mirror of https://github.com/mkerrisk/man-pages
428 lines
13 KiB
Groff
428 lines
13 KiB
Groff
.\" This is in the public domain
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.TH LD.SO 8 2012-08-26 "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 library (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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.P
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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 libraries needed by a program, prepare
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the program to run, and then run it.
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.LP
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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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.LP
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The program
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.B ld.so
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handles a.out binaries, a format used long ago;
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.B ld-linux.so*
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handles ELF (\fI/lib/ld-linux.so.1\fP for libc5, \fI/lib/ld-linux.so.2\fP
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for glibc2), which everybody has been using for years now.
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Otherwise both have the same behavior, and use the same
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support files and programs
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.BR 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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.LP
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When resolving library 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 library 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 library is loaded using that pathname.
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.LP
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If a library 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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(ELF only) 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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Except if the executable is a set-user-ID/set-group-ID binary,
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in which case it is ignored.
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.IP o
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(ELF only) 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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.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 libraries 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, libraries in the default library paths are skipped.
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Libraries installed in hardware capability directories (see below)
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are preferred to other libraries.
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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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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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.SS $ORIGIN and rpath
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.PP
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.B ld.so
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understands the string
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.I $ORIGIN
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(or equivalently
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.IR ${ORIGIN} )
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in an rpath specification (DT_RPATH or DT_RUNPATH) to mean
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the directory containing the application executable.
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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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.I gcc -Wl,-rpath,\(aq$ORIGIN/../lib\(aq
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so that it finds an associated shared library 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 libraries.
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.\" ld.so also understands $LIB, with the same meaning as $ORIGIN/lib,
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.\" it appears.
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.\"
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.\" There is also $PLATFORM. This is a kind of wildcard
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.\" of directories related to AT_PLATFORM. To get an idea of the
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.\" places that $PLATFORM would match, look at the output of the
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.\" 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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.SH OPTIONS
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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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.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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.TP
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.B \-\-library\-path PATH
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Use PATH instead of
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.B LD_LIBRARY_PATH
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environment variable setting (see below).
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.TP
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.B \-\-inhibit\-rpath LIST
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Ignore RPATH and RUNPATH information in object names in LIST.
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This option is ignored if
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.B ld.so
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is set-user-ID or set-group-ID.
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.TP
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.B \-\-audit LIST
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Use objects named in LIST as auditors.
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.SH HARDWARE CAPABILITIES
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Some libraries are compiled using hardware-specific instructions which do
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not exist on every CPU.
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Such libraries should be installed in directories whose names define the
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required hardware capabilities, such as
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.IR /usr/lib/sse2/ .
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The dynamic linker checks these directories against the hardware of the
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machine and selects the most suitable version of a given library.
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Hardware capability directories can be cascaded to combine CPU features.
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The list of supported hardware capability names depends on the CPU.
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The following names are currently recognized:
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.TP
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.B Alpha
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ev4, ev5, ev56, ev6, ev67
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.TP
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.B MIPS
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loongson2e, loongson2f, octeon, octeon2
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.TP
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.B PowerPC
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4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efpdouble, efpsingle,
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fpu, ic_snoop, mmu, notb, pa6t, power4, power5, power5+, power6x, ppc32, ppc601,
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ppc64, smt, spe, ucache, vsx
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.TP
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.B SPARC
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flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
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.TP
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.B s390
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dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
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z900, z990, z9-109, z10, zarch
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.TP
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.TP
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.B x86 (32-bit only)
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acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586, i686, mca, mmx,
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mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2, tm
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.SH ENVIRONMENT
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There are four important environment variables.
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.TP
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.B LD_BIND_NOW
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(libc5; glibc since 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 colon-separated list of directories in which to search for
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ELF libraries at execution-time.
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Similar to the
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.B PATH
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environment variable.
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Ignored in set-user-ID and set-group-ID programs.
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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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libraries to be loaded before all others.
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The items of the list can be separated by spaces or colons.
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This can be used to selectively override functions in other shared libraries.
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The libraries are searched for using the rules given under DESCRIPTION.
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For set-user-ID/set-group-ID ELF binaries,
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preload pathnames containing slashes are ignored,
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and libraries in the standard search directories are loaded
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only if the set-user-ID permission bit is enabled on the library file.
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.TP
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.B LD_TRACE_LOADED_OBJECTS
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(ELF only)
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If set to a nonempty string, causes the program to list its dynamic library
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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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.LP
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Then there are lots of more or less obscure variables,
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many obsolete or only for internal use.
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.TP
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.B LD_AOUT_LIBRARY_PATH
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(libc5)
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Version of
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.B LD_LIBRARY_PATH
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for a.out binaries only.
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Old versions of ld\-linux.so.1 also supported
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.BR LD_ELF_LIBRARY_PATH .
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.TP
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.B LD_AOUT_PRELOAD
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(libc5)
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Version of
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.B LD_PRELOAD
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for a.out binaries only.
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Old versions of ld\-linux.so.1 also supported
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.BR LD_ELF_PRELOAD .
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.TP
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.B LD_AUDIT
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(glibc since 2.4)
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A colon-separated 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).
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These libraries can be used to audit the operation of the dynamic linker.
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.B LD_AUDIT
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is ignored for set-user-ID/set-group-ID binaries.
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The dynamic linker will notify the audit
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libraries at so-called auditing checkpoints\(emfor example,
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loading a new library, 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 library.
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For details, see
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.BR rtld-audit (7).
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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
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.IR "Runtime Linker Auditing Interface" .
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.TP
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.B LD_BIND_NOT
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(glibc since 2.1.95)
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Do not update the GOT (global offset table) and PLT (procedure linkage table)
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after resolving a symbol.
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.TP
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.B LD_DEBUG
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(glibc since 2.1)
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Output verbose debugging information about the dynamic linker.
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If set to
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.B all
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prints all debugging information it has, if set to
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.B help
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prints a help message about which categories can be specified in this
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environment variable.
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Since glibc 2.3.4,
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.B LD_DEBUG
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is ignored for set-user-ID/set-group-ID binaries.
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.TP
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.B LD_DEBUG_OUTPUT
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(glibc since 2.1)
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File where
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.B LD_DEBUG
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output should be fed into, default is standard output.
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.B LD_DEBUG_OUTPUT
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is ignored for set-user-ID/set-group-ID binaries.
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.TP
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.B LD_DYNAMIC_WEAK
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(glibc since 2.1.91)
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Allow weak symbols to be overridden (reverting to old glibc behavior).
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For security reasons, since glibc 2.3.4,
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.B LD_DYNAMIC_WEAK
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is ignored for set-user-ID/set-group-ID binaries.
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.TP
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.B LD_HWCAP_MASK
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(glibc since 2.1)
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Mask for hardware capabilities.
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.TP
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.B LD_KEEPDIR
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(a.out only)(libc5)
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Don't ignore the directory in the names of a.out libraries to be loaded.
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Use of this option is strongly discouraged.
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.TP
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.B LD_NOWARN
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(a.out only)(libc5)
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Suppress warnings about a.out libraries with incompatible minor
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version numbers.
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.TP
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.B LD_ORIGIN_PATH
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(glibc since 2.1)
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Path where the binary is found (for non-set-user-ID programs).
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For security reasons, since glibc 2.4,
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.B LD_ORIGIN_PATH
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is ignored for set-user-ID/set-group-ID binaries.
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.\" Only used if $ORIGIN can't be determined by normal means
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.\" (from the origin path saved at load time, or from /proc/self/exe)?
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.TP
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.B LD_POINTER_GUARD
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(glibc since 2.4)
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Set to 0 to disable pointer guarding.
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Any other value enables pointer guarding, which is also the default.
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Pointer guarding is a security mechanism whereby some pointers to code
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stored in writable program memory (return addresses saved by
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.BR setjmp (3)
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or function pointers used by various glibc internals) are mangled
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semi-randomly to make it more difficult for an attacker to hijack
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the pointers for use in the event of a buffer overrun or
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stack-smashing attack.
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.TP
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.B LD_PROFILE
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(glibc since 2.1)
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Shared object to be profiled,
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specified either as a pathname or a soname.
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Profiling output is written to the file whose name is:
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"\fI$LD_PROFILE_OUTPUT\fP/\fI$LD_PROFILE\fP.profile".
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.TP
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.B LD_PROFILE_OUTPUT
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(glibc since 2.1)
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Directory where
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.B LD_PROFILE
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output should be written.
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If this variable is not defined, or is defined as an empty string,
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then the default is
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.IR /var/tmp .
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.B LD_PROFILE_OUTPUT
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is ignored for set-user-ID and set-group-ID programs,
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which always use
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.IR /var/profile .
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.TP
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.B LD_SHOW_AUXV
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(glibc since 2.1)
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Show auxiliary array passed up from the kernel.
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For security reasons, since glibc 2.3.5,
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.B LD_SHOW_AUXV
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is ignored for set-user-ID/set-group-ID binaries.
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.\" FIXME
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.\" Document LD_TRACE_PRELINKING (e.g.: LD_TRACE_PRELINKING=libx1.so ./prog)
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.\" Since glibc 2.3
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.\" Also enables DL_DEBUG_PRELINK
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.TP
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.B LD_USE_LOAD_BIAS
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.\" http://sources.redhat.com/ml/libc-hacker/2003-11/msg00127.html
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.\" Subject: [PATCH] Support LD_USE_LOAD_BIAS
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.\" Jakub Jelinek
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By default (i.e., if this variable is not defined)
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executables and prelinked
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shared objects will honor base addresses of their dependent libraries
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and (nonprelinked) position-independent executables (PIEs)
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and other shared objects will not honor them.
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If
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.B LD_USE_LOAD_BIAS
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is defined wit the value, both executables and PIEs
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will honor the base addresses.
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If
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.B LD_USE_LOAD_BIAS
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is defined with the value 0,
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neither executables nor PIEs will honor the base addresses.
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This variable is ignored by set-user-ID and set-group-ID programs.
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.TP
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.B LD_VERBOSE
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(glibc since 2.1)
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If set to a nonempty string,
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output symbol versioning information about the
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program if
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.B LD_TRACE_LOADED_OBJECTS
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variable has been set.
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.TP
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.B LD_WARN
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(ELF only)(glibc since 2.1.3)
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If set to a nonempty string, warn about unresolved symbols.
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.TP
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.B LDD_ARGV0
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(libc5)
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.IR argv [0]
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to be used by
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.BR ldd (1)
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when none is present.
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.SH FILES
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.PD 0
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.TP
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.I /lib/ld.so
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a.out dynamic linker/loader
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.TP
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.IR /lib/ld\-linux.so. { 1 , 2 }
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ELF dynamic linker/loader
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.TP
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.I /etc/ld.so.cache
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File containing a compiled list of directories in which to search for
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libraries and an ordered list of candidate libraries.
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.TP
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.I /etc/ld.so.preload
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File containing a whitespace-separated list of ELF shared libraries to
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be loaded before the program.
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.TP
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.B lib*.so*
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shared libraries
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.PD
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.SH NOTES
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The
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.B ld.so
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functionality is available for executables compiled using libc version
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4.4.3 or greater.
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ELF functionality is available since Linux 1.1.52 and libc5.
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.SH SEE ALSO
|
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.BR ldd (1),
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.BR getauxval (3),
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.BR rtld-audit (7),
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.BR ldconfig (8)
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.\" .SH AUTHORS
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.\" ld.so: David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
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.\" Torvalds, Lars Wirzenius and Mitch D'Souza
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.\" ld-linux.so: Roland McGrath, Ulrich Drepper and others.
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.\"
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.\" In the above, (libc5) stands for David Engel's ld.so/ld-linux.so.
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