Книга: Embedded Linux Primer: A Practical, Real-World Approach

6.2.2. Resolving Dependencies

6.2.2. Resolving Dependencies

It is not sufficient to simply include an executable such as init on your file system and expect it to boot. For every process you place on your root file system, you must also satisfy its dependencies. Most processes have two categories of dependencies: those that are needed to resolve unresolved references within a dynamically linked executable, and external configuration or data files that an application might need. We have a tool to find the former, but the latter can be supplied only by at least a cursory understanding of the application in question.

An example will help make this clear. The init process is a dynamically linked executable. To run init, we need to satisfy its library dependencies. A tool has been developed for this purpose: ldd. To understand what libraries a given application requires, simply run your cross-version of ldd on the binary:

 $ ppc_4xxFP-ldd init
        libc.so.6 => /opt/eldk/ppc_4xxFP/lib/libc.so.6
        ld.so.1 => /opt/eldk/ppc_4xxFP/lib/ld.so.1
 $

From this ldd output, we can see that the PowerPC init executable in this example is dependent on two libraries. These are the standard C library (libc.so.6) and the Linux dynamic loader (ld.so.1).

To satisfy the second category of dependencies for an executable, the configuration and data files that it might need, there is little substitute for some knowledge about how the subsystem works. For example, init expects to read its operational configuration from a data file called inittab located on /etc. Unless you are using a tool that has this knowledge built in, such as those described in the earlier Section 6.1.6, "Automated File System Build Tools," you must supply that knowledge.

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