225 lines
7.7 KiB
Groff
225 lines
7.7 KiB
Groff
.\" $NetBSD: ld.elf_so.1,v 1.15 2010/12/17 08:50:26 wiz Exp $
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.\"
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.\" Copyright (c) 2001 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Nick Hudson.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd December 17, 2010
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.Dt LD.ELF_SO 1
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.Os
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.Sh NAME
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.Nm ld.elf_so
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.Nd run-time link-editor (linker)
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.Sh DESCRIPTION
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.Nm
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is a self-contained, position independent program
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image providing run-time support for loading and
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link-editing shared objects into a process' address space.
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It uses information stored in data structures within the binary (see
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.Xr elf 5 )
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and environment variables to determine which shared objects are needed.
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These shared objects are loaded at a convenient virtual address using the
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.Xr mmap 2
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system call.
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After all shared objects have been successfully loaded,
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.Nm
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proceeds to resolve external references from both
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the main program and all objects loaded.
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Once all required references are resolved control is
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passed to the program via its entry point.
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.Ss Startup
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On the execution of a dynamically linked binary the kernel will load
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the program and its run-time linker as specified in the PT_INTERP
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section in the program header.
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At this point, instead of passing control directly to the program,
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the kernel passes control to the specified linker.
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An auxiliary vector of information is passed that includes
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the address of the program header, the size of each entry in the header,
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and the number of entries.
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The entry point of the program and the base address of where
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.Nm
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is loaded is also supplied.
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.Ss Finding objects
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Each
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.Xr elf 5
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object file may contain information in its dynamic (PT_DYNAMIC) section
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about which shared objects it requires (often referred to as dependencies).
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These dependencies are specified in the optional DT_NEEDED entry within
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the dynamic section.
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Each DT_NEEDED entry refers to a filename string of
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the shared object that is to be searched for.
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.Pp
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The linker will search for libraries in three lists of paths:
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.Bl -enum
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.It
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A user defined list of paths as specified in LD_LIBRARY_PATH and
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.Xr ld.so.conf 5 .
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.Pp
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The use of ld.so.conf should be avoided as the setting of a global search
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path can present a security risk.
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.It
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A list of paths specified within a shared object using a DT_RPATH entry in
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the dynamic section.
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This is defined at shared object link time.
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.It
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The list of default paths which is set to
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.Pa /usr/lib .
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.El
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.Pp
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.Nm
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will expand the following variables if present in the paths:
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.Bl -tag -width $PLATFORM
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.It $HWCAP
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Processor hardware capabilities, for example FPU, MMX, SSE.
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Currently unimplemented.
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.It $ISALIST
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List of instructions sets this processor can execute.
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Currently unimplemented.
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.It $ORIGIN
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The directory of the main object.
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Implemented in
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.Nm
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but the kernel support is currently disabled.
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.It $OSNAME
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The value of the
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.Dv kern.ostype
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.Xr sysctl 3 .
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.It $OSREL
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The value of the
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.Dv kern.osrelease
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.Xr sysctl 3 .
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.It $PLATFORM
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The value of the
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.Dv hw.machine_arch
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.Xr sysctl 3 .
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.El
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.Pp
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Both
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.Dv ${VARIABLE}
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and
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.Dv $VARIABLE
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are recognized.
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.Pp
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The filename string can be considered free form, however, it will almost
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always take the form lib\*[Lt]name\*[Gt].so.\*[Lt]number\*[Gt],
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where name specifies the
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.Sq library
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name and number is conceptually the library's major version number.
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.Pp
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This name and another of the form lib\*[Lt]name\*[Gt].so are normally
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symbolic links to the real shared object which has a filename of the form
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lib\*[Lt]name\*[Gt].so.\*[Lt]major\*[Gt].\*[Lt]minor\*[Gt][.\*[Lt]teeny\*[Gt]].
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This naming convention allows a versioning scheme similar to
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.Xr a.out 5 .
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.Ss Relocation
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.Nm
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will perform all necessary relocations immediately except for relocations
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relating to the Procedure Linkage Table (PLT).
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The PLT is used as a indirection method for procedure
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calls to globally defined functions.
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It allows, through the use of intermediate code, the delayed binding of
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a call to a globally defined function to be performed at procedure call time.
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This
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.Sq lazy
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method is the default (see LD_BIND_NOW).
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.Ss Initialization
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A mechanism is provided for initialization and termination routines
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to be called, on a per-object basis before execution of the program proper
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begins or after the program has completed.
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This gives a shared object an opportunity to perform
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any extra set-up or completion work.
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.Pp
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The DT_INIT and DT_FINI entries in the dynamic section specify the addresses
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of the initialization and termination functions, respectively, for
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the shared object.
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.Nm
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arranges for each initialization function to be called before control is passed
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to the program and for the termination functions to be called by using
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.Xr atexit 3 .
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.Pp
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This mechanism is exploited by the system-supplied constructor
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initialization and destructor code located in
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.Pa /usr/lib/crtbeginS.o
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and
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.Pa /usr/lib/crtendS.o .
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These files are automatically included by
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.Xr cc 1
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and
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.Xr c++ 1
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in the list of object-code files passed to
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.Xr ld 1
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when building a shared C or C++ object.
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.Sh ENVIRONMENT
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If the following environment variables exist they will be used by
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.Nm .
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.Bl -tag -width "LD_LIBRARY_PATH"
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.It Ev LD_LIBRARY_PATH
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A colon separated list of directories, overriding the default search path
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for shared libraries.
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.It Ev LD_PRELOAD
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A colon or space separated list of shared object filenames to be loaded
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.Em after
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the main program but
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.Em before
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its shared object dependencies.
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Space is allowed as a separator for backwards compatibility only.
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Support may be removed in a future release and should not be relied upon.
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.It Ev LD_BIND_NOW
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If defined immediate binding of Procedure Link Table (PLT) entries is
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performed instead of the default lazy method.
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.It Ev LD_DEBUG
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If defined a variety of debug information will be written to the standard
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error of an dynamically linked executed when it is run.
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This variable is only recognized if
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.Nm
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was compiled with debugging support
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.Sy ( -DDEBUG ) .
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.El
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.Sh FILES
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.Bl -tag -width /etc/ld.so.conf -compact
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.It Pa /etc/ld.so.conf
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library location hints supplied by the system administrator.
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.El
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.Sh SEE ALSO
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.Xr ld 1 ,
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.Xr ld.aout_so 1 ,
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.Xr dlfcn 3 ,
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.Xr elf 5
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.Sh HISTORY
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The ELF shared library model employed first appeared in Sys V R4.
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.Pp
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The path expansion variables first appeared in Solaris 10, and
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in
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.Nx 5.0 .
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.Sh SECURITY CONSIDERATIONS
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The environment variables
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.Ev LD_LIBRARY_PATH
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and
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.Ev LD_PRELOAD
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are not honored when executing in a set-user-ID or set-group-ID environment.
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This action is taken to prevent malicious substitution of shared object
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dependencies or interposition of symbols.
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