Reading from FIFO file

ineya · 07-16-2007, 02:03 PM

I have a fifo file, where another process wrote some data. I want to read this data from this file in bash script I'm trying to write.

However, all calls like "cat", "more", "tail" or "cp" are blocking and waiting forever. (because as I understand it, FIFO file "has no end").

Any suggestion, how to solve this? (It is clear to me, that I could write some piece of code myself, but I'm wondering if there is already some useful utility I could use)

Thank you.

Tux-Slack · 07-16-2007, 03:05 PM

FIFO does have and end of file. The problem would be if you would be reading data from that file and the other process would be still writing it. Then the end of file is "on the move". And as the other process is writing it it moves the end of file line by line down and therefor you can't reach the end.
This would be the same as if you would run
cat /dev/urandom > /some/file
and try to cat /some/file to the screen.
You would never reach the end untill you stoped cat /dev/urandom > /some/file

ta0kira · 07-17-2007, 12:11 AM

The read process will block until the writing process closes its descriptor. That indicates the end of the data, or "EOF". When piping in a script without using "|" you generally have to set up the read operation in the background (e.g. cat mypipe &,) execute the write operation (e.g. ls -l > mypipe,) and then wait to make sure the background process is done before continuing.

IO blocking is a state set at the file descriptor level. In a C program you would change the default blocking to non-blocking with fcntl(file_descriptor, F_SETFL, O_NONBLOCK);. That's necessary for a lot of server-type programs. If you really need a version of cat that doesn't block, you could write one in just a few lines. Is it a time limit you are concerned with? Do you want it to only wait X number of seconds when no input is given?

Code:

#include <stdio.h>
#include <fcntl.h>
#include <time.h>
#include <unistd.h>
#include <string.h>


void read_loop(int fFile, double wWait)
{
	if (fFile < 0) return;

	double max_time = wWait, total_time = 0;
	struct timespec cycle_time = { 0, 50 * 1000 * 1000 };
	double add_time = (double) cycle_time.tv_sec + (double) cycle_time.tv_nsec / 1000000000.;

	char next_line[1024];

	FILE *input_file = fdopen(fFile, "r");

	while (total_time < max_time)
	{
	while (fgets(next_line, 1024, input_file))
	 {
	write(STDOUT_FILENO, next_line, strlen(next_line));
	total_time = 0;
	 }

	nanosleep(&cycle_time, NULL);
	total_time += add_time;
	}

	fclose(input_file);
}


int main(int argc, char *argv[])
{
	if (argc < 2)
	{
	fprintf(stderr, "%s [max time] (files...)\n", argv[0]);
	return 1;
	}

	int max_wait = strtoul(argv[1],0, 10);

	if (argc == 2)
	{
	fprintf(stderr, "%s: using standard input\n", argv[0]);
	fcntl(STDIN_FILENO, F_SETFL, O_NONBLOCK);
	read_loop(STDIN_FILENO, max_wait);
	return 0;
	}

	int current = 2;

	while (current < argc)
	{
	fprintf(stderr, "%s: switch to file '%s'\n", argv[0], argv[current]);
	int next_file = open(argv[current++], O_RDONLY | O_NONBLOCK);
	read_loop(next_file, max_wait);
	close(next_file);
	}

	return 0;
}

Here is my non-blocking version of cat. You give it a time in seconds to wait between valid inputs and if it times out then it will switch files until it runs out of them. Yes, I think I can solve all problems by throwing C code at them

ta0kira

ineya · 07-17-2007, 01:22 AM

I was prepared to write it by myself if necessary :-), but you saved me a lot of work.
Thank you.

I just had to add

Code:

#include <stdlib.h>

to compile this. My debian machine had trouble with "strtoul".

Quote:

Yes, I think I can solve all problems by throwing C code at them

You surely solved mine. ;-)

ta0kira · 07-17-2007, 11:19 PM

I did have that header in when I added the function, but then I looked at it when I was done and thought, "why is THAT in there?" After a quick look, I didn't see anything that needed it so I took it out, but it still managed to compile alright.
ta0kira