...rb->write=%tx\n", rb->wr_pointer );
printf("r...rb->read=%tx\n", rb->rd_pointer );
printf("DATA READ ---> %s\n",(char *)buf);
printf("r...rb->fill_count=%d\n", rb->fill_count );
return 0;
}

Answer 1

At the producer you also need to wait on conditional variable for the has empty space condition. The both conditional variables should be signaled unconditionally, i.e. when a consumer removes an element from the ring buffer it should signal the producers; when a producer put something in the buffer it should signal the consumers.
Also, I would move this waiting/signaling logic into rb_read and rb_write implementations, so your ring buffer is a 'complete to use solution' for the rest of your program.

Answer 2

As to your questions --
1. I can't find that bug either -- in fact, I've tried your code and don't see that behavior.
2. You ask if this is logic/approach correct -- well, as far as it goes, this does implement a kind of ring buffer. Your test case happens to have an integer multiple of the size, and the record size is constant, so that's not the best test.
In trying your code, I found that there is a lot of thread starvation -- the 1st producer thread to run (the last created) hits things really hard, trying and failing after the 1st 5 times to stuff things into the buffer, not giving the consumer thread a chance to run (or even start). Then, when the consumer thread starts, it stays cranking for quite some time before it releases the cpu, and the next producer thread finally starts. That's how it works on my machine -- it will be different on different machines, I'm sure.
It's too bad that your current code doesn't have a way to end -- creating files of 10's or 100's of MB ... hard to wade through.

Answer 3

(Probably a bit later for the author, but if anyone else searches for a "multiple producers single consumer")
I think the fundamental problem in that implementation is what rb_write modifies a global state (rb->fill_count and other rb->XX) w/o doing any synchronization between multiple writers.
For alternative ideas check the: http://www.linuxjournal.com/content/lock-free-multi-producer-multi-consumer-queue-ring-buffer.