prelink(8)						      System Manager's Manual							prelink(8)

NAME

       prelink - prelink ELF shared libraries and binaries to speed up startup time

SYNOPSIS

       prelink [OPTION...] [FILES]

DESCRIPTION

       prelink	is  a program which modifies ELF shared libraries and ELF dynamically linked binaries, so that the time which dynamic linker needs
       for their relocation at startup significantly decreases and also due to fewer relocations the run-time  memory  consumption  decreases  too
       (especially  number  of	unshareable pages). Such prelinking information is only used if all its dependant libraries have not changed since
       prelinking, otherwise programs are relocated normally.

       prelink first collects ELF binaries which should be prelinked and all the ELF shared libraries they depend on. Then  it	assigns  a  unique
       virtual	address space slot for each library and relinks the shared library to that base address.  When the dynamic linker attempts to load
       such a library, unless that virtual address space slot is already occupied, it will map it into the given slot.	After this is  done,  pre-
       link with the help of dynamic linker resolves all relocations in the binary or library against its dependant libraries and stores the relo-
       cations into the ELF object.  It also stores a list of all dependant libraries together with their checksums into the  binary  or  library.
       For  binaries,  it also computes a list of conflicts (relocations which resolve differently in the binary's symbol search scope than in the
       smaller search scope in which the dependant library was resolved) and stores it into a special ELF section.

       At runtime, the dynamic linker first checks whether all dependant libraries were successfully mapped into their	designated  address  space
       slots  and  whether they have not changed since the prelinking was done.  If all checks are successful, the dynamic linker just replays the
       list of conflicts (which is usually significantly shorter than total number of relocations) instead of relocating each library.

OPTIONS

       -v --verbose
	      Verbose mode.  Print the virtual address slot assignment to libraries and print what binary or library is currently being prelinked.

       -n --dry-run
	      Don't actually prelink anything, just collect the binaries/libraries, assign them addresses and with -v print  what  would  be  pre-
	      linked.

       -a --all
	      Prelink  all binaries and dependant libraries found in directory hierarchies specified in /etc/prelink.conf.  Normally only binaries
	      specified from command line and their dependant libraries are prelinked.

       -m --conserve-memory
	      When assigning addresses to libraries, allow overlap of address space slots provided that the two libraries are not present together
	      in  any  of  the binaries or libraries. This results in smaller virtual address space range used for libraries, on the other side if
	      during incremental prelinking prelink sees a binary which puts together two libraries which were not present together in	any  other
	      binary  and were given the same virtual address space slots, then the binary cannot be prelinked.  Normally each library is assigned
	      a unique virtual address space slot.

       -R --random
	      When assigning addresses to libraries, start with random address within architecture dependant virtual address  space  range.   This
	      can  make  some  buffer overflow attacks slightly harder to exploit, because libraries are not present on the same addresses accross
	      different machines.  Normally, assigning virtual addresses starts at the bottom of architecture dependant range.

       -r --reloc-only=ADDRESS
	      Instead of prelinking, just relink given shared libraries to the specified base address.

       -N --no-update-cache
	      Don't save cache file after prelinking. Normally, list of libraries (and with -m binaries also) is  stored  into	/etc/prelink.cache
	      file  together  with  their  given address space slots and dependencies, so it can be used during incremental prelinking (prelinking
	      without -a option).

       -c --config-file=CONFIG
	      Specify alternate config file instead of default /etc/prelink.conf.

       -C --cache-file=CACHE
	      Specify alternate cache file instead of default /etc/prelink.cache.

       -f --force
	      Force re-prelinking even for already prelinked objects for which no dependencies changed. This option  causes  new  virtual  address
	      space  slots  to	be assigned to all libraries.  Normally, only binaries or libraries which are either not prelinked yet, or some of
	      their dependencies changed, are prelinked.

       -p --print-cache
	      Print the content of the cache file (normally /etc/prelink.conf) and exit.

       --dynamic-linker=LDSO
	      Specify alternate dynamic linker instead of the default.

       --ld-library-path=PATH
	      Specify special LD_LIBRARY_PATH to be used when prelink queries dynamic linker about symbol resolution details.

       --libs-only
	      Only prelink ELF shared libraries, don't prelink any binaries.

       -h --dereference
	      When processing command line directory arguments, follow symbolic links when walking directory hierarchies.

       -l --one-file-system
	      When processing command line directory arguments, limit directory tree walk to a single filesystem.

       -u --undo
	      Revert binaries and libraries to their original content before they were prelinked.  Without -a option this causes only the binaries
	      and  libraries  specified  on  the  command  line  to be reverted to their original state (and e.g. not their dependencies). If used
	      together with -a option all binaries and libraries from command line, all their dependencies,  all  binaries  found  in  directories
	      specified on command line and in config file and all their dependencies will be undone.

       -y --verify
	      Verifies	a  prelinked  binary  or  library.  This option can be used only on a single binary or library. It first applies an --undo
	      operation on the file, then prelinks just that file again and compares this with the original file. If both are identical, it prints
	      the  file after --undo operation on standard output and exit with zero status. Otherwise it exits with error status.  Thus if --ver-
	      ify operation returns zero exit status and its standard output is equal to the content of the binary or library  before  prelinking,
	      you  can	be  sure  that	nobody	modified the binaries or libraries after prelinking.  Similarly with message digests and checksums
	      (unless you trigger the unprobable case of modified file and original file having the same digest or checksum).

       -V --version
	      Print version and exit.

       -? --help
	      Print short help and exit.

ARGUMENTS

       Command line arguments should be either directory hierarchies (in which case -l and -h options apply), or particular ELF binaries or shared
       libraries.   Unlike  when  walking  directory hierarchies, specifying a shared library explicitely on the command line causes it to be pre-
       linked even if no binary is linked against it. Normally, only binaries are collected together with all libraries they depend on.

EXAMPLES

	      # /usr/sbin/prelink -avmR
       will prelink all binaries found in directories specified in /etc/prelink.conf and all their dependant libraries, assigning libraries unique
       virtual address space slots only if they ever appear together and will start assigning at random address.
	      # /usr/sbin/prelink -vm ~/bin/progx
       will prelink ~/bin/progx program and all its dependant libraries (unless they were prelinked already e.g. during prelink -a invocation).
	      # /usr/sbin/prelink -au
       will revert all binaries and libraries to their original content.
	      #  /usr/sbin/prelink  -y	/bin/prelinked_prog  >	/tmp/original_prog;  echo  $?  will verify whether /bin/prelinked_prog hasn't been
	      changed.

FILES

       /etc/prelink.cache  Binary file containing list of prelinked libraries and/or binaries together with their assigned virtual  address  space
			   slots and dependencies.  You can run /usr/sbin/prelink -p to see what is stored in there.
       /etc/prelink.conf   Configuration  file containing a list of directory hierarchies which can contain ELF shared libraries or binaries which
			   should be prelinked.  This configuration file is used in -a mode to find binaries which should be prelinked and also no
			   matter  whether  -a	is  given or not to limit which dependant shared libraries should be prelinked. If prelink finds a
			   dependant library of some binary or other library which is not present in any of the directories specified in /etc/pre-
			   link.conf  and neither in any of the directories specified on the command line, then it cannot be prelinked.  Each line
			   of the config file should be either comment starting with #, or directory name. It can be prefixed by -l switch,  mean-
			   ing	tree  walk  of	the given directory will be only limited to one filesystem, or -h switch, meaning tree walk of the
			   given directory will follow symbolic links.
SEE ALSO

       ldd(1), ld.so(8).
BUGS

       prelink Some architectures, including IA-64, HPPA and MIPS are not yet supported.
AUTHORS

       Jakub Jelinek <jakub@redhat.com>.

								   14 July 2002 							prelink(8)