Some strange error appeared after I upgraded my Ubuntu from (10.11, 11.04 i dont know) to 11.10.
I am getting an undefined reference to 'sqrt' while using math.h and linking with -lm
I'm compiling with gcc -Wall -Werror -g -Iinclude/ -lm lib/matrix.c src/analyse.c -o bin/analyse.o both source-files use and include math.h.
This code compiled without problems for and I didn't change much since the upgrade but now it won't work.
Do you have any suggestions what I can do, to find the error?
I'm sorry, if this question was asked before; there are so many posts on math linker errors and I didn't find a matching one
The library that you are using needs to be placed after the files that use it when you are using it from the command line. So place -lm on after your C files on the command line.
SOLVED, this is not the common missing -lm problem! I'm in the same situation after upgrade to (k)ubuntu 11.10!
$ whereis math.h
math: /usr/include/math.h
Makefile:
CC=gcc
CFLAGS=--std=c99 -g -pedantic -Wall -lm
uname:
Linux idefix 3.0.0-12-generic #20-Ubuntu SMP Fri Oct 7 14:56:25 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux
You really HAVE TO place the -lm swith after -o foo foo.c parameter
Output:
pidi@idefix:~/projekt1$ make
gcc -o b1 b1.c --std=c99 -g -pedantic -Wall -lm
pidi@idefix:~/projekt1$
So swap your flags in Makefile! GUYS. This is pretty new (and serious) BUG!
This is a problem due to the default activation of the gcc flag --as-needed in the linker
More information here: http://www.gentoo.org/proj/en/qa/asneeded.xml
Simple fix (worked for me at least):
Add -Wl,--no-as-needed to the linker
I found the same problem after upgrading my Ubuntu to 11.10 version. I use Netbeans for developing and solved the problem by specifying the "Mathematics" standard library as it follows:
Right click on project, click on Properties, select "Linker" on menu, click on "Libraries" and then "Add Standard Library" choosing "Mathematics".
When compiling the '-lm' option is placed after all the other options and it works. Probably this gcc version follows a specific architecture and it expects the libraries at the end of the command compiling line.
Cheers!
D.
cc filename.c -lm
just try..........☻
来源：https://stackoverflow.com/questions/7824439/c-math-linker-problems-on-ubuntu-11-10