Chapter 8. NIFs and port drivers

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.

8.1. C source code location and Erlang environment

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.

8.2. Using a custom Makefile

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

8.3. Using Erlang.mk directly

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 overriden.

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.