View source for Working with libraries

==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.
* 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==
* 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 --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===
* The presence of <tt>/etc/ld.so.nohwcap</tt> precludes use of optimized library versions

===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?===
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==
* 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
* 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==
* [[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]