mirror of https://github.com/mkerrisk/man-pages
840 lines
24 KiB
Groff
840 lines
24 KiB
Groff
.\" %%%LICENSE_START(PUBLIC_DOMAIN)
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.\" This is in the public domain
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.\" %%%LICENSE_END
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.\" Various parts:
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.\" Copyright (C) 2007-9, 2013, 2016 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.TH LD.SO 8 2021-03-22 "GNU" "Linux Programmer's Manual"
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.SH NAME
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ld.so, ld\-linux.so \- dynamic linker/loader
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.SH SYNOPSIS
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The dynamic linker can be run either indirectly by running some
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dynamically linked program or shared object
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(in which case no command-line options
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to the dynamic linker can be passed and, in the ELF case, the dynamic linker
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which is stored in the
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.B .interp
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section of the program is executed) or directly by running:
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.PP
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.I /lib/ld\-linux.so.*
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[OPTIONS] [PROGRAM [ARGUMENTS]]
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.SH DESCRIPTION
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The programs
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.B ld.so
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and
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.B ld\-linux.so*
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find and load the shared objects (shared libraries) needed by a program,
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prepare the program to run, and then run it.
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.PP
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Linux binaries require dynamic linking (linking at run time)
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unless the
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.B \-static
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option was given to
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.BR ld (1)
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during compilation.
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.PP
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The program
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.B ld.so
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handles a.out binaries, a binary format used long ago.
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The program
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.B ld\-linux.so*
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(\fI/lib/ld\-linux.so.1\fP for libc5, \fI/lib/ld\-linux.so.2\fP for glibc2)
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handles binaries that are in the more modern ELF format.
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Both programs have the same behavior, and use the same
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support files and programs
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.RB ( ldd (1),
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.BR ldconfig (8),
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and
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.IR /etc/ld.so.conf ).
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.PP
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When resolving shared object dependencies,
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the dynamic linker first inspects each dependency
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string to see if it contains a slash (this can occur if
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a shared object pathname containing slashes was specified at link time).
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If a slash is found, then the dependency string is interpreted as
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a (relative or absolute) pathname,
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and the shared object is loaded using that pathname.
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.PP
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If a shared object dependency does not contain a slash,
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then it is searched for in the following order:
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.IP o 3
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Using the directories specified in the
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DT_RPATH dynamic section attribute
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of the binary if present and DT_RUNPATH attribute does not exist.
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Use of DT_RPATH is deprecated.
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.IP o
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Using the environment variable
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.BR LD_LIBRARY_PATH ,
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unless the executable is being run in secure-execution mode (see below),
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in which case this variable is ignored.
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.IP o
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Using the directories specified in the
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DT_RUNPATH dynamic section attribute
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of the binary if present.
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Such directories are searched only to
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find those objects required by DT_NEEDED (direct dependencies) entries
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and do not apply to those objects' children,
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which must themselves have their own DT_RUNPATH entries.
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This is unlike DT_RPATH, which is applied
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to searches for all children in the dependency tree.
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.IP o
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From the cache file
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.IR /etc/ld.so.cache ,
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which contains a compiled list of candidate shared objects previously found
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in the augmented library path.
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If, however, the binary was linked with the
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.B \-z nodeflib
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linker option, shared objects in the default paths are skipped.
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Shared objects installed in hardware capability directories (see below)
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are preferred to other shared objects.
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.IP o
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In the default path
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.IR /lib ,
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and then
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.IR /usr/lib .
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(On some 64-bit architectures, the default paths for 64-bit shared objects are
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.IR /lib64 ,
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and then
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.IR /usr/lib64 .)
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If the binary was linked with the
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.B \-z nodeflib
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linker option, this step is skipped.
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.\"
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.SS Dynamic string tokens
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In several places, the dynamic linker expands dynamic string tokens:
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.IP o 3
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In the environment variables
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.BR LD_LIBRARY_PATH ,
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.BR LD_PRELOAD ,
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and
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.BR LD_AUDIT ,
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.IP o 3
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inside the values of the dynamic section tags
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.BR DT_NEEDED ,
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.BR DT_RPATH ,
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.BR DT_RUNPATH ,
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.BR DT_AUDIT ,
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and
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.BR DT_DEPAUDIT
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of ELF binaries,
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.IP o 3
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in the arguments to the
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.B ld.so
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command line options
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.BR \-\-audit ,
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.BR \-\-library\-path ,
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and
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.B \-\-preload
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(see below), and
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.IP o 3
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in the filename arguments to the
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.BR dlopen (3)
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and
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.BR dlmopen (3)
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functions.
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.PP
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The substituted tokens are as follows:
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.TP
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.IR $ORIGIN " (or equivalently " ${ORIGIN} )
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This expands to
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the directory containing the program or shared object.
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Thus, an application located in
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.I somedir/app
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could be compiled with
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.IP
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.in +4n
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.EX
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gcc \-Wl,\-rpath,\(aq$ORIGIN/../lib\(aq
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.EE
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.in
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.IP
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so that it finds an associated shared object in
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.I somedir/lib
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no matter where
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.I somedir
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is located in the directory hierarchy.
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This facilitates the creation of "turn-key" applications that
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do not need to be installed into special directories,
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but can instead be unpacked into any directory
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and still find their own shared objects.
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.TP
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.IR $LIB " (or equivalently " ${LIB} )
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This expands to
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.I lib
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or
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.I lib64
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depending on the architecture
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(e.g., on x86-64, it expands to
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.IR lib64
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and
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on x86-32, it expands to
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.IR lib ).
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.TP
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.IR $PLATFORM " (or equivalently " ${PLATFORM} )
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This expands to a string corresponding to the processor type
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of the host system (e.g., "x86_64").
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On some architectures, the Linux kernel doesn't provide a platform
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string to the dynamic linker.
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The value of this string is taken from the
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.BR AT_PLATFORM
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value in the auxiliary vector (see
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.BR getauxval (3)).
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.\" To get an idea of the places that $PLATFORM would match,
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.\" look at the output of the following:
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.\"
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.\" mkdir /tmp/d
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.\" LD_LIBRARY_PATH=/tmp/d strace -e open /bin/date 2>&1 | grep /tmp/d
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.\"
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.\" ld.so lets names be abbreviated, so $O will work for $ORIGIN;
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.\" Don't do this!!
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.PP
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Note that the dynamic string tokens have to be quoted properly when
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set from a shell,
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to prevent their expansion as shell or environment variables.
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.SH OPTIONS
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.TP
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.BR \-\-argv0 " \fIstring\fP (since glibc 2.33)"
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Set
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.I argv[0]
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to the value
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.I string
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before running the program.
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.TP
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.BI \-\-audit " list"
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Use objects named in
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.I list
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as auditors.
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The objects in
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.I list
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are delimited by colons.
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.TP
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.B \-\-inhibit\-cache
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Do not use
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.IR /etc/ld.so.cache .
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.TP
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.BI \-\-library\-path " path"
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Use
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.I path
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instead of
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.B LD_LIBRARY_PATH
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environment variable setting (see below).
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The names
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.IR ORIGIN ,
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.IR LIB ,
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and
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.IR PLATFORM
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are interpreted as for the
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.BR LD_LIBRARY_PATH
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environment variable.
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.TP
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.BI \-\-inhibit\-rpath " list"
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Ignore RPATH and RUNPATH information in object names in
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.IR list .
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This option is ignored when running in secure-execution mode (see below).
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The objects in
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.I list
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are delimited by colons or spaces.
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.TP
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.B \-\-list
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List all dependencies and how they are resolved.
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.TP
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.BR \-\-preload " \fIlist\fP (since glibc 2.30)"
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Preload the objects specified in
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.IR list .
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The objects in
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.I list
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are delimited by colons or spaces.
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The objects are preloaded as explained in the description of the
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.BR LD_PRELOAD
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environment variable below.
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.IP
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By contrast with
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.BR LD_PRELOAD ,
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the
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.BR \-\-preload
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option provides a way to perform preloading for a single executable
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without affecting preloading performed in any child process that executes
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a new program.
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.TP
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.B \-\-verify
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Verify that program is dynamically linked and this dynamic linker can handle
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it.
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.SH ENVIRONMENT
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Various environment variables influence the operation of the dynamic linker.
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.\"
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.SS Secure-execution mode
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For security reasons,
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if the dynamic linker determines that a binary should be
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run in secure-execution mode,
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the effects of some environment variables are voided or modified,
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and furthermore those environment variables are stripped from the environment,
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so that the program does not even see the definitions.
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Some of these environment variables affect the operation of
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the dynamic linker itself, and are described below.
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Other environment variables treated in this way include:
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.BR GCONV_PATH ,
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.BR GETCONF_DIR ,
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.BR HOSTALIASES ,
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.BR LOCALDOMAIN ,
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.BR LOCPATH ,
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.BR MALLOC_TRACE ,
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.BR NIS_PATH ,
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.BR NLSPATH ,
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.BR RESOLV_HOST_CONF ,
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.BR RES_OPTIONS ,
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.BR TMPDIR ,
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and
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.BR TZDIR .
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.PP
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A binary is executed in secure-execution mode if the
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.B AT_SECURE
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entry in the auxiliary vector (see
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.BR getauxval (3))
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has a nonzero value.
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This entry may have a nonzero value for various reasons, including:
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.IP * 3
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The process's real and effective user IDs differ,
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or the real and effective group IDs differ.
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This typically occurs as a result of executing
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a set-user-ID or set-group-ID program.
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.IP *
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A process with a non-root user ID executed a binary that
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conferred capabilities to the process.
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.IP *
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A nonzero value may have been set by a Linux Security Module.
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.\"
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.SS Environment variables
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Among the more important environment variables are the following:
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.TP
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.BR LD_ASSUME_KERNEL " (since glibc 2.2.3)"
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Each shared object can inform the dynamic linker of the minimum kernel ABI
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version that it requires.
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(This requirement is encoded in an ELF note section that is viewable via
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.IR "readelf\ \-n"
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as a section labeled
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.BR NT_GNU_ABI_TAG .)
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At run time,
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the dynamic linker determines the ABI version of the running kernel and
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will reject loading shared objects that specify minimum ABI versions
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that exceed that ABI version.
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.IP
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.BR LD_ASSUME_KERNEL
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can be used to
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cause the dynamic linker to assume that it is running on a system with
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a different kernel ABI version.
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For example, the following command line causes the
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dynamic linker to assume it is running on Linux 2.2.5 when loading
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the shared objects required by
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.IR myprog :
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.IP
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.in +4n
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.EX
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$ \fBLD_ASSUME_KERNEL=2.2.5 ./myprog\fP
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.EE
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.in
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.IP
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On systems that provide multiple versions of a shared object
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(in different directories in the search path) that have
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different minimum kernel ABI version requirements,
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.BR LD_ASSUME_KERNEL
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can be used to select the version of the object that is used
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(dependent on the directory search order).
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.IP
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Historically, the most common use of the
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.BR LD_ASSUME_KERNEL
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feature was to manually select the older
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LinuxThreads POSIX threads implementation on systems that provided both
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LinuxThreads and NPTL
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(which latter was typically the default on such systems);
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see
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.BR pthreads (7).
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.TP
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.BR LD_BIND_NOW " (since glibc 2.1.1)"
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If set to a nonempty string,
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causes the dynamic linker to resolve all symbols
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at program startup instead of deferring function call resolution to the point
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when they are first referenced.
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This is useful when using a debugger.
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.TP
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.B LD_LIBRARY_PATH
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A list of directories in which to search for
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ELF libraries at execution time.
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The items in the list are separated by either colons or semicolons,
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and there is no support for escaping either separator.
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A zero-length directory name indicates the current working directory.
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.IP
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This variable is ignored in secure-execution mode.
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.IP
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Within the pathnames specified in
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.BR LD_LIBRARY_PATH ,
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the dynamic linker expands the tokens
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.IR $ORIGIN ,
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.IR $LIB ,
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and
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.IR $PLATFORM
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(or the versions using curly braces around the names)
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as described above in
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.IR "Dynamic string tokens" .
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Thus, for example,
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the following would cause a library to be searched for in either the
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.I lib
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or
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.I lib64
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subdirectory below the directory containing the program to be executed:
|
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.IP
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.in +4n
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.EX
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$ \fBLD_LIBRARY_PATH=\(aq$ORIGIN/$LIB\(aq prog\fP
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.EE
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.in
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.IP
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(Note the use of single quotes, which prevent expansion of
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.I $ORIGIN
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and
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.I $LIB
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as shell variables!)
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.TP
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.B LD_PRELOAD
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A list of additional, user-specified, ELF shared
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objects to be loaded before all others.
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This feature can be used to selectively override functions
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in other shared objects.
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.IP
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The items of the list can be separated by spaces or colons,
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and there is no support for escaping either separator.
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The objects are searched for using the rules given under DESCRIPTION.
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Objects are searched for and added to the link map in the left-to-right
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order specified in the list.
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.IP
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In secure-execution mode,
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preload pathnames containing slashes are ignored.
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Furthermore, shared objects are preloaded only
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from the standard search directories and only
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if they have set-user-ID mode bit enabled (which is not typical).
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.IP
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Within the names specified in the
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.BR LD_PRELOAD
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list, the dynamic linker understands the tokens
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.IR $ORIGIN ,
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.IR $LIB ,
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and
|
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.IR $PLATFORM
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(or the versions using curly braces around the names)
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as described above in
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.IR "Dynamic string tokens" .
|
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(See also the discussion of quoting under the description of
|
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.BR LD_LIBRARY_PATH .)
|
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.\" Tested with the following:
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.\"
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.\" LD_PRELOAD='$LIB/libmod.so' LD_LIBRARY_PATH=. ./prog
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.\"
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.\" which will preload the libmod.so in 'lib' or 'lib64', using it
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.\" in preference to the version in '.'.
|
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.IP
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There are various methods of specifying libraries to be preloaded,
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and these are handled in the following order:
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.RS
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.IP (1) 4
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The
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.BR LD_PRELOAD
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environment variable.
|
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.IP (2)
|
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The
|
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.B \-\-preload
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command-line option when invoking the dynamic linker directly.
|
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.IP (3)
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The
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.I /etc/ld.so.preload
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file (described below).
|
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.RE
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.TP
|
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.BR LD_TRACE_LOADED_OBJECTS
|
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If set (to any value), causes the program to list its dynamic
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dependencies, as if run by
|
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.BR ldd (1),
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instead of running normally.
|
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.PP
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Then there are lots of more or less obscure variables,
|
|
many obsolete or only for internal use.
|
|
.TP
|
|
.BR LD_AUDIT " (since glibc 2.4)"
|
|
A list of user-specified, ELF shared objects
|
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to be loaded before all others in a separate linker namespace
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(i.e., one that does not intrude upon the normal symbol bindings that
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would occur in the process)
|
|
These objects can be used to audit the operation of the dynamic linker.
|
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The items in the list are colon-separated,
|
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and there is no support for escaping the separator.
|
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.IP
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.B LD_AUDIT
|
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is ignored in secure-execution mode.
|
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.IP
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The dynamic linker will notify the audit
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shared objects at so-called auditing checkpoints\(emfor example,
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loading a new shared object, resolving a symbol,
|
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or calling a symbol from another shared object\(emby
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calling an appropriate function within the audit shared object.
|
|
For details, see
|
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.BR rtld\-audit (7).
|
|
The auditing interface is largely compatible with that provided on Solaris,
|
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as described in its
|
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.IR "Linker and Libraries Guide" ,
|
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in the chapter
|
|
.IR "Runtime Linker Auditing Interface" .
|
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.IP
|
|
Within the names specified in the
|
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.BR LD_AUDIT
|
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list, the dynamic linker understands the tokens
|
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.IR $ORIGIN ,
|
|
.IR $LIB ,
|
|
and
|
|
.IR $PLATFORM
|
|
(or the versions using curly braces around the names)
|
|
as described above in
|
|
.IR "Dynamic string tokens" .
|
|
(See also the discussion of quoting under the description of
|
|
.BR LD_LIBRARY_PATH .)
|
|
.IP
|
|
Since glibc 2.13,
|
|
.\" commit 8e9f92e9d5d7737afdacf79b76d98c4c42980508
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in secure-execution mode,
|
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names in the audit list that contain slashes are ignored,
|
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and only shared objects in the standard search directories that
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have the set-user-ID mode bit enabled are loaded.
|
|
.TP
|
|
.BR LD_BIND_NOT " (since glibc 2.1.95)"
|
|
If this environment variable is set to a nonempty string,
|
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do not update the GOT (global offset table) and PLT (procedure linkage table)
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after resolving a function symbol.
|
|
By combining the use of this variable with
|
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.BR LD_DEBUG
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(with the categories
|
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.IR bindings
|
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and
|
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.IR symbols ),
|
|
one can observe all run-time function bindings.
|
|
.TP
|
|
.BR LD_DEBUG " (since glibc 2.1)"
|
|
Output verbose debugging information about operation of the dynamic linker.
|
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The content of this variable is one of more of the following categories,
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separated by colons, commas, or (if the value is quoted) spaces:
|
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.RS
|
|
.TP 12
|
|
.I help
|
|
Specifying
|
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.IR help
|
|
in the value of this variable does not run the specified program,
|
|
and displays a help message about which categories can be specified in this
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|
environment variable.
|
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.TP
|
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.I all
|
|
Print all debugging information (except
|
|
.IR statistics
|
|
and
|
|
.IR unused ;
|
|
see below).
|
|
.TP
|
|
.I bindings
|
|
Display information about which definition each symbol is bound to.
|
|
.TP
|
|
.I files
|
|
Display progress for input file.
|
|
.TP
|
|
.I libs
|
|
Display library search paths.
|
|
.TP
|
|
.I reloc
|
|
Display relocation processing.
|
|
.TP
|
|
.I scopes
|
|
Display scope information.
|
|
.TP
|
|
.I statistics
|
|
Display relocation statistics.
|
|
.TP
|
|
.I symbols
|
|
Display search paths for each symbol look-up.
|
|
.TP
|
|
.I unused
|
|
Determine unused DSOs.
|
|
.TP
|
|
.I versions
|
|
Display version dependencies.
|
|
.RE
|
|
.IP
|
|
Since glibc 2.3.4,
|
|
.B LD_DEBUG
|
|
is ignored in secure-execution mode, unless the file
|
|
.IR /etc/suid\-debug
|
|
exists (the content of the file is irrelevant).
|
|
.TP
|
|
.BR LD_DEBUG_OUTPUT " (since glibc 2.1)"
|
|
By default,
|
|
.B LD_DEBUG
|
|
output is written to standard error.
|
|
If
|
|
.B LD_DEBUG_OUTPUT
|
|
is defined, then output is written to the pathname specified by its value,
|
|
with the suffix "." (dot) followed by the process ID appended to the pathname.
|
|
.IP
|
|
.B LD_DEBUG_OUTPUT
|
|
is ignored in secure-execution mode.
|
|
.TP
|
|
.BR LD_DYNAMIC_WEAK " (since glibc 2.1.91)"
|
|
By default, when searching shared libraries to resolve a symbol reference,
|
|
the dynamic linker will resolve to the first definition it finds.
|
|
.IP
|
|
Old glibc versions (before 2.2), provided a different behavior:
|
|
if the linker found a symbol that was weak,
|
|
it would remember that symbol and
|
|
keep searching in the remaining shared libraries.
|
|
If it subsequently found a strong definition of the same symbol,
|
|
then it would instead use that definition.
|
|
(If no further symbol was found,
|
|
then the dynamic linker would use the weak symbol that it initially found.)
|
|
.IP
|
|
The old glibc behavior was nonstandard.
|
|
(Standard practice is that the distinction between
|
|
weak and strong symbols should have effect only at static link time.)
|
|
In glibc 2.2,
|
|
.\" More precisely 2.1.92
|
|
.\" See weak handling
|
|
.\" https://www.sourceware.org/ml/libc-hacker/2000-06/msg00029.html
|
|
.\" To: GNU libc hacker <libc-hacker at sourceware dot cygnus dot com>
|
|
.\" Subject: weak handling
|
|
.\" From: Ulrich Drepper <drepper at redhat dot com>
|
|
.\" Date: 07 Jun 2000 20:08:12 -0700
|
|
.\" Reply-To: drepper at cygnus dot com (Ulrich Drepper)
|
|
the dynamic linker was modified to provide the current behavior
|
|
(which was the behavior that was provided by most other implementations
|
|
at that time).
|
|
.IP
|
|
Defining the
|
|
.B LD_DYNAMIC_WEAK
|
|
environment variable (with any value) provides
|
|
the old (nonstandard) glibc behavior,
|
|
whereby a weak symbol in one shared library may be overridden by
|
|
a strong symbol subsequently discovered in another shared library.
|
|
(Note that even when this variable is set,
|
|
a strong symbol in a shared library will not override
|
|
a weak definition of the same symbol in the main program.)
|
|
.IP
|
|
Since glibc 2.3.4,
|
|
.B LD_DYNAMIC_WEAK
|
|
is ignored in secure-execution mode.
|
|
.TP
|
|
.BR LD_HWCAP_MASK " (since glibc 2.1)"
|
|
Mask for hardware capabilities.
|
|
.TP
|
|
.BR LD_ORIGIN_PATH " (since glibc 2.1)"
|
|
Path where the binary is found.
|
|
.\" Used only if $ORIGIN can't be determined by normal means
|
|
.\" (from the origin path saved at load time, or from /proc/self/exe)?
|
|
.IP
|
|
Since glibc 2.4,
|
|
.B LD_ORIGIN_PATH
|
|
is ignored in secure-execution mode.
|
|
.TP
|
|
.BR LD_POINTER_GUARD " (glibc from 2.4 to 2.22)"
|
|
Set to 0 to disable pointer guarding.
|
|
Any other value enables pointer guarding, which is also the default.
|
|
Pointer guarding is a security mechanism whereby some pointers to code
|
|
stored in writable program memory (return addresses saved by
|
|
.BR setjmp (3)
|
|
or function pointers used by various glibc internals) are mangled
|
|
semi-randomly to make it more difficult for an attacker to hijack
|
|
the pointers for use in the event of a buffer overrun or
|
|
stack-smashing attack.
|
|
Since glibc 2.23,
|
|
.\" commit a014cecd82b71b70a6a843e250e06b541ad524f7
|
|
.B LD_POINTER_GUARD
|
|
can no longer be used to disable pointer guarding,
|
|
which is now always enabled.
|
|
.TP
|
|
.BR LD_PROFILE " (since glibc 2.1)"
|
|
The name of a (single) shared object to be profiled,
|
|
specified either as a pathname or a soname.
|
|
Profiling output is appended to the file whose name is:
|
|
"\fI$LD_PROFILE_OUTPUT\fP/\fI$LD_PROFILE\fP.profile".
|
|
.IP
|
|
Since glibc 2.2.5,
|
|
.BR LD_PROFILE
|
|
is ignored in secure-execution mode.
|
|
.TP
|
|
.BR LD_PROFILE_OUTPUT " (since glibc 2.1)"
|
|
Directory where
|
|
.B LD_PROFILE
|
|
output should be written.
|
|
If this variable is not defined, or is defined as an empty string,
|
|
then the default is
|
|
.IR /var/tmp .
|
|
.IP
|
|
.B LD_PROFILE_OUTPUT
|
|
is ignored in secure-execution mode; instead
|
|
.IR /var/profile
|
|
is always used.
|
|
(This detail is relevant only before glibc 2.2.5,
|
|
since in later glibc versions,
|
|
.B LD_PROFILE
|
|
is also ignored in secure-execution mode.)
|
|
.TP
|
|
.BR LD_SHOW_AUXV " (since glibc 2.1)"
|
|
If this environment variable is defined (with any value),
|
|
show the auxiliary array passed up from the kernel (see also
|
|
.BR getauxval (3)).
|
|
.IP
|
|
Since glibc 2.3.4,
|
|
.B LD_SHOW_AUXV
|
|
is ignored in secure-execution mode.
|
|
.TP
|
|
.BR LD_TRACE_PRELINKING " (since glibc 2.4)"
|
|
If this environment variable is defined,
|
|
trace prelinking of the object whose name is assigned to
|
|
this environment variable.
|
|
(Use
|
|
.BR ldd (1)
|
|
to get a list of the objects that might be traced.)
|
|
If the object name is not recognized,
|
|
.\" (This is what seems to happen, from experimenting)
|
|
then all prelinking activity is traced.
|
|
.TP
|
|
.BR LD_USE_LOAD_BIAS " (since glibc 2.3.3)"
|
|
.\" http://sources.redhat.com/ml/libc-hacker/2003-11/msg00127.html
|
|
.\" Subject: [PATCH] Support LD_USE_LOAD_BIAS
|
|
.\" Jakub Jelinek
|
|
By default (i.e., if this variable is not defined),
|
|
executables and prelinked
|
|
shared objects will honor base addresses of their dependent shared objects
|
|
and (nonprelinked) position-independent executables (PIEs)
|
|
and other shared objects will not honor them.
|
|
If
|
|
.B LD_USE_LOAD_BIAS
|
|
is defined with the value 1, both executables and PIEs
|
|
will honor the base addresses.
|
|
If
|
|
.B LD_USE_LOAD_BIAS
|
|
is defined with the value 0,
|
|
neither executables nor PIEs will honor the base addresses.
|
|
.IP
|
|
Since glibc 2.3.3, this variable is ignored in secure-execution mode.
|
|
.TP
|
|
.BR LD_VERBOSE " (since glibc 2.1)"
|
|
If set to a nonempty string,
|
|
output symbol versioning information about the
|
|
program if the
|
|
.B LD_TRACE_LOADED_OBJECTS
|
|
environment variable has been set.
|
|
.TP
|
|
.BR LD_WARN " (since glibc 2.1.3)"
|
|
If set to a nonempty string, warn about unresolved symbols.
|
|
.TP
|
|
.BR LD_PREFER_MAP_32BIT_EXEC " (x86-64 only; since glibc 2.23)"
|
|
According to the Intel Silvermont software optimization guide, for 64-bit
|
|
applications, branch prediction performance can be negatively impacted
|
|
when the target of a branch is more than 4\ GB away from the branch.
|
|
If this environment variable is set (to any value),
|
|
the dynamic linker
|
|
will first try to map executable pages using the
|
|
.BR mmap (2)
|
|
.BR MAP_32BIT
|
|
flag, and fall back to mapping without that flag if that attempt fails.
|
|
NB: MAP_32BIT will map to the low 2\ GB (not 4\ GB) of the address space.
|
|
.IP
|
|
Because
|
|
.B MAP_32BIT
|
|
reduces the address range available for address space layout
|
|
randomization (ASLR),
|
|
.B LD_PREFER_MAP_32BIT_EXEC
|
|
is always disabled in secure-execution mode.
|
|
.SH FILES
|
|
.TP
|
|
.I /lib/ld.so
|
|
a.out dynamic linker/loader
|
|
.TP
|
|
.IR /lib/ld\-linux.so. { 1 , 2 }
|
|
ELF dynamic linker/loader
|
|
.TP
|
|
.I /etc/ld.so.cache
|
|
File containing a compiled list of directories in which to search for
|
|
shared objects and an ordered list of candidate shared objects.
|
|
See
|
|
.BR ldconfig (8).
|
|
.TP
|
|
.I /etc/ld.so.preload
|
|
File containing a whitespace-separated list of ELF shared objects to
|
|
be loaded before the program.
|
|
See the discussion of
|
|
.BR LD_PRELOAD
|
|
above.
|
|
If both
|
|
.BR LD_PRELOAD
|
|
and
|
|
.I /etc/ld.so.preload
|
|
are employed, the libraries specified by
|
|
.BR LD_PRELOAD
|
|
are preloaded first.
|
|
.I /etc/ld.so.preload
|
|
has a system-wide effect,
|
|
causing the specified libraries to be preloaded for
|
|
all programs that are executed on the system.
|
|
(This is usually undesirable,
|
|
and is typically employed only as an emergency remedy, for example,
|
|
as a temporary workaround to a library misconfiguration issue.)
|
|
.TP
|
|
.I lib*.so*
|
|
shared objects
|
|
.SH NOTES
|
|
.SS Hardware capabilities
|
|
Some shared objects are compiled using hardware-specific instructions which do
|
|
not exist on every CPU.
|
|
Such objects should be installed in directories whose names define the
|
|
required hardware capabilities, such as
|
|
.IR /usr/lib/sse2/ .
|
|
The dynamic linker checks these directories against the hardware of the
|
|
machine and selects the most suitable version of a given shared object.
|
|
Hardware capability directories can be cascaded to combine CPU features.
|
|
The list of supported hardware capability names depends on the CPU.
|
|
The following names are currently recognized:
|
|
.\" Presumably, this info comes from sysdeps/i386/dl-procinfo.c and
|
|
.\" similar files
|
|
.TP
|
|
.B Alpha
|
|
ev4, ev5, ev56, ev6, ev67
|
|
.TP
|
|
.B MIPS
|
|
loongson2e, loongson2f, octeon, octeon2
|
|
.TP
|
|
.B PowerPC
|
|
4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efpdouble, efpsingle,
|
|
fpu, ic_snoop, mmu, notb, pa6t, power4, power5, power5+, power6x, ppc32, ppc601,
|
|
ppc64, smt, spe, ucache, vsx
|
|
.TP
|
|
.B SPARC
|
|
flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
|
|
.TP
|
|
.B s390
|
|
dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
|
|
z900, z990, z9-109, z10, zarch
|
|
.TP
|
|
.B x86 (32-bit only)
|
|
acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586, i686, mca, mmx,
|
|
mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2, tm
|
|
.SH SEE ALSO
|
|
.BR ld (1),
|
|
.BR ldd (1),
|
|
.BR pldd (1),
|
|
.BR sprof (1),
|
|
.BR dlopen (3),
|
|
.BR getauxval (3),
|
|
.BR elf (5),
|
|
.BR capabilities (7),
|
|
.BR rtld\-audit (7),
|
|
.BR ldconfig (8),
|
|
.BR sln (8)
|
|
.\" .SH AUTHORS
|
|
.\" ld.so: David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
|
|
.\" Torvalds, Lars Wirzenius and Mitch D'Souza
|
|
.\" ld\-linux.so: Roland McGrath, Ulrich Drepper and others.
|
|
.\"
|
|
.\" In the above, (libc5) stands for David Engel's ld.so/ld\-linux.so.
|