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What is /boot/System.map ???

Posted on 2002-05-24
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Last Modified: 2013-12-16
What does this file do? (Redhat 7.x)
The kernel README doesnt mention it, but some other documents do. If I am booting various kernels for testing, etc, often the System.map does'nt match the kernel I booted, does that matter, if not- why is it there?  
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Question by:medent
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jlevie earned 100 total points
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The System.ma file is simply a loader map of the kernel and lists the addresses of kernel variables and functions. It is used by the klogd daemon if a protection fault occurs to translate the addresses into meaningful symbols. There are some drivers that need use System.map and some utilities, like lsof and dosemu (and probably others). The usual way to be able to have more than one kernel installed is to also have that kernel's System.map with some unique name, like perhaps System.map-2.4.18-test1 and use a symlink for System.map.
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by:medent
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It doesnt seem to make sense that I can setup grub.conf to boot multiple kernels, and initrds, but the not System.map .
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by:jlevie
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The the kernel and initrd can be specified to grub and it will use that definition once to boot the system. And since the kernel knows what its ID is it can find the modules dir as needed. The other, application level, things that use System.map have no easy way to figure out what system map is associated with the running kernel.

In a case where one is playing with kernels and has several around to boot with it would be possible to add some code to rc.local that figures out what kernel has just booted from 'uname -r' and adjusts the symlink as appropriate.
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