使用gdb调试glibc
最近在研究signal的过程中，发现很多问题对源码进行一些简单的分析就可以迎刃而解。其实之前已经类似的想法了，能不能用gdb在调试程序的同时也对库函数进行调试。不过这种方法最深只能调试到系统调用，对于内核的运行情况，直接用gdb还是无法调试。就我现在看到的方法来说，跟踪调试内核的运行情况必须要虚拟机。
好了书归正传，今天让我们来看看如何使用gdb调试glibc。开始之前还是要先感谢我参考的原blog：http://blog.chinaunix.net/uid-24774106-id-3642925.html 最近我的很多文章都是参考这位大神的blog，在此也向大家安利以下这位大神的blog。
1. 首先安装glibc的符号表，命令如下：
sudo apt-get install libc6-dbg其中libc6-dbg的内容是：“This package contains the detached debugging symbols for the GNU C library.” 翻译过来就是这个包的内容是被分离的GNU C库的调试符号。
对于libc6的其他包可参考如下网址：https://launchpad.net/ubuntu/+source/glibc/2.21-0ubuntu4
2. 再来是安装glibc的源文件，命令如下：
sudo apt-get source libc6-dev其中libc6-dev的内容是：“Contains the symlinks, headers, and object files needed to compile and link programs which use the standard C library.” 翻译以下吧，这个包包含了symlinks（是啥不知道）、头文件与目标文件，以上文件是使用标准c库编译链接程序时所需，这个包最为关键，有了头文件、库文件我们写的程序就能运行了。
其中source选项是指下载该包的源代码，对于apt-get命令的其他选项，请见这篇文章：http://jingyan.baidu.com/article/22a299b51648e09e19376ae7.html
以上命令运行结果如下：
sudo apt-get source libc6-dev
正在读取软件包列表... 完成
正在分析软件包的依赖关系树
正在读取状态信息... 完成
选择 glibc 作为源代码包而非 libc6-dev
提示：glibc 的打包工作被维护于以下位置的 Svn 版本控制系统中：
svn://svn.debian.org/pkg-glibc/glibc-package/
需要下载 13.7 MB 的源代码包。
获取：1 http://cn.archive.ubuntu.com/ubuntu/ vivid/main glibc 2.21-0ubuntu4 (dsc) [8,672 B]
获取：2 http://cn.archive.ubuntu.com/ubuntu/ vivid/main glibc 2.21-0ubuntu4 (tar) [12.7 MB]
获取：3 http://cn.archive.ubuntu.com/ubuntu/ vivid/main glibc 2.21-0ubuntu4 (diff) [994 kB]
下载 13.7 MB，耗时 12秒 (1,068 kB/s)
gpgv: 于 2015年03月26日 星期四 14时09分22秒 CST 创建的签名，使用 RSA，钥匙号 B1CDE58F
gpgv: 无法检查签名：找不到公钥
dpkg-source: 警告: 对 ./glibc_2.21-0ubuntu4.dsc 校验签名失败
dpkg-source: info: extracting glibc in glibc-2.21
dpkg-source: info: unpacking glibc_2.21.orig.tar.xz
dpkg-source: info: unpacking glibc_2.21-0ubuntu4.debian.tar.xz
dpkg-source: info: applying git-updates.diff
dpkg-source: info: applying locale/check-unknown-symbols.diff
dpkg-source: info: applying locale/fix-LC_COLLATE-rules.diff
dpkg-source: info: applying locale/preprocessor-collate-uli-sucks.diff
dpkg-source: info: applying locale/preprocessor-collate.diff
dpkg-source: info: applying locale/locale-print-LANGUAGE.diff
dpkg-source: info: applying locale/LC_IDENTIFICATION-optional-fields.diff
dpkg-source: info: applying locale/LC_COLLATE-keywords-ordering.diff
dpkg-source: info: applying localedata/local-all-no-archive.diff
dpkg-source: info: applying localedata/sort-UTF8-first.diff
dpkg-source: info: applying localedata/supported.diff
dpkg-source: info: applying localedata/locale-eo_EO.diff
dpkg-source: info: applying localedata/locale-eu_FR.diff
dpkg-source: info: applying localedata/new-valencian-locale.diff
dpkg-source: info: applying localedata/locale-ku_TR.diff
dpkg-source: info: applying localedata/tl_PH-yesexpr.diff
dpkg-source: info: applying localedata/fo_FO-date_fmt.diff
dpkg-source: info: applying localedata/locales_CH.diff
dpkg-source: info: applying localedata/locales-fr.diff
dpkg-source: info: applying localedata/locale-en_DK.diff
dpkg-source: info: applying localedata/locale-csb_PL.diff
dpkg-source: info: applying localedata/locale-zh_TW.diff
dpkg-source: info: applying localedata/locale-se_NO.diff
dpkg-source: info: applying localedata/tailor-iso14651_t1.diff
dpkg-source: info: applying localedata/locale-hsb_DE.diff
dpkg-source: info: applying localedata/tailor-iso14651_t1-common.diff
dpkg-source: info: applying localedata/fix-lang.diff
dpkg-source: info: applying localedata/fr_CA-first_weekday.diff
dpkg-source: info: applying localedata/fr_CH-first_weekday.diff
dpkg-source: info: applying localedata/locale-de_LI.diff
dpkg-source: info: applying localedata/submitted-bz9725-locale-sv_SE.diff
dpkg-source: info: applying localedata/first_weekday.diff
dpkg-source: info: applying localedata/locale-C.diff
dpkg-source: info: applying localedata/locale-nb_NO.diff
dpkg-source: info: applying localedata/submitted-en_AU-date_fmt.diff
dpkg-source: info: applying localedata/submitted-es_MX-decimal_point.diff
dpkg-source: info: applying localedata/submitted-locale-bo.diff
dpkg-source: info: applying localedata/submitted-it_IT-thousands_sep.diff
dpkg-source: info: applying alpha/local-gcc4.1.diff
dpkg-source: info: applying alpha/submitted-dl-support.diff
dpkg-source: info: applying alpha/submitted-rtld-fPIC.diff
dpkg-source: info: applying alpha/local-string-functions.diff
dpkg-source: info: applying amd64/local-blacklist-on-TSX-Haswell.diff
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dpkg-source: info: applying arm/unsubmitted-ldso-abi-check.diff
dpkg-source: info: applying arm/local-soname-hack.diff
dpkg-source: info: applying arm/local-vfp-sysdeps.diff
dpkg-source: info: applying arm/unsubmitted-ldso-multilib.diff
dpkg-source: info: applying arm/cvs-arm-sfi_breg.diff
dpkg-source: info: applying arm/cvs-memcpy-memmove-always-bx.diff
dpkg-source: info: applying arm/local-arm-futex.diff
dpkg-source: info: applying hppa/local-inlining.diff
dpkg-source: info: applying hppa/local-stack-grows-up.diff
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dpkg-source: info: applying hppa/local-fcntl-osync.diff
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dpkg-source: info: applying hurd-i386/local-enable-ldconfig.diff
dpkg-source: info: applying hurd-i386/tg-libpthread_depends.diff
dpkg-source: info: applying i386/local-biarch.diff
dpkg-source: info: applying i386/local-cmov.diff
dpkg-source: info: applying i386/local-cpuid-level2.diff
dpkg-source: info: applying i386/submitted-i686-timing.diff
dpkg-source: info: applying i386/unsubmitted-quiet-ldconfig.diff
dpkg-source: info: applying kfreebsd/submitted-waitid.diff
dpkg-source: info: applying kfreebsd/submitted-auxv.diff
dpkg-source: info: applying kfreebsd/local-config_h_in.patch
dpkg-source: info: applying kfreebsd/local-grantpt.diff
dpkg-source: info: applying kfreebsd/local-sysdeps.diff
dpkg-source: info: applying kfreebsd/local-fbtl.diff
dpkg-source: info: applying kfreebsd/local-fbtl-depends.diff
dpkg-source: info: applying kfreebsd/local-scripts.diff
dpkg-source: info: applying kfreebsd/local-getaddrinfo-freebsd-kernel.diff
dpkg-source: info: applying kfreebsd/local-no-execfn.diff
dpkg-source: info: applying m68k/local-dwarf2-buildfix.diff
dpkg-source: info: applying m68k/local-fpic.diff
dpkg-source: info: applying m68k/local-reloc.diff
dpkg-source: info: applying m68k/submitted-gcc34-seccomment.diff
dpkg-source: info: applying mips/local-r10k.diff
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dpkg-source: info: applying powerpc/local-powerpc8xx-dcbz.diff
dpkg-source: info: applying powerpc/local-tune-power8.diff
dpkg-source: info: applying powerpc/cvs-ppc-sqrt.diff
dpkg-source: info: applying powerpc/cvs-ppc-sqrtf.diff
dpkg-source: info: applying powerpc/cvs-ppc-pow.diff
dpkg-source: info: applying powerpc/cvs-ppc-feraiseexcept.diff
dpkg-source: info: applying powerpc/cvs-power7-strncpy.diff
dpkg-source: info: applying powerpc/cvs-ppc-tabort-le.diff
dpkg-source: info: applying s390/submitted-nexttowardf.diff
dpkg-source: info: applying sh4/local-fpscr_values.diff
dpkg-source: info: applying sparc/local-sparcv9-target.diff
dpkg-source: info: applying sparc/submitted-timing.diff
dpkg-source: info: applying all/local-alias-UTF-8.diff
dpkg-source: info: applying all/local-alias-et_EE.diff
dpkg-source: info: applying all/local-remove-manual.diff
dpkg-source: info: applying all/local-ru_RU.diff
dpkg-source: info: applying all/local-ldd.diff
dpkg-source: info: applying all/local-nis-shadow.diff
dpkg-source: info: applying all/submitted-po-fr-fixes.diff
dpkg-source: info: applying any/local-rtlddir-cross.diff
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dpkg-source: info: applying any/local-bindresvport_blacklist.diff
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dpkg-source: info: applying any/local-fhs-linux-paths.diff
dpkg-source: info: applying any/local-fhs-nscd.diff
dpkg-source: info: applying any/local-ld-multiarch.diff
dpkg-source: info: applying any/local-ldso-disable-hwcap.diff
dpkg-source: info: applying any/local-ldconfig.diff
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dpkg-source: info: applying any/local-libgcc-compat-abilists.diff
dpkg-source: info: applying any/local-mktemp.diff
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dpkg-source: info: applying any/submitted-bits-fcntl_h-at.diff
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dpkg-source: info: applying any/submitted-nptl-invalid-td.patch
dpkg-source: info: applying any/local-dlfptr.diff
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dpkg-source: info: applying any/local-ldconfig-multiarch.diff
dpkg-source: info: applying any/local-tst-writev.diff
dpkg-source: info: applying any/local-disable-libnss-db.diff
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dpkg-source: info: applying any/local-tst-eintr1-eagain.diff
dpkg-source: info: applying any/unsubmitted-ldso-machine-mismatch.diff
dpkg-source: info: applying any/unsubmitted-dlopen-static-crash.diff
dpkg-source: info: applying any/unsubmitted-scanf-includes.diff
dpkg-source: info: applying any/local-ldconfig-ignore-ld.so.diff
dpkg-source: info: applying any/local-no-malloc-backtrace.diff
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dpkg-source: info: applying any/local-libpic.diff
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dpkg-source: info: applying any/cvs-ldconfig-aux-cache.diff
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dpkg-source: info: applying any/local-tester-gcc-4.9.diff
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dpkg-source: info: applying any/cvs-localplt-new-readelf.diff
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dpkg-source: info: applying ubuntu/local-warn-bsd-source.diff
内容挺多的，一条一条的看
glibc 2.21-0ubuntu4 (dsc)
dpkg-source: 警告: 对 ./glibc_2.21-0ubuntu4.dsc 校验签名失败
对比这两条输出，基本可以判定 .dsc 文件是dsc加密所需的文件。与源代码无关。
glibc 2.21-0ubuntu4 (tar) [12.7 MB]从文件格式与大小判断，这就是glibc的源代码。
glibc 2.21-0ubuntu4 (diff) [994 kB]这就是补丁文件，这个补丁文件不需要我们手动下载，它会随程序的源文件一起下载，关于diff文件，请参考这篇blog： http://www.cnblogs.com/cute/archive/2011/04/29/2033011.html 在下惭愧，这篇文章我还没看了 。
好了一切万事俱备，接下来该调试一下了。
还是使用咱们的signal程序，启动gdb进行调试：
gdb ./test_signal_2
start
遇到signal函数进入
(gdb) step
34 if(signal(SIGINT,signal_handler) == SIG_ERR)
(gdb) step
__bsd_signal (sig=2, handler=0x40099b )
at ../sysdeps/posix/signal.c:36
36 ../sysdeps/posix/signal.c: 没有那个文件或目录.
此时发现没有那个文件，使用directory命令加入
directory /home/glibc-2.21/sysdeps/posix/之所以没有在一开始就使用directory命令，是因为directory命令无法对源文件所在目录进行递归搜索。但在很多时候我们无法知道某个函数的实现在哪个文件中，因此我采用这个方法：先由gdb定位文件，再使用directory命令。
以上。