Undefined reference to math functions when linking with gcc

Question

I'm getting an error when using make to install a program. The full step that it fails on with error looks like this:

gcc  -g -O2 -fopenmp -L/usr/lib -lcfitsio -lm  -o lenstool_tab e_nfwg.o lenstool_tab.o midpnt.o nrutil.o polint.o qromo.o read_bin.o lenstool_tab.o: In function `main':
/usr/local/src/lenstool-6.8/table_src/lenstool_tab.c:73: undefined reference to `log'
/usr/local/src/lenstool-6.8/table_src/lenstool_tab.c:73: undefined reference to `log'
e_nfwg.o: In function `surfdens2':
/usr/local/src/lenstool-6.8/table_src/e_nfwg.c:133: undefined reference to `pow'
/usr/local/src/lenstool-6.8/table_src/e_nfwg.c:130: undefined reference to `sin'
/usr/local/src/lenstool-6.8/table_src/e_nfwg.c:138: undefined reference to `pow'
/usr/local/src/lenstool-6.8/table_src/e_nfwg.c:140: undefined reference to `sin'
/usr/local/src/lenstool-6.8/table_src/e_nfwg.c:140: undefined reference to `pow'
/usr/local/src/lenstool-6.8/table_src/e_nfwg.c:144: undefined reference to `sin'
/usr/local/src/lenstool-6.8/table_src/e_nfwg.c:144: undefined reference to `pow'
e_nfwg.o: In function `nfwg_kappa':
/usr/local/src/lenstool-6.8/table_src/e_nfwg.c:63: undefined reference to `pow'
e_nfwg.o: In function `scmass':
/usr/local/src/lenstool-6.8/table_src/e_nfwg.c:165: undefined reference to `pow'
collect2: error: ld returned 1 exit status
make[1]: *** [lenstool_tab] Error 1
make[1]: Leaving directory `/usr/local/src/lenstool-6.8/table_src'
make: *** [all-recursive] Error 1

So obviously there is a problem with linking to math library. I checked that these codes do contain #include<math.h>. The main cause of this problem seems to usually be from a lack of -lm at the compile command, but as you can see it appears in the above. I added the -lm to various parts of the makefile that seemed appropriate, but it didn't help. I'm including the makefile here in case it helps. Should I be modifying the makefile to fix this? I searched this problem on the web but no answers seemed to consider the makefile. I read a suggestion in another thread to modify config.status to include -lm, but it was not clear where to do that. I've installed this program on a Mac before without issue but now I'm using Ubuntu 14 and I get this error. Any help is much appreciated! Thanks.

Solution

The libraries being linked to should be specified after there is a reference to them. Thus, you will change the command to:

gcc  -g -O2 -fopenmp -L/usr/lib  -o lenstool_tab e_nfwg.o lenstool_tab.o midpnt.o nrutil.o polint.o qromo.o read_bin.o lenstool_tab.o -lcfitsio -lm

This should fix your problem. You can possibly fix the problem in your Makefile so that the libraries are specified later.

Comments

[c0der]

I suspected that might be an issue, thanks for pointing that out. But any ideas about how can I implement this fix? You can see that -lcfitsio -lm comes from line 301 in the makefile, how can I set it so that it comes at the end of the gcc commands? Actually I dug a little deeper and found another makefile in the directory with those c programs, and found a similar line to put -lm on, but it's still not showing up at the end of that command. Or if I could do this step manually without modifying the makefile, I'm not sure how to do it.

[unxnut]

You will have to check where the final target is being built. I am sure the libraries have been defined using some macros and you just need to move the variables around in the statement.

[c0der]

Well I spent a lot of time altering the makefiles but I couldn't get it to work. In the end I just entered that gcc command manually and used make again to finish it off.