Working with libraries - revision history

Dank: /* Building with libraries */

2020-08-12T08:17:16Z

Building with libraries

← Older revision		Revision as of 08:17, 12 August 2020
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	==Building with libraries==		==Building with libraries==
	* If the library supports [~~http://pkg-config.freedesktop.org~~ pkg-config], and you're willing to depend on that tool being installed, proper compilation flags can be lifted from it:		* If the library supports [[pkg-config]], and you're willing to depend on that tool being installed, proper compilation flags can be lifted from it:
	** <tt>pkg-config --cflags ''pkgname''</tt> will pull the preprocessor and compilation flags		** <tt>pkg-config --cflags ''pkgname''</tt> will pull the preprocessor and compilation flags
	** <tt>pkg-config --libs ''pkgname''</tt> will pull the linking flags		** <tt>pkg-config --libs ''pkgname''</tt> will pull the linking flags

Dank: /* Files that control ld.so */

2009-12-14T13:43:32Z

Files that control ld.so

← Older revision		Revision as of 13:43, 14 December 2009
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	===Environment variables that control ld.so===		===Environment variables that control ld.so===
	===Files that control ld.so===		===Files that control ld.so===
			* The presence of <tt>/etc/ld.so.nohwcap</tt> precludes use of optimized library versions

	===Interpositioning===		===Interpositioning===
	* Using the GNU linker, a particular object can wrap a symbol via <tt>-Wl,--wrap,symbolname</tt>. This will result in calls to <tt>symbolname</tt> resolving to <tt>__wrap_symbolname</tt>, and calls to <tt>__real_symbolname</tt> to resolve <tt>symbolname</tt>.		* Using the GNU linker, a particular object can wrap a symbol via <tt>-Wl,--wrap,symbolname</tt>. This will result in calls to <tt>symbolname</tt> resolving to <tt>__wrap_symbolname</tt>, and calls to <tt>__real_symbolname</tt> to resolve <tt>symbolname</tt>.

Dank: /* What the hell is linux-gate.so.1? */

2009-09-13T01:58:28Z

What the hell is linux-gate.so.1?

← Older revision		Revision as of 01:58, 13 September 2009
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	===What the hell is linux-gate.so.1?===		===What the hell is linux-gate.so.1?===
	The SYSENTER/SYSEXIT instructions of some Pentium processors provides a faster way to switch rings (a callgate, hence the name). [http://www.trilithium.com/johan/2005/08/linux-gate/ This page] explains pretty well what's going on.		The [[MSR\|SYSENTER/SYSEXIT]] instructions of some Pentium processors provides a faster way to switch rings (a callgate, hence the name). [http://www.trilithium.com/johan/2005/08/linux-gate/ This page] explains pretty well what's going on.

	==Quality Assurance==		==Quality Assurance==

Dank: /* Interpositioning */

2009-07-08T20:56:50Z

Interpositioning

← Older revision		Revision as of 20:56, 8 July 2009
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	===Files that control ld.so===		===Files that control ld.so===
	===Interpositioning===		===Interpositioning===
			* Using the GNU linker, a particular object can wrap a symbol via <tt>-Wl,--wrap,symbolname</tt>. This will result in calls to <tt>symbolname</tt> resolving to <tt>__wrap_symbolname</tt>, and calls to <tt>__real_symbolname</tt> to resolve <tt>symbolname</tt>.
			* LD_PRELOAD can force an object to be loaded early in dynamic linking. Export a symbol with the same identifier as the function to be wrapped, and use <tt>dlsym(RTLD_NEXT,symbolname)</tt> to access the wrapped symbol.
			** Doing this with no per-call checking, in a thread-safe manner, is left as an exercise for the reader '''FIXME'''

	===What the hell is linux-gate.so.1?===		===What the hell is linux-gate.so.1?===
	The SYSENTER/SYSEXIT instructions of some Pentium processors provides a faster way to switch rings (a callgate, hence the name). [http://www.trilithium.com/johan/2005/08/linux-gate/ This page] explains pretty well what's going on.		The SYSENTER/SYSEXIT instructions of some Pentium processors provides a faster way to switch rings (a callgate, hence the name). [http://www.trilithium.com/johan/2005/08/linux-gate/ This page] explains pretty well what's going on.

Dank at 04:17, 7 July 2009

2009-07-07T04:17:21Z

← Older revision		Revision as of 04:17, 7 July 2009
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	* <tt>-rpath=''path''</tt> (<tt>-R ''directory''</tt> on some linkers; GNU ld supports this) sets the DT_RUNPATH (DT_RPATH without new dtags) dtag, controlling [[rpaths]]		* <tt>-rpath=''path''</tt> (<tt>-R ''directory''</tt> on some linkers; GNU ld supports this) sets the DT_RUNPATH (DT_RPATH without new dtags) dtag, controlling [[rpaths]]
	** <tt>--enable-new-dtags</tt> is required to generate DT_RUNPATH or DT_FLAGS dtags specified by newer [[ELF]] systems.		** <tt>--enable-new-dtags</tt> is required to generate DT_RUNPATH or DT_FLAGS dtags specified by newer [[ELF]] systems.
			* Set an [[ELF]] entry point via -Wl,-e,<tt>your exported symbol</tt>
			** This is how [[glibc\|libc6.so]] prints diagnostic information when executed

	==Building with libraries==		==Building with libraries==

Dank: /* Quality Assurance */

2009-07-07T04:15:36Z

Quality Assurance

← Older revision		Revision as of 04:15, 7 July 2009
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	[recombinator](1) $</pre>		[recombinator](1) $</pre>
	* ld warn options, especially <tt>--warn-shared-textrel</tt>, <tt>--fatal-warnings</tt>, <tt>--warn-common</tt>, etc		* ld warn options, especially <tt>--warn-shared-textrel</tt>, <tt>--fatal-warnings</tt>, <tt>--warn-common</tt>, etc
			* Your libraries oughtn't be installed executable unless they can be meaningfully executed.
			** An [[ELF]] object without an entry point, when executed, will coredump
			** Either don't set executable bits, or run each installed library and ensure it exits cleanly

	==See also==		==See also==

Dank at 07:27, 3 May 2009

2009-05-03T07:27:13Z

← Older revision		Revision as of 07:27, 3 May 2009
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	===Files that control ld.so===		===Files that control ld.so===
	===Interpositioning===		===Interpositioning===
			===What the hell is linux-gate.so.1?===
			The SYSENTER/SYSEXIT instructions of some Pentium processors provides a faster way to switch rings (a callgate, hence the name). [http://www.trilithium.com/johan/2005/08/linux-gate/ This page] explains pretty well what's going on.

	==Quality Assurance==		==Quality Assurance==

Dank at 09:23, 9 March 2009

2009-03-09T09:23:21Z

← Older revision		Revision as of 09:23, 9 March 2009
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	===Environment variables that control ld.so===		===Environment variables that control ld.so===
	===Files that control ld.so===		===Files that control ld.so===
			===Interpositioning===

	==Quality Assurance==		==Quality Assurance==

WikiSysop: 1 revision

2009-01-28T07:32:16Z

1 revision

← Older revision	Revision as of 07:32, 28 January 2009
(No difference)

Dank: /* See also */

2008-11-10T12:03:09Z

See also

New page

==Building libraries==
===[[gcc]] issues===
* For predictable results, the same set of options used to generate code (see "Code Gen Options" and "Submodel Options" in the [[gcc]] Info pages) must be used to link (see "Link Options" in the [[gcc]] Info pages)!
* <tt>-shared</tt> should be supplied with the link step to produce a shared object.
* Regarding <tt>-fpic</tt> vs <tt>-fPIC</tt>: <tt>-fPIC</tt> is only necessary in certain architecture-dependent situations; [[gcc]] will let you know if it's needed, and fail out, should you provide <tt>-fpic</tt>
* Use <tt>-Wl,''ldopt''</tt> to pass options to <tt>ld</tt> ([[Working with libraries#ld issues|see below]])
===ld issues===
* Generally, either the packaging system or build process needs invoke <tt>ldconfig</tt> with suitable arguments so that the linker cache is updated.
* The <tt>-t</tt> option (<tt>-Wl,-t</tt> as a [[gcc]] option) traces input files to ld, and can be useful for debugging, as can <tt>-y ''symbol''</tt> to trace a given symbol's introduction and use.m
* <tt>soname=''name''</tt> sets the internal DT_SONAME field of an [[ELF]] object
* <tt>-fini ''name''</tt> and <tt>-init ''name''</tt> to specify alternative values for the [[ELF]] DT_INIT and DT_FINI initialization and finalization fields. By default, <tt>_init</tt> and <tt>_finit</tt> are used.
* <tt>-rpath=''path''</tt> (<tt>-R ''directory''</tt> on some linkers; GNU ld supports this) sets the DT_RUNPATH (DT_RPATH without new dtags) dtag, controlling [[rpaths]]
** <tt>--enable-new-dtags</tt> is required to generate DT_RUNPATH or DT_FLAGS dtags specified by newer [[ELF]] systems.

==Building with libraries==
* If the library supports [http://pkg-config.freedesktop.org pkg-config], and you're willing to depend on that tool being installed, proper compilation flags can be lifted from it:
** <tt>pkg-config --cflags ''pkgname''</tt> will pull the preprocessor and compilation flags
** <tt>pkg-config --libs ''pkgname''</tt> will pull the linking flags
** <tt>pkg-config --list-all</tt> lists all libraries registered with <tt>pkg-config</tt>

==Runtime linking==
===Environment variables that control ld.so===
===Files that control ld.so===

==Quality Assurance==
* Use <tt>ldd</tt>'s command line options to check the library.
** Discover unused direct dependencies with <tt>-u -r</tt>:
<pre>[recombinator](1) $ ldd -u -r /usr/lib/libapr-1.so.0
Unused direct dependencies:

/lib/librt.so.1
/lib/libcrypt.so.1
[recombinator](1) $ ldd -u -r .out/libdank/libdank.so
Unused direct dependencies:

[recombinator](1) $</pre>
* ld warn options, especially <tt>--warn-shared-textrel</tt>, <tt>--fatal-warnings</tt>, <tt>--warn-common</tt>, etc

==See also==
* [[ELF]] / [[rpaths]]
* [[Interesting libraries]] / [[Glibc]]
* [http://wiki.linuxquestions.org/wiki/Library-related_Commands_and_Files Library] entry on [http://wiki.linuxquestions.org LQWiki]
* Ulrich Drepper's excellent paper, "[http://people.redhat.com/drepper/dsohowto.pdf How to Write Shared Libraries]"
* [http://sourceware.org/ml/binutils/2008-03/msg00162.html Gold], a new linker introduced to binutils in March 2008. As of November 2008, it's not been a part of any binutils release.
* Solaris 9 [http://docsun.cites.uiuc.edu/sun_docs/C/solaris_9/SUNWdev/LLM/p55.html dtag documentation] from Sun
* This 1997-11 [http://www.linuxfocus.org/English/November1997/article6.html LinuxFocus article] is old but pretty good
* The Linux Documentation Project's [http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html Shared Library HOWTO]