Working with libraries

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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>--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 [~~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 --libs ''pkgname''</tt> will pull the linking flags

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===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==

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[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==

Working with libraries: Difference between revisions

Dank (talk | contribs)

@@ Line 12: / Line 12: @@
 * <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 [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 --libs ''pkgname''</tt> will pull the linking flags
@@ Line 22: / Line 24: @@
 ===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==
@@ Line 36: / Line 47: @@
 [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==