man8/ld.so.8

   1 .\" %%%LICENSE_START(PUBLIC_DOMAIN)
   2 .\" This is in the public domain
   3 .\" %%%LICENSE_END
   4 .\"
   5 .TH LD.SO 8 2015-08-08 "GNU" "Linux Programmer's Manual"
   6 .SH NAME
   7 ld.so, ld-linux.so* \- dynamic linker/loader
   8 .SH SYNOPSIS
   9 The dynamic linker can be run either indirectly by running some
  10 dynamically linked program or shared object
  11 (in which case no command-line options
  12 to the dynamic linker can be passed and, in the ELF case, the dynamic linker
  13 which is stored in the
  14 .B .interp
  15 section of the program is executed) or directly by running:
  16 .P
  17 .I /lib/ld-linux.so.*
  18 [OPTIONS] [PROGRAM [ARGUMENTS]]
  19 .SH DESCRIPTION
  20 The programs
  21 .B ld.so
  22 and
  23 .B ld-linux.so*
  24 find and load the shared objects (shared libraries) needed by a program,
  25 prepare the program to run, and then run it.
  26 .LP
  27 Linux binaries require dynamic linking (linking at run time)
  28 unless the
  29 .B \-static
  30 option was given to
  31 .BR ld (1)
  32 during compilation.
  33 .LP
  34 The program
  35 .B ld.so
  36 handles a.out binaries, a format used long ago;
  37 .B ld-linux.so*
  38 handles ELF (\fI/lib/ld-linux.so.1\fP for libc5, \fI/lib/ld-linux.so.2\fP
  39 for glibc2), which everybody has been using for years now.
  40 Otherwise, both have the same behavior, and use the same
  41 support files and programs
  42 .BR ldd (1),
  43 .BR ldconfig (8),
  44 and
  45 .IR /etc/ld.so.conf .
  46 .LP
  47 When resolving shared object dependencies,
  48 the dynamic linker first inspects each dependency
  49 string to see if it contains a slash (this can occur if
  50 a shared object pathname containing slashes was specified at link time).
  51 If a slash is found, then the dependency string is interpreted as
  52 a (relative or absolute) pathname,
  53 and the shared object is loaded using that pathname.
  54 .LP
  55 If a shared object dependency does not contain a slash,
  56 then it is searched for in the following order:
  57 .IP o 3
  58 (ELF only) Using the directories specified in the
  59 DT_RPATH dynamic section attribute
  60 of the binary if present and DT_RUNPATH attribute does not exist.
  61 Use of DT_RPATH is deprecated.
  62 .IP o
  63 Using the environment variable
  64 .BR LD_LIBRARY_PATH .
  65 Except if the executable is a set-user-ID/set-group-ID binary,
  66 in which case it is ignored.
  67 .IP o
  68 (ELF only) Using the directories specified in the
  69 DT_RUNPATH dynamic section attribute
  70 of the binary if present.
  71 .IP o
  72 From the cache file
  73 .IR /etc/ld.so.cache ,
  74 which contains a compiled list of candidate shared objects previously found
  75 in the augmented library path.
  76 If, however, the binary was linked with the
  77 .B \-z nodeflib
  78 linker option, shared objects in the default paths are skipped.
  79 Shared objects installed in hardware capability directories (see below)
  80 are preferred to other shared objects.
  81 .IP o
  82 In the default path
  83 .IR /lib ,
  84 and then
  85 .IR /usr/lib .
  86 (On some 64-bit archiectures, the default paths for 64-bit shared objects are
  87 .IR /lib64 ,
  88 and then
  89 .IR /usr/lib64 .)
  90 If the binary was linked with the
  91 .B \-z nodeflib
  92 linker option, this step is skipped.
  93 .SS Rpath token expansion
  94 .PP
  95 .B ld.so
  96 understands certain strings in an rpath specification (DT_RPATH or DT_RUNPATH); those strings are substituted as follows
  97 .TP
  98 .IR $ORIGIN " (or equivalently " ${ORIGIN} )
  99 This expands to
 100 the directory containing the program or shared object.
 101 Thus, an application located in
 102 .I somedir/app
 103 could be compiled with
 104
 105     gcc \-Wl,\-rpath,\(aq$ORIGIN/../lib\(aq
 106
 107 so that it finds an associated shared object in
 108 .I somedir/lib
 109 no matter where
 110 .I somedir
 111 is located in the directory hierarchy.
 112 This facilitates the creation of "turn-key" applications that
 113 do not need to be installed into special directories,
 114 but can instead be unpacked into any directory
 115 and still find their own shared objects.
 116 .TP
 117 .IR $LIB " (or equivalently " ${LIB} )
 118 This expands to
 119 .I lib
 120 or
 121 .I lib64
 122 depending on the architecture
 123 (e.g., on x86-64, it expands to
 124 .IR lib64
 125 and
 126 on x86-32, it expands to
 127 .IR lib ).
 128 .TP
 129 .IR $PLATFORM " (or equivalently " ${PLATFORM} )
 130 This expands to a string corresponding to the processor type
 131 of the host system (e.g., "x86_64").
 132 On some architectures, the Linux kernel doesn't provide a platform
 133 string to the dynamic linker.
 134 The value of this string is taken from the
 135 .BR AT_PLATFORM
 136 value in the auxiliary vector (see
 137 .BR getauxval (3)).
 138 .\" To get an idea of the places that $PLATFORM would match,
 139 .\" look at the output of the following:
 140 .\"
 141 .\"     mkdir /tmp/d
 142 .\"     LD_LIBRARY_PATH=/tmp/d strace -e open /bin/date 2>&1 | grep /tmp/d
 143 .\"
 144 .\" ld.so lets names be abbreviated, so $O will work for $ORIGIN;
 145 .\" Don't do this!!
 146 .SH OPTIONS
 147 .TP
 148 .B \-\-list
 149 List all dependencies and how they are resolved.
 150 .TP
 151 .B \-\-verify
 152 Verify that program is dynamically linked and this dynamic linker can handle
 153 it.
 154 .TP
 155 .B \-\-inhibit-cache
 156 Do not use
 157 .IR /etc/ld.so.cache .
 158 .TP
 159 .BI \-\-library\-path " path"
 160 Use
 161 .I path
 162 instead of
 163 .B LD_LIBRARY_PATH
 164 environment variable setting (see below).
 165 .TP
 166 .BI \-\-inhibit\-rpath " list"
 167 Ignore RPATH and RUNPATH information in object names in
 168 .IR list .
 169 This option is ignored if
 170 .B ld.so
 171 is set-user-ID or set-group-ID.
 172 .TP
 173 .BI \-\-audit " list"
 174 Use objects named in
 175 .I list
 176 as auditors.
 177 .SH ENVIRONMENT
 178 Among the more important environment variables are the following:
 179 .TP
 180 .B LD_ASSUME_KERNEL
 181 (glibc since 2.2.3)
 182 Each shared object can inform the dynamic linker of the minimum kernel ABI
 183 version that it requires.
 184 (This requirement is encoded in an ELF note section that is viewable via
 185 .IR "readelf\ \-n"
 186 as a section labeled
 187 .BR NT_GNU_ABI_TAG .)
 188 At run time,
 189 the dynamic linker determines the ABI version of the running kernel and
 190 will reject loading shared objects that specify minimum ABI versions
 191 that exceed that ABI version.
 192
 193 .BR LD_ASSUME_KERNEL
 194 can be used to
 195 cause the dynamic linker to assume that it is running on a system with
 196 a different kernel ABI version.
 197 For example, the following command line causes the
 198 dynamic linker to assume it is running on Linux 2.2.5 when loading
 199 the shared objects required by
 200 .IR myprog :
 201
 202 .in +4n
 203 .nf
 204 $ \fBLD_ASSUME_KERNEL=2.2.5 ./myprog\fP
 205 .fi
 206 .in
 207
 208 On systems that provide multiple versions of a shared object
 209 (in different directories in the search path) that have
 210 different minimum kernel ABI version requirements,
 211 .BR LD_ASSUME_KERNEL
 212 can be used to select the version of the object that is used
 213 (dependent on the directory search order).
 214 Historically, the most common use of the
 215 .BR LD_ASSUME_KERNEL
 216 feature was to manually select the older
 217 LinuxThreads POSIX threads implementation on systems that provided both
 218 LinuxThreads and NPTL
 219 (which latter was typically the default on such systems);
 220 see
 221 .BR pthreads (7).
 222 .TP
 223 .B LD_BIND_NOW
 224 (libc5; glibc since 2.1.1)
 225 If set to a nonempty string,
 226 causes the dynamic linker to resolve all symbols
 227 at program startup instead of deferring function call resolution to the point
 228 when they are first referenced.
 229 This is useful when using a debugger.
 230 .TP
 231 .B LD_LIBRARY_PATH
 232 A list of directories in which to search for
 233 ELF libraries at execution-time.
 234 The items in the list are separated by either colons or semicolons.
 235 Similar to the
 236 .B PATH
 237 environment variable.
 238 Ignored in set-user-ID and set-group-ID programs.
 239 .TP
 240 .B LD_PRELOAD
 241 A list of additional, user-specified, ELF shared
 242 objects to be loaded before all others.
 243 The items of the list can be separated by spaces or colons.
 244 This can be used to selectively override functions in other shared objects.
 245 The objects are searched for using the rules given under DESCRIPTION.
 246 For set-user-ID/set-group-ID ELF binaries,
 247 preload pathnames containing slashes are ignored,
 248 and shared objects in the standard search directories are loaded
 249 only if the set-user-ID mode bit is enabled on the shared object file.
 250 .TP
 251 .B LD_TRACE_LOADED_OBJECTS
 252 (ELF only)
 253 If set to a nonempty string, causes the program to list its dynamic
 254 dependencies, as if run by
 255 .BR ldd (1),
 256 instead of running normally.
 257 .LP
 258 Then there are lots of more or less obscure variables,
 259 many obsolete or only for internal use.
 260 .TP
 261 .B LD_AOUT_LIBRARY_PATH
 262 (libc5)
 263 Version of
 264 .B LD_LIBRARY_PATH
 265 for a.out binaries only.
 266 Old versions of ld\-linux.so.1 also supported
 267 .BR LD_ELF_LIBRARY_PATH .
 268 .TP
 269 .B LD_AOUT_PRELOAD
 270 (libc5)
 271 Version of
 272 .B LD_PRELOAD
 273 for a.out binaries only.
 274 Old versions of ld\-linux.so.1 also supported
 275 .BR LD_ELF_PRELOAD .
 276 .TP
 277 .B LD_AUDIT
 278 (glibc since 2.4)
 279 A colon-separated list of user-specified, ELF shared objects
 280 to be loaded before all others in a separate linker namespace
 281 (i.e., one that does not intrude upon the normal symbol bindings that
 282 would occur in the process).
 283 These objects can be used to audit the operation of the dynamic linker.
 284 .B LD_AUDIT
 285 is ignored for set-user-ID/set-group-ID binaries.
 286
 287 The dynamic linker will notify the audit
 288 shared objects at so-called auditing checkpoints\(emfor example,
 289 loading a new shared object, resolving a symbol,
 290 or calling a symbol from another shared object\(emby
 291 calling an appropriate function within the audit shared object.
 292 For details, see
 293 .BR rtld-audit (7).
 294 The auditing interface is largely compatible with that provided on Solaris,
 295 as described in its
 296 .IR "Linker and Libraries Guide" ,
 297 in the chapter
 298 .IR "Runtime Linker Auditing Interface" .
 299 .TP
 300 .B LD_BIND_NOT
 301 (glibc since 2.1.95)
 302 Do not update the GOT (global offset table) and PLT (procedure linkage table)
 303 after resolving a symbol.
 304 .TP
 305 .B LD_DEBUG
 306 (glibc since 2.1)
 307 Output verbose debugging information about the dynamic linker.
 308 If set to
 309 .B all
 310 prints all debugging information it has, if set to
 311 .B help
 312 prints a help message about which categories can be specified in this
 313 environment variable.
 314 Since glibc 2.3.4,
 315 .B LD_DEBUG
 316 is ignored for set-user-ID/set-group-ID binaries.
 317 .TP
 318 .B LD_DEBUG_OUTPUT
 319 (glibc since 2.1)
 320 File in which
 321 .B LD_DEBUG
 322 output should be written.
 323 The default is standard error.
 324 .B LD_DEBUG_OUTPUT
 325 is ignored for set-user-ID/set-group-ID binaries.
 326 .TP
 327 .B LD_DYNAMIC_WEAK
 328 (glibc since 2.1.91)
 329 Allow weak symbols to be overridden (reverting to old glibc behavior).
 330 .\" See weak handling
 331 .\"     https://www.sourceware.org/ml/libc-hacker/2000-06/msg00029.html
 332 .\"     To: GNU libc hacker <libc-hacker at sourceware dot cygnus dot com>
 333 .\"     Subject: weak handling
 334 .\"     From: Ulrich Drepper <drepper at redhat dot com>
 335 .\"     Date: 07 Jun 2000 20:08:12 -0700
 336 .\"     Reply-To: drepper at cygnus dot com (Ulrich Drepper)
 337 For security reasons, since glibc 2.3.4,
 338 .B LD_DYNAMIC_WEAK
 339 is ignored for set-user-ID/set-group-ID binaries.
 340 .TP
 341 .B LD_HWCAP_MASK
 342 (glibc since 2.1)
 343 Mask for hardware capabilities.
 344 .TP
 345 .B LD_KEEPDIR
 346 (a.out only)(libc5)
 347 Don't ignore the directory in the names of a.out libraries to be loaded.
 348 Use of this option is strongly discouraged.
 349 .TP
 350 .B LD_NOWARN
 351 (a.out only)(libc5)
 352 Suppress warnings about a.out libraries with incompatible minor
 353 version numbers.
 354 .TP
 355 .B LD_ORIGIN_PATH
 356 (glibc since 2.1)
 357 Path where the binary is found (for non-set-user-ID programs).
 358 For security reasons, since glibc 2.4,
 359 .B LD_ORIGIN_PATH
 360 is ignored for set-user-ID/set-group-ID binaries.
 361 .\" Only used if $ORIGIN can't be determined by normal means
 362 .\" (from the origin path saved at load time, or from /proc/self/exe)?
 363 .TP
 364 .B LD_POINTER_GUARD
 365 (glibc from 2.4 to 2.22)
 366 Set to 0 to disable pointer guarding.
 367 Any other value enables pointer guarding, which is also the default.
 368 Pointer guarding is a security mechanism whereby some pointers to code
 369 stored in writable program memory (return addresses saved by
 370 .BR setjmp (3)
 371 or function pointers used by various glibc internals) are mangled
 372 semi-randomly to make it more difficult for an attacker to hijack
 373 the pointers for use in the event of a buffer overrun or
 374 stack-smashing attack.
 375 Since glibc 2.23,
 376 .\" commit a014cecd82b71b70a6a843e250e06b541ad524f7
 377 .B LD_POINTER_GUARD
 378 can no longer be used to disable pointer guarding,
 379 which is now always enabled.
 380 .TP
 381 .B LD_PROFILE
 382 (glibc since 2.1)
 383 The name of a (single) shared object to be profiled,
 384 specified either as a pathname or a soname.
 385 Profiling output is appended to the file whose name is:
 386 "\fI$LD_PROFILE_OUTPUT\fP/\fI$LD_PROFILE\fP.profile".
 387 .TP
 388 .B LD_PROFILE_OUTPUT
 389 (glibc since 2.1)
 390 Directory where
 391 .B LD_PROFILE
 392 output should be written.
 393 If this variable is not defined, or is defined as an empty string,
 394 then the default is
 395 .IR /var/tmp .
 396 .B LD_PROFILE_OUTPUT
 397 is ignored for set-user-ID and set-group-ID programs,
 398 which always use
 399 .IR /var/profile .
 400 .TP
 401 .B LD_SHOW_AUXV
 402 (glibc since 2.1)
 403 Show auxiliary array passed up from the kernel.
 404 For security reasons, since glibc 2.3.5,
 405 .B LD_SHOW_AUXV
 406 is ignored for set-user-ID/set-group-ID binaries.
 407 .TP
 408 .B LD_TRACE_PRELINKING
 409 (glibc since 2.4)
 410 Trace prelinking of the object whose name is assigned to
 411 this environment variable.
 412 (Use
 413 .BR ldd (1)
 414 to get a list of the objects that might be traced.)
 415 If the object name is not recognized,
 416 .\" (This is what seems to happen, from experimenting)
 417 then all prelinking activity is traced.
 418 .TP
 419 .B LD_USE_LOAD_BIAS
 420 .\" http://sources.redhat.com/ml/libc-hacker/2003-11/msg00127.html
 421 .\" Subject: [PATCH] Support LD_USE_LOAD_BIAS
 422 .\" Jakub Jelinek
 423 By default (i.e., if this variable is not defined),
 424 executables and prelinked
 425 shared objects will honor base addresses of their dependent shared objects
 426 and (nonprelinked) position-independent executables (PIEs)
 427 and other shared objects will not honor them.
 428 If
 429 .B LD_USE_LOAD_BIAS
 430 is defined with the value 1, both executables and PIEs
 431 will honor the base addresses.
 432 If
 433 .B LD_USE_LOAD_BIAS
 434 is defined with the value 0,
 435 neither executables nor PIEs will honor the base addresses.
 436 This variable is ignored by set-user-ID and set-group-ID programs.
 437 .TP
 438 .B LD_VERBOSE
 439 (glibc since 2.1)
 440 If set to a nonempty string,
 441 output symbol versioning information about the
 442 program if the
 443 .B LD_TRACE_LOADED_OBJECTS
 444 environment variable has been set.
 445 .TP
 446 .B LD_WARN
 447 (ELF only)(glibc since 2.1.3)
 448 If set to a nonempty string, warn about unresolved symbols.
 449 .TP
 450 .B LDD_ARGV0
 451 (libc5)
 452 .IR argv [0]
 453 to be used by
 454 .BR ldd (1)
 455 when none is present.
 456 .SH FILES
 457 .PD 0
 458 .TP
 459 .I /lib/ld.so
 460 a.out dynamic linker/loader
 461 .TP
 462 .IR /lib/ld\-linux.so. { 1 , 2 }
 463 ELF dynamic linker/loader
 464 .TP
 465 .I /etc/ld.so.cache
 466 File containing a compiled list of directories in which to search for
 467 shared objects and an ordered list of candidate shared objects.
 468 .TP
 469 .I /etc/ld.so.preload
 470 File containing a whitespace-separated list of ELF shared objects to
 471 be loaded before the program.
 472 .TP
 473 .B lib*.so*
 474 shared objects
 475 .PD
 476 .SH NOTES
 477 The
 478 .B ld.so
 479 functionality is available for executables compiled using libc version
 480 4.4.3 or greater.
 481 ELF functionality is available since Linux 1.1.52 and libc5.
 482 .SS Hardware capabilities
 483 Some shared objects are compiled using hardware-specific instructions which do
 484 not exist on every CPU.
 485 Such objects should be installed in directories whose names define the
 486 required hardware capabilities, such as
 487 .IR /usr/lib/sse2/ .
 488 The dynamic linker checks these directories against the hardware of the
 489 machine and selects the most suitable version of a given shared object.
 490 Hardware capability directories can be cascaded to combine CPU features.
 491 The list of supported hardware capability names depends on the CPU.
 492 The following names are currently recognized:
 493 .TP
 494 .B Alpha
 495 ev4, ev5, ev56, ev6, ev67
 496 .TP
 497 .B MIPS
 498 loongson2e, loongson2f, octeon, octeon2
 499 .TP
 500 .B PowerPC
 501 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp, efpdouble, efpsingle,
 502 fpu, ic_snoop, mmu, notb, pa6t, power4, power5, power5+, power6x, ppc32, ppc601,
 503 ppc64, smt, spe, ucache, vsx
 504 .TP
 505 .B SPARC
 506 flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
 507 .TP
 508 .B s390
 509 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
 510 z900, z990, z9-109, z10, zarch
 511 .TP
 512 .B x86 (32-bit only)
 513 acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486, i586, i686, mca, mmx,
 514 mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2, tm
 515 .SH SEE ALSO
 516 .BR ld (1),
 517 .BR ldd (1),
 518 .BR pldd (1),
 519 .BR sprof (1),
 520 .BR dlopen (3),
 521 .BR getauxval (3),
 522 .BR rtld-audit (7),
 523 .BR ldconfig (8),
 524 .BR sln (8)
 525 .\" .SH AUTHORS
 526 .\" ld.so: David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
 527 .\"  Torvalds, Lars Wirzenius and Mitch D'Souza
 528 .\" ld-linux.so: Roland McGrath, Ulrich Drepper and others.
 529 .\"
 530 .\" In the above, (libc5) stands for David Engel's ld.so/ld-linux.so.