Linux中的[g]libc和libc.so.6

本文最后更新于2015年4月18日,已超过 1 年没有更新,如果文章内容失效,还请反馈给我,谢谢!

在操作的时候犯了大错,不想多说,一句话:系统无法正常登录了……之前还不清楚libc或glibc在Linux系统上的重要性(现在真的是切切实实的认识到了,沉痛的经验教训啊!)

搜索关键字:

  • linux libc.so.
  • linux libc.so.6 stackoverflow.com
  • site:stackoverflow.com LD_LIBRARY_PATH LD_PRELOAD

==

libc implements both standard C functions like strcpy() and POSIX functions (which may be system calls) like getpid(). Note that not all standard C functions are in libc – most math functions are in libm.

You cannot directly make system calls in the same way that you call normal functions because calls to the kernel aren’t normal function calls, so they can’t be resolved by the linker. Instead, architecture-specific assembly language thunks are used to call into the kernel – you can of course write these directly in your own program too, but you don’t need to because libc provides them for you.

Note that in Linux it is the combination of the kernel and libc that provides the POSIX API. libc adds a decent amount of value – not every POSIX function is necessarily a system call, and for the ones that are, the kernel behaviour isn’t always POSIX conforming.

libc is a single library file (both .so and .a versions are available) and in most cases resides in /usr/lib. However, the glibc (GNU libc) project provides more than just libc – it also provides the libm mentioned earlier, and other core libraries like libpthread. So libc is just one of the libraries provided by glibc – and there are other alternate implementations of libc other than glibc.

=

Why glibc binary is called libc.so.6 not a libc.so.1 or libc.so.4?

Those SONAMEs have already been used. Since the new version of the library presents a break in the interface, a new major SONAME number must be used for it.

=

The term “libc” is commonly used as a shorthand for the “standard C library“, a library of standard functions that can be used by all C programs (and sometimes by programs in other languages).  Because of some history (see below), use of the term “libc” to refer to the standard C library is somewhat ambiguous on Linux.

glibc

By far the most widely used C library on Linux is the GNU C Library <http://www.gnu.org/software/libc/>, often referred to as glibc.

This is the C library that is nowadays used in all major Linux distributions.  It is also the C library whose details are documented in the relevant pages of the man-pages project (primarily in Section 3 of the manual).  Documentation of glibc is also available in the glibc manual, available via the command info libc.  Release 1.0 of glibc was made in September 1992.  (There were earlier 0.x releases.)

The next major release of glibc was 2.0, at the beginning of 1997.

The pathname /lib/libc.so.6 (or something similar) is normally a symbolic link that points to the location of the glibc library, and executing this pathname will cause glibc to display various information about the version installed on your system.

Linux libc

In the early to mid 1990s, there was for a while Linux libc, a fork of glibc 1.x created by Linux developers who felt that glibc development at the time was not sufficing for the needs of Linux.  Often, this library was referred to (ambiguously) as just “libc”.  Linux libc released major versions 2, 3, 4, and 5 (as well as many minor versions of those releases).  For a while, Linux libc was the standard C library in many Linux distributions.

However, notwithstanding the original motivations of the Linux libc effort, by the time glibc 2.0 was released (in 1997), it was clearly superior to Linux libc, and all major Linux distributions that had been using Linux libc soon switched back to glibc.  Since this switch occurred long ago, man-pages no longer takes care to document Linux libc details.  Nevertheless, the history is visible in vestiges of information about Linux libc that remain in some manual pages, in particular, references to libc4 and libc5.

Other C libraries

There are various other less widely used C libraries for Linux.

These libraries are generally smaller than glibc, both in terms of features and memory footprint, and often intended for building small binaries, perhaps targeted at development for embedded Linux systems.

Among such libraries are uClibc <http://www.uclibc.org/>, dietlibc <http://www.fefe.de/dietlibc/>, and musl libc <http://www.musl-libc.org/>.  Details of these libraries are covered by the man-pages project, where they are known.

==

参考链接:

声明: 除非注明,ixyzero.com文章均为原创,转载请以链接形式标明本文地址,谢谢!
https://ixyzero.com/blog/archives/2074.html

《Linux中的[g]libc和libc.so.6》上有11条评论

  1. 如何拯救一台glibc被干掉的Linux服务器?
    https://zhuanlan.zhihu.com/p/20062978

    今天正要下班,旁边部门的PM过来问我:
    『我们有个小伙子把生产系统上的glibc给删了,现在什么命令都跑不了了,还有救吗?』

    我说,glibc也能弄掉,莫不是rpm --force?
    回答是肯定的。

    glibc被卸载,负责加载所有.so的ld.so也就没了,因此运行几乎所有外部命令时都会得到一句『找不到ld-linux-x-y-z.so.2』的出错提示。比如ls,比如cp,以及所有动态链接的命令。

    printf和busybox拯救世界!

发表评论

电子邮件地址不会被公开。 必填项已用*标注