gcc编译生成动态链接库,静态库步骤

时间:2022-01-26 15:50:06

废话不说,直接干货:

首先touch三个文件test.h,test.c,main.c

test.c文件内容:

int add(int a, int b)
{
return a + b;
}

int sub(int a, int b)
{
return a - b;
}

int mul(int a, int b)
{
return a * b;
}

int div(int a, int b)
{
return a / b;
}

test.h文件内容:

#ifndef _TEST_H_

#define _TEST_H_

extern int add(int a, int b);
extern int sub(int a, int b);
extern int mul(int a, int b);
extern int div(int a, int b);

#endif

main.c文件内容:

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

int main ()
{
int a, b;
printf("please input a and b\n");
scanf("%d%d", &a, &b);

printf("The add : %d\n", add(a, b));
printf("The sub : %d\n", sub(a, b));
printf("The mul : %d\n", mul(a, b));
printf("The div : %d\n", div(a, b));

return 0;
}

总结使用gcc生成动态库及使用动态库的方法:

step 1: 生成test.o目标文件,使用如下命令。此处需要添加-fPIC参数,该参数用于生成位置无关代码已共生成动态库使用:

[xgqin@xgqin-desktop so]$ gcc -c -o test.o -fPIC test.c 
[xgqin@xgqin-desktop so]$ ls
app_static libtest.a main.c test.c test.h test.o
[xgqin@xgqin-desktop so]$ file test.o
test.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

step 2: 使用-shared参数生成动态库,使用如下命令:

[xgqin@xgqin-desktop so]$ gcc -shared -o libmyshare.so test.o 
[xgqin@xgqin-desktop so]$ ls
app_static libmyshare.so libtest.a main.c test.c test.h test.o
[xgqin@xgqin-desktop so]$ file libmyshare.so
libmyshare.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=0x4053caa2d2d48b9b026576a69929a4a44abedcb0, not stripped
上述两个命令可以合在一块,如下所示

[xgqin@xgqin-desktop so]$ gcc -shared -fPIC -o libmyshare.so test.c 
[xgqin@xgqin-desktop so]$ file libmyshare.so
libmyshare.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=0x4053caa2d2d48b9b026576a69929a4a44abedcb0, not stripped

step 3: 编译main.c,使用libmyshare.so动态库:

[xgqin@xgqin-desktop so]$ gcc -o app_share main.c -L. -lmyshare
[xgqin@xgqin-desktop so]$ ls
app_share app_static libmyshare.so libtest.a main.c test.c test.h test.o
[xgqin@xgqin-desktop so]$ ldd app_share
linux-vdso.so.1 => (0x00007fff3972c000)
libmyshare.so => not found
libc.so.6 => /lib64/libc.so.6 (0x0000003039600000)
/lib64/ld-linux-x86-64.so.2 (0x0000003039200000)
使用ldd命令查看app_share使用的动态库,发现提示libmyshare无法找到,如果直接执行app_share,则出现如下错误:

[xgqin@xgqin-desktop so]$ ./app_share 
./app_share: error while loading shared libraries: libmyshare.so: cannot open shared object file: No such file or directory
查看相关资料后,提示与LD_LIBRARY_PATH相关,首先使用export LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH将当前目录加入LD_LIBRARY_PATH变量中。再次运行ldd app_share,如下:
[xgqin@xgqin-desktop so]$ gcc -o app_share main.c -L. -lmyshare[xgqin@xgqin-desktop so]$ ldd app_share 	linux-vdso.so.1 =>  (0x00007fff26b70000)	libmyshare.so => not found	libc.so.6 => /lib64/libc.so.6 (0x0000003039600000)	/lib64/ld-linux-x86-64.so.2 (0x0000003039200000)[xgqin@xgqin-desktop so]$ export LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH[xgqin@xgqin-desktop so]$ echo $LD_LIBRARY_PATH.:[xgqin@xgqin-desktop so]$ ldd app_share 	linux-vdso.so.1 =>  (0x00007fff55dbe000)	libmyshare.so => ./libmyshare.so (0x00007fb8c3b18000)	libc.so.6 => /lib64/libc.so.6 (0x0000003039600000)	/lib64/ld-linux-x86-64.so.2 (0x0000003039200000)[xgqin@xgqin-desktop so]$ ./app_share please input a and b4 2The add : 6The sub : 2The mul : 8The div : 2
 注:注意观察export前后的两次ldd执行时,libmyshare.so的指向情况。

另一种编译main.c,并链接libmyshare.so的方式如下(该方式通过./libmyshare.so直接指定使用当前目录下的libmyshare.so文件:

[xgqin@xgqin-desktop so]$ gcc -o app_share main.c ./libmyshare.so 
[xgqin@xgqin-desktop so]$ ldd app_share
linux-vdso.so.1 => (0x00007fff066c3000)
./libmyshare.so (0x00007f945b6b0000)
libc.so.6 => /lib64/libc.so.6 (0x0000003039600000)
/lib64/ld-linux-x86-64.so.2 (0x0000003039200000)
[xgqin@xgqin-desktop so]$ ./app_share
please input a and b
4 2
The add : 6
The sub : 2
The mul : 8
The div : 2
[xgqin@xgqin-desktop so]$


总结使用gcc生成静态库及使用静态库的方法:

在此例中,test.c用于编译生成静态库libtest.a,test.h为libtest.a对应的头文件。

    step 1: 生成test.o目标文件,使用如下命令:

[xgqin@xgqin-desktop so]$ ls
main.c test.c test.h
[xgqin@xgqin-desktop so]$ gcc -c test.c -o test.o
[xgqin@xgqin-desktop so]$ ls
main.c test.c test.h test.o

在第一步中使用gcc -c test.c -o test.o首先生成test.o目标文件。

    step 2: 使用ar将test.o打包成libtest.a静态库,使用如下命令:

[xgqin@xgqin-desktop so]$ ar rcs -o libtest.a test.o 
[xgqin@xgqin-desktop so]$ ls
libtest.a main.c test.c test.h test.o

step 3:生成libtest.a静态库后,可以使用ar t参数查看libtest.a文件中包含哪些文件:

[xgqin@xgqin-desktop so]$ ar t libtest.a test.o
[xgqin@xgqin-desktop so]$

step 4: 编译main.c,并使用libtest.a静态库,链接时-l参数后不加空格指定所需链接的库,这里库名是libtest.a,但是只需要给出-ltest即可,ld会以libtest作为库的实际名字(例如-lm参数,实际表示链接libm库,也就是数学库):

[xgqin@xgqin-desktop so]$ gcc -o app_static main.c -L. -ltest[xgqin@xgqin-desktop so]$ lsapp_static  libtest.a  main.c  test.c  test.h  test.o

或使用如下方式编译main.c,并使用libtest.a静态库:

[xgqin@xgqin-desktop so]$ gcc -o app_static main.c libtest.a
[xgqin@xgqin-desktop so]$ ls
app_static libtest.a main.c test.c test.h test.o

step 5: 运行app_static:

[xgqin@xgqin-desktop so]$ ./app_static 
please input a and b
4 2
The add : 6
The sub : 2
The mul : 8
The div : 2
[xgqin@xgqin-desktop so]$

step 6: 使用readelf 查看app_static的符号表,观察sub, add, mul, div等函数是否处于app_static的.text段中。注意,section headers中指出 .text代码段的编号是13,而再symbol table '.symtab'中则显示add, div, mul, sub均处于13段中,也就是.text代码段中。因此libtest.a静态库经过链接后对应的函数代码已加入app_static程序代码段中,此为静态链接。
[xgqin@xgqin-desktop so]$ readelf -a app_static     .... 
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
....
[13] .text PROGBITS 0000000000400520 00000520
0000000000000274 0000000000000000 AX 0 0 16
[14] .fini PROGBITS 0000000000400794 00000794
0000000000000009 0000000000000000 AX 0 0 4
[15] .rodata PROGBITS 00000000004007a0 000007a0
0000000000000062 0000000000000000 A 0 0 8
....
[29] .strtab STRTAB 0000000000000000 00001fb8
000000000000027b 0000000000000000 0 0 1
Symbol table '.symtab' contains 72 entries:
Num: Value Size Type Bind Vis Ndx Name
....
49: 00000000004006c4 20 FUNC GLOBAL DEFAULT 13 add
....
53: 0000000000400701 19 FUNC GLOBAL DEFAULT 13 div
....
64: 0000000000400610 179 FUNC GLOBAL DEFAULT 13 main
65: 00000000004006ee 19 FUNC GLOBAL DEFAULT 13 mul
....
70: 00000000004006d8 22 FUNC GLOBAL DEFAULT 13 sub
....
注:《Linux C程序设计大全》中给出的三种链接静态库的方法在实际使用时均出现报错,如下所示

[xgqin@xgqin-desktop so]$ gcc main.c -llibtest.a -o app
/usr/bin/ld: cannot find -llibtest.a
collect2: error: ld returned 1 exit status
[xgqin@xgqin-desktop so]$ gcc main.c -ltest.a -o app
/usr/bin/ld: cannot find -ltest.a
collect2: error: ld returned 1 exit status
[xgqin@xgqin-desktop so]$ gcc main.c -ltest -o app
/usr/bin/ld: cannot find -ltest
collect2: error: ld returned 1 exit status
[xgqin@xgqin-desktop so]$ gcc -L. main.c -o app
/tmp/ccEpq6Fp.o: In function `main':
main.c:(.text+0x37): undefined reference to `add'
main.c:(.text+0x57): undefined reference to `sub'
main.c:(.text+0x77): undefined reference to `mul'
collect2: error: ld returned 1 exit status
[xgqin@xgqin-desktop so]$ gcc main.c -static ./libtest.a -o app
/usr/bin/ld: cannot find -lc
collect2: error: ld returned 1 exit status
[xgqin@xgqin-desktop so]$
具体出错原因,个人推测应与ld相关。