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Automatic Dependencies

When a package is built by RPM, if any file in the package's %files list is a shared library, the library's soname is automatically added to the list of capabilities the package provides. The soname is the name used to determine compatibility between different versions of a library.

Note that this is not a filename. In fact, no aspect of RPM's dependency processing is based on filenames. Many people new to RPM often make the assumption that a failed dependency represents a missing file. This is not the case.

Remember that RPM's dependency processing is based on knowing what capabilities are provided by a package and what capabilities a package requires. We've seen how RPM automatically determines what shared library resources a package provides. But does it automatically determine what shared libraries a package requires?

Yes! RPM does this by running ldd on every executable program in a package's %files list. Since ldd provides a list of the shared libraries each program requires, both halves of the equation are complete — that is, the packages that make shared libraries available, and the packages that require those shared libraries, are tracked by RPM. RPM can then take that information into account when packages are installed, upgraded, or erased.

The Automatic Dependency Scripts

RPM uses two scripts to handle automatic dependency processing. They reside in /usr/bin and are called find-requires, and find-provides. We'll take a look at them in a minute, but first let's look at why there are scripts to do this sort of thing. Wouldn't it be better to have this built into RPM itself?

Actually, creating scripts for this sort of thing is a better idea. The reason? RPM has already been ported to a variety of different operating systems. Determining what shared libraries an executable requires, and the soname of shared libraries, is simple, but the exact steps required vary widely from one operating system to another. Putting this part of RPM into a script makes it easier to port RPM.

Let's take a look at the scripts that are used by RPM under the Linux operating system.

find-requires — Automatically Determine Shared Library Requirements

The find-requires script for Linux is quite simple:

# note this works for both a.out and ELF executables

ulimit -c 0

filelist=`xargs -r file | fgrep executable | cut -d: -f1 `

for f in $filelist; do
    ldd $f | awk '/=>/ { print $1 }'
done | sort -u | xargs -r -n 1 basename | sort -u

This script first creates a list of executable files. Then, for each file in the list, ldd determines the file's shared library requirements, producing a list of sonames. Finally, the list of sonames is sanitized by removing duplicates, and removing any paths.

find-provides — Automatically Determine Shared Library Sonames

The find-provides script for Linux is a bit more complex, but still pretty straightforward:

# This script reads filenames from STDIN and outputs any relevant
# provides information that needs to be included in the package.

filelist=$(grep ".so" | grep -v "^/lib/ld.so" | 
xargs file -L 2>/dev/null | grep "ELF.*shared object" | cut -d: -f1)

for f in $filelist; do
    soname=$(objdump -p $f | awk '/SONAME/ {print $2}')

    if [ "$soname" != "" ]; then
        if [ ! -L $f ]; then
            echo $soname
        echo ${f##*/}
done | sort -u

First, a list of shared libraries is created. Then, for each file on the list, the soname is extracted, cleaned up, and duplicates removed.

Automatic Dependencies: An Example

Let's take a widely used program, ls, the directory lister, as an example. On a Red Hat Linux system, ls is part of the fileutils package and is installed in /bin. Let's play the part of RPM during fileutils' package build and run find-requires on /bin/ls. Here's what we'll see:
# find-requires
The find-requires script returned libc.so.5. Therefore, RPM should add a requirement for libc.so.5 when the fileutils package is built. We can verify that RPM did add ls' requirement for libc.so.5 by using RPM's --requires option to display fileutils' requirements:
# rpm -q --requires fileutils
OK, that's the first half of the equation — RPM automatically detecting a package's shared library requirements. Now let's look at the second half of the equation -- RPM detecting packages that provide shared libraries. Since the libc package includes, among others, the shared library /lib/libc.so.5.3.12, RPM would obtain its soname. We can simulate this by using find-provides to print out the library's soname:
# find-provides
OK, so /lib/libc.so.5.3.12's soname is libc.so.5. Let's see if the libc package really does "provide" the libc.so.5 soname:
# rpm -q --provides libc

Yes, there it is, along with the soname of another library contained in the package. In this way, RPM can ensure that any package requiring libc.so.5 will have a compatible library available as long as the libc package, which provides libc.so.5, is installed.

In most cases, automatic dependencies are enough to fill the bill. However, there are circumstances when the package builder has to manually add dependency information to a package. Fortunately, RPM's approach to manual dependencies is both simple and flexible.

The autoreqprov Tag — Disable Automatic Dependency Processing

There may be times when RPM's automatic dependency processing is not desired. In these cases, the autoreqprov tag may be used to disable it. This tag takes a yes/no or 0/1 value. For example, to disable automatic dependency processing, the following line may be used:
AutoReqProv: no

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