Erlang.mk can not only build Erlang projects, but also the C code that some projects come with, like NIFs and port drivers.
There are two ways to build the C code: using a custom Makefile,
or making Erlang.mk do it directly. The C code will be built
as needed when you run make.
The C source code should be located in the $(C_SRC_DIR) directory. It defaults to c_src/. Should you need to modify it, all you need to do is to set the variable in your Makefile before including Erlang.mk:
C_SRC_DIR = $(CURDIR)/my_nif_source
When this directory exists, Erlang.mk will automatically create a file named $(C_SRC_ENV). This file defaults to $(C_SRC_DIR)/env.mk. This can also be changed:
C_SRC_ENV = $(C_SRC_DIR)/erlang_env.mk
It contains a few variable definitions for the environment used for the build:
-
ERTS_INCLUDE_DIR - Path to the ERTS include files (erl_driver.h, erl_nif.h and more).
-
ERL_INTERFACE_INCLUDE_DIR - Path to the Erl_Interface include files (ei.h and related).
-
ERL_INTERFACE_LIB_DIR - Path to the Erl_Interface static libraries.
Erlang.mk will automatically run make if it detects a Makefile
in $(C_SRC_DIR)/Makefile.
The Makefile should have at least two targets: a default target
(which can be anything, for example all) which is invoked when
building the C code, and a clean target invoked when cleaning
it.
You can include the env.mk file to benefit from the Erlang environment detection:
include env.mk
You don’t need to write a Makefile to build C source code, however. Erlang.mk comes with rules to build both shared libraries and executables, using the source files it finds in $(C_SRC_DIR).
By default, Erlang.mk will create a shared library. To change this and create an executable instead, put this in your Makefile before including Erlang.mk:
C_SRC_TYPE = executable
The generated file name varies depending on the type of project you have (shared library or executable) and on the platform you build the project on.
For shared libraries, the generated file name will be
$(C_SRC_OUTPUT)$(C_SRC_SHARED_EXTENSION), with the default
being $(CURDIR)/priv/$(PROJECT) followed by the extension:
.dll on Windows, .so everywhere else.
For executables, the generated file name is
$(C_SRC_OUTPUT)$(C_SRC_EXECUTABLE_EXTENSION), with the same
default except for the extension: .exe on Windows, and otherwise
nothing.
Erlang.mk sets appropriate compile and linker flags by default. These flags vary depending on the platform, and can of course be overridden.
-
CC - The compiler to be used.
-
CFLAGS - C compiler flags.
-
CXXFLAGS - C++ compiler flags.
-
LDFLAGS - Linker flags.
-
LDLIBS - Libraries to link against.
The source files are automatically gathered from the contents
of $(C_SRC_DIR). Erlang.mk looks for .c, .C, .cc and .cpp
source files. You can define the variable SOURCES to manually
list the files to compile.
In some cases it might be necessary to propagate the flags you just defined to the sub-Makefiles of your local project. You generally can’t just export those as this could impact the building of dependencies.
Makefiles allow you to export variables for specific targets. When doing this, the variables will be exported only when this target runs, and not for other targets. It is therefore possible to export them when building the C code without impacting other build steps.
By adding this to your Makefile all five variables will be made available to sub-Makefiles when building C code:
app-c_src: export CC += app-c_src: export CFLAGS += app-c_src: export CPPFLAGS += app-c_src: export LDFLAGS += app-c_src: export LDLIBS +=
Appending an empty string to the existing value is necessary because Makefiles expect an assignment for target-specific exports. Alternatively you can set a new value:
app-c_src: export CFLAGS = -O3