Automatic Dependencies

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.

#!/bin/sh

# 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
            

#!/bin/bash

# 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
        fi
    else
        echo ${f##*/}
    fi
done | sort -u
            

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
/bin/ls
<ctrl-d>
libc.so.5
#
          

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
libc.so.5
#
          

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
/lib/libc.so.5.3.12
Ctrl-D
libc.so.5
#
          

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
libm.so.5
libc.so.5
#
          

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, autoreq, and autoprov Tags — Disable Automatic Dependency Processing

There may be times when RPM's automatic dependency processing is not desired. In these cases, the autoreqprov, autoreq, and autoprov tags 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
          

The autoreq and autoprov tags can be used to disable automatic processing of requirements or "provides" only, respectively.