How to use LDFLAGS in makefile

Question

I am new to Linux OS  I am trying to compile a  c file using a makefile  The math library has to be linked  My makefile looks like this   CC gcc CFLAGS -Wall -lm  all client   PHONY  clean clean      rm      o client   When I run make  I get the following error    undefined reference to rint    So it is not able to link the math library   But when I compile explicitly using  gcc client c -lm -o client   it successfully compiles   So how should I change my makefile such that it works  I have already tried adding LDFLAGS -lm  But I get the same error   I should also add that when I run make  it expands to  gcc -Wall -lm client c -o client    notice that when I run gcc explicitly with -lm at the end  it works

User · Answer

Seems like the order of the linking flags was not an issue in older versions of gcc. Eg gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-16) comes with Centos-6.7 happy with linker option before inputfile; but gcc with ubuntu 16.04 gcc (Ubuntu 5.3.1-14ubuntu2.1) 5.3.1 20160413 does not allow.

Its not the gcc version alone, I has got something to with the distros

User · Answer

Your linker  ld  obviously doesn t like the order in which make arranges the GCC arguments so you ll have to change your Makefile a bit   CC gcc CFLAGS -Wall LDFLAGS -lm   PHONY  all all  client   PHONY  clean clean        RM       o client  OBJECTS client o client    OBJECTS        CC    CFLAGS    OBJECTS  -o client   LDFLAGS    In the line defining the client target change the order of   LDFLAGS  as needed

User · Answer

In more complicated build scenarios  it is common to break compilation into stages  with compilation and assembly happening first  output to object files   and linking object files into a final executable or library afterward--this prevents having to recompile all object files when their source files haven t changed  That s why including the linking flag -lm isn t working when you put it in CFLAGS  CFLAGS is used in the compilation stage    The convention for libraries to be linked is to place them in either LOADLIBES or LDLIBS  GNU make includes both  but your mileage may vary    LDLIBS -lm   This should allow you to continue using the built-in rules rather than having to write your own linking rule  For other makes  there should be a flag to output built-in rules  for GNU make  this is -p   If your version of make does not have a built-in rule for linking  or if it does not have a placeholder for -l directives   you ll need to write your own   client o  client c       CC    CFLAGS    CPPFLAGS    TARGET ARCH  -c -o      lt   client  client o       CC    LDFLAGS    TARGET ARCH       LOADLIBES    LDLIBS  -o

[makefile] How to use LDFLAGS in makefile

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