Using Dependencies

A typical package Makefile will contain a section like:

define Package/tcpdump/default
  SECTION:=net
  CATEGORY:=Network
  DEPENDS:=+libpcap
  TITLE:=Network monitoring and data acquisition tool
  URL:=http://www.tcpdump.org/
endef

This page talks about what the DEPENDS:= line should look like.

• If you specify a package name without any ornaments then that means that the current package cannot be selected unless the package named in enabled. e.g., in tcpdump above,

DEPENDS:=libpcap

would mean that tcpdump would not be shown as possible to be selected unless libpcap were already selected

• If you say +package that means if the current package is selected, it will cause package to be selected. This is the case with tcpdump above. It says that if tcpdump is selected, then select libpcap. e.g.

DEPENDS:=+libpcap

• If you say +PACKAGE_packageb:packagec, it means that packagec will only get selected by the current package (e.g. tcpdump) if packageb is enabled. e.g.

DEPENDS:=+PACKAGE_arpd:libpcap

would mean that if tcpdump was selected, then if arpd was also selected, select libpcap.

Useful is the negation ! to only select a Package if it's not build into busybox

DEPENDS:=+!BUSYBOX_CONFIG_HOSTNAME:net-tools-hostname

This means select net-tools-hostname only if hostname is not build into busybox. But don't try it the other way around, see the last case.

• If you say @SYMBOL that means that CONFIG_SYMBOL must be defined by OpenWrt in order for the package to be available for selection. e.g

DEPENDS:=@USB_SUPPORT

would mean that the package wouldn't even appear unless config symbol CONFIG_USB_SUPPORT was defined, for example by the package usb-core

• If you say +@SYMBOL that means if the current package is selected, it will cause CONFIG_SYMBOL to be selected.

DEPENDS:=+@KERNEL_DEBUG_FS

would mean that the if you select the package, it would automatically set the config symbol CONFIG_KERNEL_DEBUG_FS. Beware: This works while compiling your own image. If you install this package on a running box with opkg, this dependency is not checked!

• Don't be so clever and combine everything:

DEPENDS:=+@!PACKAGE_net-tools-hostname:BUSYBOX_CONFIG_HOSTNAME

Here we select the Symbol BUSYBOX_CONFIG_HOSTNAME if the package net-tools-hostname is not selected. The @ belongs to BUSYBOX_CONFIG_HOSTNAME. This works while compiling your own image, but the resulting ipkg depends on the unresolvable BUSYBOX_CONFIG_HOSTNAME, so your package cannot be installed later on a box with opkg.

As busybox can never be extended on a working box, always select an installable package when a busybox applet is not selected.

DEPENDS:=+!BUSYBOX_CONFIG_HOSTNAME:net-tools-hostname

It is possible to do select and depend on packages outside the usual DEPENDS definition. If, in the package Makefile, you have a definition:

define Package/package-name/config
    ...config stuff
endef

then you can include the following directives.

The author is not aware of the ability to do depends SYMBOL.

Note that the if directives may also include &&, ||, !, and uses parentheses () to use logical expression for AND (&&), OR (||) and NOT (!), and for order of operations ('()').

There is also a default 'y' SYMBOL directive but the author is hazy on the details.

It is important to note that using select bar in Package/foo/config directives will select bar for building, but it will not be included in the dependencies when opkg installs foo, nor will it guarantee that bar will be built before foo. Using DEPENDS will take care of all that.

Dependencies MAY NOT be circular (i.e. Package A may not depend on Package B, which in turn depends on Package A). If a circular dependency is created (whether directly or through another package), strange make menuconfig behaviour will be the result.

- The DEPENDS:@SYMBOL and DEPENDS:@SYMBOL:package syntax have good support for boolean operators, including parentheses, negation (!), && and ||.

- The support for operators in the DEPENDS:+SYMBOL:package syntax is limited. Negation ! is only supported to negate the whole condition. Parentheses are ignored, so use them only for readability. Like C, && has a higher precedence than ||. So +(YYY||FOO&&BAR):package will select package if CONFIG_YYY is set or if both CONFIG_FOO and CONFIG_BAR are set. You may use +YYY||(FOO&&BAR):package for readability, but writing +(YYY||FOO)&&BAR:package will not change the condition to select package if either YYY or FOO is selected and BAR is selected.

There may be bugs in the OpenWrt build system wrt to dependencies. If you can confirm that you have a dependency problem, please report a bug. The developers would appreciate the following information:

• the relevant portions of $TOPDIR/tmp/.config-package.in (where $TOPDIR is the directory where you checked out OpenWrt and where you initiate your builds)

NB Many apparent bugs are caused by circular dependencies. The OpenWrt build system doesn't like circular dependencies.