Mkfifo in GNU Octave - octave

Question

I have yet to find a complete example for using the mkfifo() function online. I am able to make the fifo like this:
mkfifo("file",777)
But when I fopen() this file, Octave just hangs. What is the proper way to create, queue, and dequeue bytes from a mkfifo object?
I would like to create an in-memory fifo in Octave (on-disk is fine too) and read and write it from the same Octave script. My project is running in real time, and so I need a buffer so that I can fill and drain from the same Octave script. I've searched for a fifo library with zero results. Even just creating a vector and pushing and popping will suit my needs. I tried this myself, but I'm running into object oriented programming design problems because Octave does not allow pass by reference or pointers.

Answer 1

There are two issues. First: mkfifo expects mode as integer with base 10, if you write "777" you think in octal, base 8. Second: mkfifo uses umask to modify the permissions to (mode & ~umask) (See man 3)
As example:
fn=tempname
[ERR, MSG] = mkfifo (fn, base2dec("744", 8))
stat(fn)
fn = /tmp/oct-83UCBR
ERR = 0
MSG =
ans =
scalar structure containing the fields:
dev = 2053
ino = 3408172
mode = 4580
modestr = prwxr--r--
nlink = 1
uid = 1000
gid = 1000
rdev = 0
size = 0
atime = 1.4311e+09
mtime = 1.4311e+09
ctime = 1.4311e+09
blksize = 4096
blocks = 0
As you can see the modestr is now prwxr--r-- as you would expect from octal 744
Now you can open one end of the FIFO:
fid = fopen (fn, "r")
Of course this blocks until the other end of the fifo gets connected.

Answer 2

The fifo class works, but only up to a certain size. The max size in bytes of a fifo can be found by running:
cat /proc/sys/fs/pipe-max-size
1048576
Below is the code that I wrote for an in-memory fifo. It's fairly crude but it works well:
1; % Prevent Octave from thinking that this is a function file
global fifoCount fifoSamples fifoFiles fifoFids fifoDataType
fifoSamples = zeros(0);
fifoCount = 0;
fifoFiles = cell(1);
fifoFids = zeros(0);
fifoDataType = 'single';
fifoDataTypeSize = 4;
fifoMaxBytes = 1048576; % this is operating system enforced, changing here will not help
function [] = o_fifo_write(index, data)
global fifoCount fifoSamples fifoFiles fifoFids fifoDataType
wrcount = fwrite(fifoFids(index), data, fifoDataType);
[sz,~] = size(data);
fifoSamples(index) = fifoSamples(index) + sz;
if( sz ~= wrcount )
disp(sprintf('o_fifo_write was given %d samples but wrote %d', sz, wrcount));
end
if( ~iscolumn(data) )
disp('data must be columnar in o_fifo_write');
end
end
function [data] = o_fifo_read(index, count)
global fifoCount fifoSamples fifoFiles fifoFids fifoDataType
[data, rdcount] = fread(fifoFids(index), count, fifoDataType);
[sz,~] = size(data);
fifoSamples(index) = fifoSamples(index) - sz;
if( sz ~= rdcount || sz ~= count )
disp(sprintf('in o_fifo_read %d %d %d should all be the same', sz, rdcount, count));
end
end
function [avail] = o_fifo_avail(index)
global fifoCount fifoSamples fifoFiles fifoFids fifoDataType
avail = fifoSamples(index);
end
function index = o_fifo_new()
global fifoCount fifoSamples fifoFiles fifoFids fifoDataType
fifoCount = fifoCount + 1;
index = fifoCount;
fifoSamples(index) = 0;
fifoFiles{index} = tempname;
[ERR, MSG] = mkfifo(fifoFiles{index}, base2dec('744',8));
fifoFids(index) = fopen(fifoFiles{index}, 'a+');
% fcntl(fifoFids(index), F_SETFL, O_NONBLOCK); % uncomment to avoid hangs when trying to overfill fifo
end
% ---- usage -----
txfifo = o_fifo_new();
disp(o_fifo_avail(txfifo));
o_fifo_write(txfifo, [1.243 pi 2*pi 4/3*pi]');
disp(o_fifo_avail(txfifo));
disp(o_fifo_read(txfifo, 4));
disp(o_fifo_avail(txfifo));