linux中的ldd命令简介

大家好，又见面了，我是你们的朋友全栈君。

在linux中，有些命令是大家通用的，比如ls, rm, mv, cp等等，这些我觉得没有必要再细说了。而有些命令，只有开发人员才会用到的，这类命令，作为程序员的我们，是有必要了解的，有的甚至需要熟练使用。

有的人总说，这些命令不重要，用的时候去查就行了，这么多么扯淡的说法啊。具体用法细节是可以可查，但至少得知道有ldd这个东西吧。连ldd都不知道，怎么知道ldd是干啥的呢？

在本文中，我们来介绍一下ldd命令，尽管它非常简单。哦，我突然想起来，我有个朋友，她的名字的是三个字，首写字母刚好是l, d, d, 有点意思。在linux中， ldd是list, dynamic, dependencies的缩写，意思是，列出动态库依赖关系。当然，你也可以用ldd –help或者man ldd来看其用法。下面，我们也来看看：

test.h的内容为：

void print();

test.c的内容为：

#include <stdio.h>
#include "test.h"

void print()
{
printf("rainy days\n");
}

main.c的内容为：

#include "test.h"

int main()
{
	print();
	return 0;
}

进行一系列的编译，并用ldd命令，得到：

[taoge@localhost learn_ldd]$ ls
main.c  test.c  test.h
[taoge@localhost learn_ldd]$ gcc -c main.c test.c
[taoge@localhost learn_ldd]$ gcc main.o test.o
[taoge@localhost learn_ldd]$ ls
a.out  main.c  main.o  test.c  test.h  test.o
[taoge@localhost learn_ldd]$ ./a.out 
rainy days
[taoge@localhost learn_ldd]$ 
[taoge@localhost learn_ldd]$ 
[taoge@localhost learn_ldd]$ 
[taoge@localhost learn_ldd]$ ldd *
a.out:
	linux-gate.so.1 =>  (0x00ba1000)
	libc.so.6 => /lib/libc.so.6 (0x0087e000)
	/lib/ld-linux.so.2 (0x00858000)
main.c:
ldd: warning: you do not have execution permission for `./main.c'
	not a dynamic executable
main.o:
ldd: warning: you do not have execution permission for `./main.o'
	not a dynamic executable
test.c:
ldd: warning: you do not have execution permission for `./test.c'
	not a dynamic executable
test.h:
ldd: warning: you do not have execution permission for `./test.h'
lddlibc4: cannot read header from `./test.h'
test.o:
ldd: warning: you do not have execution permission for `./test.o'
	not a dynamic executable
[taoge@localhost learn_ldd]$

可以看到a.out依赖于libc.so.6这个库，而这个库的路径为/lib/libc.so.6

我们继续看使用静态链接库的情形：

[taoge@localhost learn_ldd]$ ls
main.c  test.c  test.h
[taoge@localhost learn_ldd]$ gcc -c test.c
[taoge@localhost learn_ldd]$ ar rcs libtest.a test.o
[taoge@localhost learn_ldd]$ gcc main.c -L. -ltest
[taoge@localhost learn_ldd]$ ls
a.out  libtest.a  main.c  test.c  test.h  test.o
[taoge@localhost learn_ldd]$ ./a.out
rainy days
[taoge@localhost learn_ldd]$ 
[taoge@localhost learn_ldd]$ 
[taoge@localhost learn_ldd]$ 
[taoge@localhost learn_ldd]$ ldd *
a.out:
	linux-gate.so.1 =>  (0x00e7c000)
	libc.so.6 => /lib/libc.so.6 (0x0087e000)
	/lib/ld-linux.so.2 (0x00858000)
libtest.a:
ldd: warning: you do not have execution permission for `./libtest.a'
	not a dynamic executable
main.c:
ldd: warning: you do not have execution permission for `./main.c'
	not a dynamic executable
test.c:
ldd: warning: you do not have execution permission for `./test.c'
	not a dynamic executable
test.h:
ldd: warning: you do not have execution permission for `./test.h'
lddlibc4: cannot read header from `./test.h'
test.o:
ldd: warning: you do not have execution permission for `./test.o'
	not a dynamic executable
[taoge@localhost learn_ldd]$

这次用静态库，结果还是差不多，就没什么好说的了。

我们继续看使用动态链接库时的情形：

[taoge@localhost learn_ldd]$ ls
main.c  test.c  test.h
[taoge@localhost learn_ldd]$ gcc -c test.c
[taoge@localhost learn_ldd]$ gcc -shared -fPIC -o libtest.so test.o
[taoge@localhost learn_ldd]$ gcc main.c -L. -ltest
[taoge@localhost learn_ldd]$ ls
a.out  libtest.so  main.c  test.c  test.h  test.o
[taoge@localhost learn_ldd]$ ./a.out 
./a.out: error while loading shared libraries: libtest.so: cannot open shared object file: No such file or directory
[taoge@localhost learn_ldd]$ 
[taoge@localhost learn_ldd]$ 
[taoge@localhost learn_ldd]$ 
[taoge@localhost learn_ldd]$ ldd *
a.out:
	linux-gate.so.1 =>  (0x00f3d000)
	libtest.so => not found
	libc.so.6 => /lib/libc.so.6 (0x0087e000)
	/lib/ld-linux.so.2 (0x00858000)
libtest.so:
	linux-gate.so.1 =>  (0x0031d000)
	libc.so.6 => /lib/libc.so.6 (0x00110000)
	/lib/ld-linux.so.2 (0x00858000)
main.c:
ldd: warning: you do not have execution permission for `./main.c'
	not a dynamic executable
test.c:
ldd: warning: you do not have execution permission for `./test.c'
	not a dynamic executable
test.h:
ldd: warning: you do not have execution permission for `./test.h'
lddlibc4: cannot read header from `./test.h'
test.o:
ldd: warning: you do not have execution permission for `./test.o'
	not a dynamic executable
[taoge@localhost learn_ldd]$ 
[taoge@localhost learn_ldd]$ su root
Password: 
[root@localhost learn_ldd]# cp libtest.so /usr/lib/
[root@localhost learn_ldd]# ./a.out 
rainy days
[root@localhost learn_ldd]# exit
exit
[taoge@localhost learn_ldd]$ ./a.out 
rainy days
[taoge@localhost learn_ldd]$
[taoge@localhost learn_ldd]$
[taoge@localhost learn_ldd]$
[taoge@localhost learn_ldd]$ ldd a.out 
	linux-gate.so.1 =>  (0x00510000)
	libtest.so => /usr/libtest.so (0x00fe3000)
	libc.so.6 => /lib/libc.so.6 (0x0087e000)
	/lib/ld-linux.so.2 (0x00858000)
[taoge@localhost learn_ldd]$

首先，我们可以看到， a.out依赖于libtest.so这个库，但是，结果是not found，找不到。为什么呢？因为在/usr/lib下面没有libtest.so，后来，我把libtest.so拷贝过去（需要root权限），就OK了。另外，我们也应该看到， libtest.so的依赖库也是可以通过ldd命令找到的。

当然，如果不想自己写程序，但想试一下ldd命令，那也可以，直接如下：

[taoge@localhost learn_ldd]$ ldd /bin/ls
	linux-gate.so.1 =>  (0x0052b000)
	libselinux.so.1 => /lib/libselinux.so.1 (0x00b52000)
	librt.so.1 => /lib/librt.so.1 (0x00a5c000)
	libcap.so.2 => /lib/libcap.so.2 (0x0489c000)
	libacl.so.1 => /lib/libacl.so.1 (0x048c9000)
	libc.so.6 => /lib/libc.so.6 (0x0087e000)
	libdl.so.2 => /lib/libdl.so.2 (0x00a0c000)
	/lib/ld-linux.so.2 (0x00858000)
	libpthread.so.0 => /lib/libpthread.so.0 (0x00a13000)
	libattr.so.1 => /lib/libattr.so.1 (0x04d99000)
[taoge@localhost learn_ldd]$ ldd /bin/mv
	linux-gate.so.1 =>  (0x00944000)
	libselinux.so.1 => /lib/libselinux.so.1 (0x00b52000)
	librt.so.1 => /lib/librt.so.1 (0x00a5c000)
	libacl.so.1 => /lib/libacl.so.1 (0x048c9000)
	libattr.so.1 => /lib/libattr.so.1 (0x04d99000)
	libc.so.6 => /lib/libc.so.6 (0x00110000)
	libdl.so.2 => /lib/libdl.so.2 (0x00a0c000)
	/lib/ld-linux.so.2 (0x00858000)
	libpthread.so.0 => /lib/libpthread.so.0 (0x00a13000)
[taoge@localhost learn_ldd]$

在实际linux开发与调试中，要经常查看动态库依赖关系， ldd用得还是比较多的，特别是出现故障的时候。OK, ldd命令就简单介绍到这里了，虽然简单，但很实用，故不可不知。

好了，今天也写了不少文章了，就此搁笔。收拾一下，看看书，早点休息，迎接明天的周一。

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