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makefileembeddedopenwrt

Makefiles in openwrt, the difference between cp and $(CP)?

I am following a tutorial for compiling my own package in openwrt.

In the /package/helloworld directory:

.../packege/helloworld$ ls
src Makefile
.../packege/helloworld$ ls src
hello.c main.c Makefile
.../packege/helloworld$vi Makefile

#helloworld makefile
include $(TOPDIR)/rules.mk

PKG_NAME:=helloworld
PKG_RELEASE:=1
PKG_VERSION:=0.1

PKG_BUILD_DEPENDS:=

include $(INCLUDE_DIR)/package.mk

define Package/helloworld
  SECTION:=utils
  CATEGORY:=Utilities
  DEPENDS:=@TARGET_etrax
  TITLE:=Yet Another Helloworld Application
endef

define Package/helloworld/description
 This is helloworld :p
endef

define Build/Prepare
mkdir -p $(PKG_BUILD_DIR)
$(CP) ./src/* $(PKG_BUILD_DIR)/
endef

define Build/Compile
$(MAKE) -C $(PKG_BUILD_DIR) \
    $(TARGET_CONFIGURE_OPTS) \
    CFLAGS="$(TARGET_CFLAGS)"
endef

define Package/helloworld/install
$(INSTALL_DIR) $(1)/usr/bin
$(INSTALL_BIN) $(PKG_BUILD_DIR)/helloworld $(1)/usr/bin/
endef

$(eval $(call BuildPackage,helloworld))

I have two questions about this Makefile:

  1. I found there are commands, such as mkdir, $(CP),$(MAKE). I changed $(CP) to cp, and the compiling goes well. So I don't understand why these two kinds of formats exsits.

  2. Where the parameters, such as $(PKG_BUILD_DIR), $(INSTALL_DIR), are defined in openwrt? I just found the place where $(TOPDIR) is defined but not the others.

Thanks

Solution

    1. These are not different kinds of formats, cp is a Linux command, $(CP) is a makefile construction for "getting the value of make variable CP". Thus, under Linux it should expand to cp (i.e. should be initialized with this value somewhere), and , most probably, to copy under Windows (this is all particular setup-dependent since cp is not fully the same as copy). The same with $(MKDIR) and other system tools.

      1.1. $(MAKE) is actually another thing - this is a special make variable which expands to make tool name with arguments/flags passed from command line. Read this.

    2. These are all variables controlling where to build and where to install. See description here.