Shulou(Shulou.com)06/01 Report--

Today, I will talk to you about the learning and thinking of lib files under linux, which may not be well understood by many people. in order to make you understand better, the editor has summarized the following contents for you. I hope you can get something according to this article.

Speaking of this LIB file, let's start with a glitch.

One day the developer said that a virtual machine for testing can PING and SSH can not be connected. The operation and maintenance students hastened to check that the SSHD_CONFIG configuration files were all correct and there were no mistakes at all, so why?

Under the test, both yourself and other machines reported an error.

Note here that you have a libcom_err.so.2 shared library file that cannot be found.

Ask the developer to understand that they test a software and accidentally delete a library file.

Then, under the view of the normal virtual machine with the same virtual machine, and then compared with the error virtual machine, it is found that 2 library files are missing.

Just mount the system CD or copy these two files from the normal virtual machine and put them under lib64.

Try again. Normal.

This glitch is easy to fix, so how do you understand the library files in linux? Study without missing out on work.

The library files under Linux are divided into two categories: shared libraries and static libraries. The difference between them is only when the code needed for program execution is dynamically loaded at run time or statically loaded at compile time.

Linux libraries are usually recorded in the / lib or / usr/lib directory, and if it is a 64-bit system, there will be a lib64 directory. Lib is the abbreviation of Library. It mainly stores the linked library files of the system. Without this directory, the system will not be able to operate normally. The / lib directory stores the shared libraries that the program uses when it runs. Through shared libraries, many programs can reuse the same code, and these libraries can be stored in a common location, thus reducing the size of the running program. This directory contains the libraries that the program uses when linking.

Knowledge of the library

1. Naming of the library

The naming of libraries is relatively simple, the first feature is that all libraries start with lib, and the GCC command automatically adds lib before the file name specified in the-l option.

The second feature is a library whose filename ends in .a.

The third feature is that the .so library is a shared library (the shared library is dynamically loaded at run time). By default, GCC gives priority to shared libraries when linking, and static libraries are considered only if the shared library does not exist.

2. Number of the library

The number format of the library is as follows:

Library_name .major.num .minor _ .min .pathch _ num

For example, the GUN database of author Red Hat Linux 9.0 is libgdbm.so.0.0.2, which is described in detail as follows:

◆ library_name is libc.so (standard C library)

◆ major_num is 2 (major version number)

◆ minor_.min is 0 (minor version number)

◆ pathch_num is 0 (patch alias, also known as issue numbers).

3. Operation commands of the library

Linux library operations can be done using commands, the most commonly used commands are ldd and ldconfig.

Ldd is an acronym for Library Dependency Display and its purpose is to show a shared library that an executable must use.

(1) Command format

Ldd [option] file name

(2) main parameters

-d performs relocation and reports missing functions.

-r performs the relocation of functions and data objects and reports missing functions and data objects.

(3) Application examples

For example, to query which shared libraries are available in the Perl language, you can first use the find command to query the absolute path of the program, and then use the ldd command:

# find-name perl

Ldd / usr/bin/perl

$ldd test

By executing test, you can see how it calls functions in the dynamic library.

2.ldconfig

The purpose of the ldconfig command is to determine the running links required for shared libraries located in the directories / usr/lib and / lib. These links keep the existing Libs saved in the / et/ld.so.conf file. Search for shareable dynamic link libraries (format as described above, lib*.so*), and then create links and cache files needed by the dynamic loader (ld.so). The cache file defaults to / etc / ld.so.cache, which holds a sorted list of dynamic-link library names.

(1) Command format

Ldconfig [options] [libs]

(2) main options

Either-v or-- verbose ldconfig will display the directory being scanned, the dynamic link library searched, and the name of the connection it created.

-f CONF specifies that the configuration file of the dynamic link library is CONF, and the system defaults to / etc/ld.so.conf.

-C CACHE specifies that the generated cache file is CACHE, and the system defaults to / etc/ld.so.cache, which holds a sorted list of shareable dynamic link libraries.

-p or-- print-cache asks ldconfig to print out the names of all shared libraries saved by the current cache file.

-r ROOT changes the root directory of the application to ROOT.

"- n ldconfig scans only the directories specified on the command line, not the default directories (/ lib, / usr/lib), nor the directories listed in the configuration file / etc/ld.so.conf."

Used to update cache files when running the ldconfig command with no options. This command is mainly used to cache the DNS server (Caching DNS Server). The principle of caching DNS server is to provide historical records of queries and use these records to improve the efficiency of queries.

When a query is sent to the cached DNS server for the first time, the cached DNS server records the whole process of the query and uses it to answer all the same queries within a certain period of time, thus reducing the burden of the whole DNS system and improving the query speed.

(3) Application examples

If the user wants to know which dynamic link libraries are in the system, or if there is a dynamic link library in the system, use the-p option to let ldconfig output the list of dynamic link libraries in the cache file. For example:

Ldconfig-p

998 libs found in cache `/ etc/ld.so.cache'

Libzvt.so.2 (libc6) = > / usr/lib/libzvt.so.2

Libzvt.so (libc6) = > / usr/lib/libzvt.so

……

Add:

Compilation and use of static link library * .a

Create .a library files and .o library files:

[yufei@localhost perl_c2] $pwd

/ home/yufei/perl_c2

[yufei@localhost perl_c2] $cat mylib.c

# include

# include

Void hello () {

Printf ("success call from perl to c library\ n")

}

[yufei@localhost perl_c2] $cat mylib.h

Extern void hello ()

[yufei@localhost perl_c2] $gcc-c mylib.c

[yufei@localhost perl_c2] $dir

Mylib.c mylib.h mylib.o

[yufei@localhost perl_c2] $ar-r mylib.a mylib.o

Ar: creating mylib.a

[yufei@localhost perl_c2] $dir

Mylib.a mylib.c mylib.h mylib.o

* how to use .a

The simplest thing is to compile .a directly as a normal source code.

Gcc main.cpp. / lib/libInfo.a-o exec

Compilation and use of dynamic link library * .so--

Dynamic library * .so is often encountered when programming with c and C++ under linux. Here is a note to provide a little help to other brothers who are struggling with dynamic library link libraries.

1. Compilation of dynamic library

Here is an example of how to generate a dynamic library. Here is a header file: so_test.h, three .c files: test_a.c, test_b.c, and test_c.c, which we compile into a dynamic library: libtest.so.

So_test.h:

# include

# include

Void test_a ()

Void test_b ()

Void test_c ()

Test_a.c:

# include "so_test.h"

Void test_a ()

{

Printf ("this is intest_a...\ n")

}

Test_b.c:

# include "so_test.h"

Void test_b ()

{

Printf ("this is intest_b...\ n")

}

Test_c.c:

# include "so_test.h"

Void test_c ()

{

Printf ("this is intest_c...\ n")

}

Compile these files into a dynamic library: libtest.so

$gcc test_a.c test_b.c test_c.c-fPIC-shared-o libtest.so

2. Links to dynamic libraries

In 1, we have successfully generated our own dynamic link library libtest.so, let's use a program to call the functions in this library. The source file of the program is test.c.

Test.c:

# include "so_test.h"

Int main ()

{

Test_a ()

Test_b ()

Test_c ()

Return 0

}

Link test.c with the dynamic library libtest.so to generate the execution file test:

$gcc test.c-L. -l test-o test

L test whether the dynamic connection. If libtest.so is listed, then the connection should be normal.

$ldd test

L executes test, and you can see how it calls functions in the dynamic library.

Summary:

1. The shared library is especially suitable for multiple programs to share code, upgrade some functional modules of the program, and realize the "plug-in" function of the program.

On the other hand, static libraries are done once and for all, and they do not need to run with a bunch of library files after compilation, and they can run normally no matter where they are placed.

2. When both the static version and the shared version of the library exist in the searched library file directory, the linker gives priority to the shared version .so. In this case, you can use the-static link option to specify the link static version .a.

3. The dynamic library can export two special functions: _ init and _ fini. The former is called after the dynamic library is loaded, and the latter is called before the dynamic library is unloaded.

We can use these two functions to do some special work. It is important to note that when compiling after defining these two functions, you need to use the

-nostartfiles option, otherwise the compiler reports a duplicate definition error.

4. The ldd command is used to view the shared library on which the program depends, and it is also convenient for us to determine whether the shared library has been found.

The nm command looks at the identities (functions, variables) in the obj file (.so is also an obj).

After reading the above, do you have any further understanding of the learning and thinking of the lib file under linux? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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