动态链接库中的.symtab和.dynsym_.dynsym和'.symtab

// 编写一个简单的 a.c
$ cat a.c
void func(void)
{
        printf("call func()\n");
}
 
$ nm a.o
00000000 T func
         U puts
 
// 编写一个简单的 main.c
$ cat main.c
#include <stdio.h>
extern void func(void);
int main(
{
        func();
        return 0;
}
 
$ nm main.o
         U func
         00000000 T main
 
// 正常情况下
$ gcc main.o a.o -o main
$ ./main
call func()
 
// 为了验证symbol对于linker来说是必需品, 我做如下操作
$ file a.o
a.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped
$ strip a.o
$ file a.o
a.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), stripped
$ gcc main.o a.o -o main
main.o: In function `main':
/home/xan/lab/main.c:7: undefined reference to `func'
collect2: ld returned 1 exit status

// 仍用上面实验的代码
$ file main
main: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.18, not stripped
$ ./main
call func()
$ strip main
$ file main
main: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.18, stripped
$ ./main
call func()
$ 

$ ldd /bin/ls
        linux-gate.so.1 =>  (0xb7711000)
        libselinux.so.1 => /lib/libselinux.so.1 (0xb76e5000)
        librt.so.1 => /lib/i686/cmov/librt.so.1 (0xb76dc000)
        libacl.so.1 => /lib/libacl.so.1 (0xb76d4000)
        libc.so.6 => /lib/i686/cmov/libc.so.6 (0xb758d000)
        libdl.so.2 => /lib/i686/cmov/libdl.so.2 (0xb7589000)
        /lib/ld-linux.so.2 (0xb7712000)
        libpthread.so.0 => /lib/i686/cmov/libpthread.so.0 (0xb7570000)
        libattr.so.1 => /lib/libattr.so.1 (0xb756b000)
$ nm /lib/i686/cmov/libc.so.6 
nm: /lib/i686/cmov/libc.so.6: no symbols    --> libattr, libacl也一样, 都显示"no symbols"
 
// 而libpthread有一大串
$ nm /lib/i686/cmov/libpthread.so.0 | tail
        U twalk@@GLIBC_2.0
        U uname@@GLIBC_2.0
        U unlink@@GLIBC_2.0
0000c930 t unwind_cleanup
0000c970 t unwind_stop
0000cfd0 W vfork
0000de90 W wait
0000df50 W waitpid
0000c140 t walker
0000d020 W write

$ sudo strip /lib/i686/cmov/libpthread-2.11.3.so
$ file /lib/i686/cmov/libpthread-2.11.3.so
/lib/i686/cmov/libpthread-2.11.3.so: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.18, stripped
$ ls
...(输出正确结果)
 
$ nm /lib/i686/cmov/libpthread-2.11.3.so
nm: /lib/i686/cmov/libpthread-2.11.3.so: no symbols
 
$ readelf -s /lib/i686/cmov/libpthread-2.11.3.so | tail
322: 0000b6a0   292 FUNC    GLOBAL DEFAULT   13 __pthread_clock_gettime@@GLIBC_PRIVATE
323: 0000ec30    46 FUNC    GLOBAL DEFAULT   13 pthread_mutex_consistent_@@GLIBC_2.4
324: 0000b3a0    50 FUNC    GLOBAL DEFAULT   13 pthread_testcancel@@GLIBC_2.0
325: 0000d6b0   111 FUNC    WEAK   DEFAULT   13 fsync@@GLIBC_2.0
326: 0000d1f0   180 FUNC    WEAK   DEFAULT   13 fcntl@@GLIBC_2.0
327: 0000dde0   176 FUNC    WEAK   DEFAULT   13 tcdrain@@GLIBC_2.0
328: 00009390     7 FUNC    GLOBAL DEFAULT   13 pthread_mutexattr_destroy@@GLIBC_2.0
329: 00006de0    23 FUNC    GLOBAL DEFAULT   13 pthread_yield@@GLIBC_2.2
330: 000077c0   259 FUNC    GLOBAL DEFAULT   13 pthread_mutex_init@@GLIBC_2.0
331: 000093c0    49 FUNC    GLOBAL DEFAULT   13 pthread_mutexattr_setpsha@@GLIBC_2.2
 
$ readelf -s /lib/libattr.so.1.1.0 | tail
48: 00002f50    50 FUNC    GLOBAL DEFAULT   13 lremovexattr@@ATTR_1.0
49: 00003010    57 FUNC    GLOBAL DEFAULT   13 llistxattr@@ATTR_1.0
50: 00002ae0    50 FUNC    GLOBAL DEFAULT   13 attr_copy_check_permissio@@ATTR_1.1
51: 00001b50   259 FUNC    GLOBAL DEFAULT   13 attr_set@@ATTR_1.0
52: 00002b20  1002 FUNC    GLOBAL DEFAULT   13 attr_copy_action
53: 000031f0    71 FUNC    GLOBAL DEFAULT   13 setxattr@@ATTR_1.0
54: 00001380   543 FUNC    GLOBAL DEFAULT   13 attr_list@@ATTR_1.2
55: 000030d0    64 FUNC    GLOBAL DEFAULT   13 lgetxattr@@ATTR_1.0
56: 00002fd0    57 FUNC    GLOBAL DEFAULT   13 flistxattr@@ATTR_1.0
57: 00002f10    50 FUNC    GLOBAL DEFAULT   13 fremovexattr@@ATTR_1.0