Berkeley CSUA MOTD:Entry 21582
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2021/12/06 [General] UID:1000 Activity:popular
12/6    

2001/6/21-22 [Computer/HW/Memory, Computer/SW/OS/VM] UID:21582 Activity:moderate
6/20    I have an OS question.  Most programs make use of shared
        libraries and use a dynamic link loader to relocate branch
        targets at run-time.  The problem is that if you have
        multiple copies of, say, emacs running under their own
        user space wouldn't those relocated addresses (either relative
        or absolute) conflict under each process?  For example, if
        multiple programs were using OpenGL they would create virtual
        memory entries for libGL.so in their own process space. The
        location of libGL within this process differs from program to
        program so if libGL calls libm.so the dynamic link loader will
        place that library in a different location for each process
        and the branch targets under one process won't correspond to
        those of the other.
        \_ The shared bits are mapped to the same virtual addresses in
           each address space.
        \_ Not sure what you're asking.  But only the text portion of
           the shared lib is shared amongst the different processes.  All
           processes that need to use libGL.so have addresses that point to
           just one copy of the text portion of libGL.so.  It's the OS's
           job to keep the program counter and the VM straight.  Why would
           there be any conflict?
           \_ That's the problem. If you only have one copy of libGL.so
              in memory there would be a conflict in the outgoing branches
              from libGL.  For example, if we were calling glVertex
              within libGl. That function would have a jump an link
              to another absolute address to the libm math library. The
              dynamic link loader is responsible for resolving the branch
              addresses.  The problem is that if one user was running
              Quake while another person was  running Doom or something
              like that then the jump target addresses would be different.
              Let's just say, for example, that glVertex called the pow
              function:
              Quake                             Doom
              -----                             ----
              0x00000000 main                   0x00000000 main
              0x00003fff end of quake           0x00001fff end of doom
              0x00004000 libGL                  0x00002000 libGL
              0x00005000 libm                   0x00003000 libm
              0x00005040  pow()                 0x00003040  pow()
              If we were to have a jump and link to the pow() function
              then the addresses would be different in both copies of
              libGL.
        \_ Also, most, if not all, shared libraries are compiled to be
           position-independent.
2021/12/06 [General] UID:1000 Activity:popular
12/6    

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