ld: undefined reference to symbol 'log2@@GLIBC_2.2.5'

Question

Whats wrong with

for (level = 1; level <= log2((double)size); level++)
                         ^

Seems like its from using log2() but whats wrong? I am using it with OpenMPI code actually, but commenting this line fixes things.

Full Source(http://pastie.org/7559178) see line 40

[jiewmeng@JM Assign3]$ mpicc -o cpi cpi.c && mpirun -np 16 cpi
/usr/bin/ld: /tmp/cca9x4he.o: undefined reference to symbol 'log2@@GLIBC_2.2.5'
/usr/bin/ld: note: 'log2@@GLIBC_2.2.5' is defined in DSO /usr/lib/libm.so.6 so try adding it to the linker command line
/usr/lib/libm.so.6: could not read symbols: Invalid operation
collect2: error: ld returned 1 exit status

Seems like log2(4) will work but I cant pass in a variable?

Answer 1

In in order to link libm you need to add -lm argument, as this document; MPI under Linux in the Math Department says:

If your code includes mathematical functions (like exp, cos, etc.), you need to link to the mathematics library libm.so. This is done, just like for serial compiling, by adding -lm to the end of your compile command, that is,

mpicc -o sample sample.c -lm