Dependencies should be familiar to anyone who has used an .rpm and .deb based desktop distribution. A dependency is something that a package requires either to run the package (a run-time dependency) or to build the package (a build-time or compile-time, dependency).
There are two variables provided to allow the specifications of dependencies:
Specifies build-time dependencies, via a list of bitbake recipes to build prior to build the recipe. These are programs (flex-native) or libraries (libpcre) that are required in order to build the package.
Specifies run-time dependencies, via a list of packages to install prior to installing the current package. These are programs or libraries that are required in order to run the program. Note that libraries which are dynamically linked to an application will be automatically detected and added to RDEPENDS and therefore do not need to be explicitly declared. If a library was dynamically loaded then it would need to be explicitly listed.
If we take openssh for an example, it requires zlib and openssl in order to both built and run. In the recipe we have:
DEPENDS = "zlib openssl"
This tells bitbake that it will need to build and stage zlib and openssl prior to trying to build openssh, since openssh requires both of them. Note that there is no RDEPENDS even though openssh requires both of them to run. The run time dependencies on libz1 (the name of the package containing the zlib library) and libssl0 (the name of the package containing the ssl library) are automatically determined and added via the auto shared libs dependency code.