Libc - revision history

Dank: /* Using an alternate Glibc */

2020-07-05T16:22:05Z

Using an alternate Glibc

← Older revision		Revision as of 16:22, 5 July 2020
Line 44:		Line 44:
	* provide frame pointers (none for inlined code, omitted with most [[gcc]] optimization levels)		* provide frame pointers (none for inlined code, omitted with most [[gcc]] optimization levels)
	==Using an alternate Glibc==		==Using an alternate Glibc==
			Acquire the glibc sources, create a build directory, and run <tt>configure</tt> from that build directory. You are advised to run with <tt>--prefix=/usr</tt>, and then install to an alternative directory using e.g. <tt>make install DESTDIR=$HOME/glibc</tt>. Use <tt>--with-headers</tt> to specify kernel headers:

			<pre>
			[schwarzgerat](0) $ pwd
			/home/dank/src/glibc/build
			[schwarzgerat](0) $ ../configure --prefix=/usr --with-headers=/usr/src/linux-headers-5.7.7nlb/include/generated/uapi/
			</pre>

	So you've installed a local build of Glibc to, say, $HOME/glibc, eh?		So you've installed a local build of Glibc to, say, $HOME/glibc, eh?

Dank at 16:14, 5 July 2020

2020-07-05T16:14:48Z

← Older revision		Revision as of 16:14, 5 July 2020
Line 43:		Line 43:
	* be linked with -rdynamic/--export-dynamic, and		* be linked with -rdynamic/--export-dynamic, and
	* provide frame pointers (none for inlined code, omitted with most [[gcc]] optimization levels)		* provide frame pointers (none for inlined code, omitted with most [[gcc]] optimization levels)
			==Using an alternate Glibc==
			So you've installed a local build of Glibc to, say, $HOME/glibc, eh?

			Trying to run with <tt>LD_LIBRARY_PATH=$HOME/glibc/lib</tt> will result in all dynamically-linked executables segfaulting immediately. That's because they're using a mismatched loader. Try running the correct loader directly to sanity-check your library install, e.g.:
			<pre>
			[schwarzgerat](0) $ /bin/echo
			Segmentation fault (core dumped)
			[schwarzgerat](139) $ ~/glibc/lib/ld-linux-x86-64.so.2 /bin/echo this will work
			this will work
			[schwarzgerat](0) $
			</pre>

Dank at 06:46, 24 September 2010

2010-09-24T06:46:08Z

← Older revision		Revision as of 06:46, 24 September 2010
Line 40:		Line 40:
	* <tt>backtrace_symbols</tt>, given the output of <tt>backtrace</tt>, will populate its argv-style return value with any symbols it can look up		* <tt>backtrace_symbols</tt>, given the output of <tt>backtrace</tt>, will populate its argv-style return value with any symbols it can look up
	In order to extract symbols, appropriate binaries must:		In order to extract symbols, appropriate binaries must:
	* not be stripped (or linked with -s), ~~and~~		* not be stripped (or linked with -s),
	* be linked with -rdynamic/--export-dynamic		* be linked with -rdynamic/--export-dynamic, and
			* provide frame pointers (none for inlined code, omitted with most [[gcc]] optimization levels)

Dank at 06:44, 24 September 2010

2010-09-24T06:44:13Z

← Older revision		Revision as of 06:44, 24 September 2010
Line 35:		Line 35:
	same name contained in libraries that have already been loaded.		same name contained in libraries that have already been loaded.
	This flag is not specified in POSIX.1-2001.</pre>		This flag is not specified in POSIX.1-2001.</pre>
			==Generating a stack trace==
			The <tt>backtrace</tt>, <tt>backtrace_symbols</tt> and <tt>backtrace_symbols_fd</tt> glibc calls can be used to generate a stack trace from within a program.
			* <tt>backtrace</tt> generates a list of addresses
			* <tt>backtrace_symbols</tt>, given the output of <tt>backtrace</tt>, will populate its argv-style return value with any symbols it can look up
			In order to extract symbols, appropriate binaries must:
			* not be stripped (or linked with -s), and
			* be linked with -rdynamic/--export-dynamic

Dank at 03:51, 9 January 2010

2010-01-09T03:51:19Z

← Older revision		Revision as of 03:51, 9 January 2010
Line 1:		Line 1:
			The [[Linux APIs]] page lists the GNU libc versions corresponding to various Linux-specific functionality.
	== Determining glibc version ==		== Determining glibc version ==
	* From [[C]], use confstr(3) with _CS_GNU_LIBC_VERSION and _CS_GNU_LIBPTHREAD_VERSION.		* From [[C]], use confstr(3) with _CS_GNU_LIBC_VERSION and _CS_GNU_LIBPTHREAD_VERSION.

WikiSysop: 1 revision

2009-01-28T07:32:14Z

1 revision

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

Dank at 00:32, 19 October 2008

2008-10-19T00:32:14Z

New page

== Determining glibc version ==
* From [[C]], use confstr(3) with _CS_GNU_LIBC_VERSION and _CS_GNU_LIBPTHREAD_VERSION.
** [[libdank]] provides confstr_dyn(), a convenience wrapper
* On the command line, getconf(1) can be used:
** <tt>getconf GNU_LIBC_VERSION</tt>, <tt>getconf GNU_LIBPTHREAD_VERSION</tt>

== Memory Utilities ==
* What is mudflap? ''FIXME''
* [http://www.gnu.org/software/libc/manual/html_node/Allocation-Debugging.html#Allocation-Debugging Memory traces]: if code calls <code>mtrace()</code>, then the environment variable MALLOC_TRACE is checked for a filename. If this file can be written to/created, it'll be truncated, and hooks will be installed so that all allocations and deallocations are dumped to the file. The <code>mtrace(1)</code> tool (part of glibc) can then be used to generate human-readable output from this file, especially if the source code is available.
* [http://www.gnu.org/software/libc/manual/html_node/Heap-Consistency-Checking.html#Heap-Consistency-Checking Heap consistency checking]: A less efficient allocation system can be used, which detects certain classes of errors. Link with -lmcheck, or call <code>mcheck(3)</code> prior to any allocations, or define MALLOC_CHECK_ to be 1 (to print a warning) or 2 (to generate SIGABRT). <code>mprobe(3)</code> is available in this mode, which upon invocation performs extra checks on specified blocks.
* [http://www.gnu.org/software/libc/manual/html_node/Statistics-of-Malloc.html#Statistics-of-Malloc Statistics]: <code>mallinfo(3)</code> can be called at any time to collect statistics from the allocator. Beware: this function returns a large struct on the stack (unless its prototype is ever changed), and can thus be '''incredibly''' slow!

== Dynamic Symbols ==
* <tt>dlopen(3dl)</tt> is since glibc 2.2 extended by the RTLD_NODELETE and RTLD_NOLOAD directives. From the man page:
<pre> RTLD_NODELETE (since glibc 2.2)
Do not unload the library during dlclose(). Consequently, the
library’s static variables are not reinitialized if the library
is reloaded with dlopen() at a later time. This flag is not
specified in POSIX.1-2001.

RTLD_NOLOAD (since glibc 2.2)
Don’t load the library. This can be used to test if the library
is already resident (dlopen() returns NULL if it is not, or the
library’s handle if it is resident). This flag can also be used
to promote the flags on a library that is already loaded. For
example, a library that was previously loaded with RTLD_LOCAL
can be re-opened with RTLD_NOLOAD | RTLD_GLOBAL. This flag is
not specified in POSIX.1-2001.</pre>
* <tt>dlopen(3dl)</tt> is since glibc 2.3.4 extended by the RTLD_DEEPBIND flag. From the man page:
<pre> RTLD_DEEPBIND (since glibc 2.3.4)
Place the lookup scope of the symbols in this library ahead of
the global scope. This means that a self-contained library will
use its own symbols in preference to global symbols with the
same name contained in libraries that have already been loaded.
This flag is not specified in POSIX.1-2001.</pre>