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README.rst

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Why use make

  • automates compilations for code, LaTeX docs
  • tracks dependencies and (re)compiles as necessary
  • parallel compilation (also serves as a bug check on the Makefile)

Writing a Makefile

make needs rules to specify three things:

  1. what is the target
  2. what are the pre-requisites to make the target
  3. how to create the target once all the pre-requisites are met

Anatomy of a Makefile

  1. Macros contain variable names. For instance, SRC := main.c utils.c is a macro
  2. Targets are specified by a :. For instance, all: MYEXE specifies the target all
  3. Names that follow the : are pre-requisites. (Left of the : are targets, right of the : are pre-reqs). MYEXE was the pre-req in the previous line.
  4. Line following the target specifies the rule to create the target based on the pre-reqs. This rule MUST be initialized by a new-line followed by a <TAB>, like so
MYEXE: prereq1 prereq2
<TAB> $(CC) -o MYEXE prereq1 prereq2
  1. It is considered bad practice to repeat names, so the in the real-world the previous line would be written as
OBJS := prereq1 prereq2   # OBJS is a macro
MYEXE: $(OBJS)
<TAB> $(CC) -o $@ $(OBJS)

where, $@ is a special make variable that contains the name of the target, $(OBJS) contains the names of all your object files, $(CC) is your compiler.

note Targets that are not a file, should be declared as .PHONY (for instance, all, clean). Otherwise, if a file called all or clean exists in the directory, make will not do anything.

C/C++/Fortran Compile Conventions

  • CFLAGS/CPPFLAGS/FCFLAGS is automatically picked from the environment by the compiler to create the object file
  • LDFLAGS is used during linking to produce the final shared/static library or executable.
  • -lgsl is short-hand for a file called libgsl.so somewhere in the LD_LIBRARY_PATH, similarly, -lm is short-hand for libm.so (that contains all your standard math)

Basic Makefile

There is a basic Makefile already in the repo, you can use that for simple single directory jobs that will only run on linux. This Makefile should be sufficient for all (small) personal code-bases.

Important Makefile tricks and Compile Flags

  • Automatically pick up where GSL is located by using gsl-config --prefix
  • Embed library paths into the executable by using the compiler flag -Xlinker -rpath -Xlinker /path/to/the/library.

For instance:

GSL_DIR := $(shell gsl-config --prefix)
GSL_CFLAGS := $(shell gsl-config --cflags)
GSL_LIBS := $(shell gsl-config --libs)
GSL_LIBDIR := $(GSL_DIR)/lib

MYEXE: $(OBJS)
<TAB> $(CC) -o $@ $(OBJS) $(GSL_LIBS) -Xlinker -rpath -Xlinker $(GSL_LIBDIR)

With this extra flag, your exectuable will always pick up the compile time library, irrespective of your runtime environment. You can completely eliminate the runtime error of library not found.