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undefined reference to yywrap error when compiling sysmon

I needed to add some monitoring of one of my customer’s mail servers and compiled sysmon on a CentOS Linux machine to do the monitoring. When compiling sysmon I got the error message "undefined reference to `yywrap’". This post looks at the full error message and the solution.

How to install sysmon

The steps to install sysmon are as follows:

wget ftp://puck.nether.net/pub/jared/sysmon-0.92.2.tar.gz
tar -zxf sysmon-0.92.2.tar.gz
cd sysmon-0.92.2
cd src
sudo make install

The error message

At the "sudo make install" step I got the following error message and sysmon would not install:

/bin/rm -f sysmond core sysmond.core
gcc -L/usr/local//lib  -o sysmond heartbeat.o syswatch.o page.o tcp.o udp.o icmp.o pop3.o umichX500.o loadconfig.o smtp.o nntp.o talktcp.o imap.o lib.o textfile.o http.o srvclient.o dnscache.o ircd.o sysmon.o radius.o md5.o parser.o snpp.o snmp.o pingv6.o logging.o dns.o ssh.o -lwrap -lbsd -lresolv -lpthread -lnsl
parser.o: In function `sysmon_conf_yylex':
/home/chris/install/sysmon-0.92.2/src/parser.l:2212: undefined reference to `yywrap'
collect2: ld returned 1 exit status
make: *** [sysmond] Error 1

Read configure’s output better…

If I’d read the output from ./configure better I would have seen this in there:

checking for flex... no
checking for lex... no
configure: WARNING: without lex or flex sysmon may not compile but give it a try
configure: WARNING: We may not compile as we did not find yywrap.  I suggest
installing flex. ftp://ftp.gnu.org/gnu/non-gnu/flex/

The solution

The solution to this error message when compiling sysmon is to ensure you have flex installed. On a CentOS machine do this:

sudo yum install flex

Debian, or a Debian derived distro such as Ubuntu, would do this:

sudo apt-get install flex

After flex has been installed, you’ll need to start over from ./configure – if you attempt to make clean, make, make install it will still error out because it still doesn’t know flex is there:

cd ..
cd src
sudo make install


sysmon needs flex to compile. If it isn’t already installed you’ll get the error message detailed in this post. Install flex, then configure and make again.