Named Pipes (FIFOs) on Unix with multiple readers
The O in FIFO means "out". Once your data is "out", it's gone. :-) So naturally if another process comes along and someone else has already issued a read, the data isn't going to be there twice.
To accomplish what you suggest you should look into Unix domain sockets. Manpage here. You can write a server which can write to client processes, binding to a filesystem path. See also socket(), bind(), listen(), accept(), connect(), all of which you'll want to use with PF_UNIX, AF_UNIX, and struct sockaddr_un.
I don't think that the behaviour you observed is more than coincidental. Consider this trace, which uses 'sed' as the two readers and a loop as the writer:
Osiris JL: mkdir fifoOsiris JL: cd fifoOsiris JL: mkfifo fifoOsiris JL: sed 's/^/1: /' < fifo &[1] 4235Osiris JL: sed 's/^/2: /' < fifo &[2] 4237Osiris JL: while read line ; do echo $line; done > fifo < /etc/passwd1: ##1: # User Database1: #1: # Note that this file is consulted directly only when the system is running1: # in single-user mode. At other times this information is provided by1: # Open Directory.1: #1: # This file will not be consulted for authentication unless the BSD local node1: # is enabled via /Applications/Utilities/Directory Utility.app1: #1: # See the DirectoryService(8) man page for additional information about1: # Open Directory.1: ##1: nobody:*:-2:-2:Unprivileged User:/var/empty:/usr/bin/false1: root:*:0:0:System Administrator:/var/root:/bin/sh1: daemon:*:1:1:System Services:/var/root:/usr/bin/false1: _uucp:*:4:4:Unix to Unix Copy Protocol:/var/spool/uucp:/usr/sbin/uucico1: _lp:*:26:26:Printing Services:/var/spool/cups:/usr/bin/false2: _postfix:*:27:27:Postfix Mail Server:/var/spool/postfix:/usr/bin/false2: _mcxalr:*:54:54:MCX AppLaunch:/var/empty:/usr/bin/false2: _pcastagent:*:55:55:Podcast Producer Agent:/var/pcast/agent:/usr/bin/false2: _pcastserver:*:56:56:Podcast Producer Server:/var/pcast/server:/usr/bin/false2: _serialnumberd:*:58:58:Serial Number Daemon:/var/empty:/usr/bin/false2: _devdocs:*:59:59:Developer Documentation:/var/empty:/usr/bin/false2: _sandbox:*:60:60:Seatbelt:/var/empty:/usr/bin/false2: _mdnsresponder:*:65:65:mDNSResponder:/var/empty:/usr/bin/false2: _ard:*:67:67:Apple Remote Desktop:/var/empty:/usr/bin/false2: _www:*:70:70:World Wide Web Server:/Library/WebServer:/usr/bin/false2: _eppc:*:71:71:Apple Events User:/var/empty:/usr/bin/false2: _cvs:*:72:72:CVS Server:/var/empty:/usr/bin/false2: _svn:*:73:73:SVN Server:/var/empty:/usr/bin/false2: _mysql:*:74:74:MySQL Server:/var/empty:/usr/bin/false2: _sshd:*:75:75:sshd Privilege separation:/var/empty:/usr/bin/false2: _qtss:*:76:76:QuickTime Streaming Server:/var/empty:/usr/bin/false2: _cyrus:*:77:6:Cyrus Administrator:/var/imap:/usr/bin/false2: _mailman:*:78:78:Mailman List Server:/var/empty:/usr/bin/false2: _appserver:*:79:79:Application Server:/var/empty:/usr/bin/false2: _clamav:*:82:82:ClamAV Daemon:/var/virusmails:/usr/bin/false2: _amavisd:*:83:83:AMaViS Daemon:/var/virusmails:/usr/bin/false2: _jabber:*:84:84:Jabber XMPP Server:/var/empty:/usr/bin/false2: _xgridcontroller:*:85:85:Xgrid Controller:/var/xgrid/controller:/usr/bin/false2: _xgridagent:*:86:86:Xgrid Agent:/var/xgrid/agent:/usr/bin/false2: _appowner:*:87:87:Application Owner:/var/empty:/usr/bin/false2: _windowserver:*:88:88:WindowServer:/var/empty:/usr/bin/false2: _spotlight:*:89:89:Spotlight:/var/empty:/usr/bin/false2: _tokend:*:91:91:Token Daemon:/var/empty:/usr/bin/false2: _securityagent:*:92:92:SecurityAgent:/var/empty:/usr/bin/false2: _calendar:*:93:93:Calendar:/var/empty:/usr/bin/false2: _teamsserver:*:94:94:TeamsServer:/var/teamsserver:/usr/bin/false2: _update_sharing:*:95:-2:Update Sharing:/var/empty:/usr/bin/false2: _installer:*:96:-2:Installer:/var/empty:/usr/bin/false2: _atsserver:*:97:97:ATS Server:/var/empty:/usr/bin/false2: _unknown:*:99:99:Unknown User:/var/empty:/usr/bin/falseOsiris JL: jobs[1]- Running sed 's/^/1: /' < fifo &[2]+ Done sed 's/^/2: /' < fifoOsiris JL: echo > fifo1: Osiris JL: jobs[1]+ Done sed 's/^/1: /' < fifoOsiris JL: As you can see, both readers got to read some of the data. Which reader was scheduled at any time depended on the whim of the o/s. Note that I carefully used an echo to print each line of the file; those were atomic writes that were read atomically.
Had I used a Perl script with, for example, a delay after reading and echoing a line, then I might well have seen more determinate behaviour with (generally) two lines from Reader 1 for every 1 line from Reader 2.
perl -n -e 'while(<>){ print "1: $_"; sleep 1; }' < fifo &perl -n -e 'while(<>){ print "2: $_"; sleep 2; }' < fifo &Experimentation done on MacOS X 10.5.8 (Leopard) - but likely to be similar most places.