远端gdbserver已经起来，本地使用gdb已经连接上gdbserver，但是本地提示找不到符号文件：
warning: Could not load vsyscall page because no executable was specified
try using the "file" command first.
那么使用file命令，指定被调试的可执行文件即可：
file /opt/linuxshare/helloworld/main
这里要注意一下，file指定的main已经是一个ELF可执行文件，也就是正在被调试的程序，另外，这个文件其实是存在于远端磁盘上，但是已经被mount到了本地机器上。
file成功后即可看到源码：
(gdb) file /opt/linuxshare/helloworld/main
A program is being debugged already.
Are you sure you want to change the file? (y or n) y
Reading symbols from /opt/linuxshare/helloworld/main...done.
(gdb) b main
Breakpoint 1 at 0x40055c: file main.c, line 3.
(gdb) info b
Num Type Disp Enb Address What
1 breakpoint keep y 0x000000000040055c in main at main.c:3
(gdb) l
1 #include <stdio.h>
2 int main() {
3 printf("Hello World\n");
4
5 int i = 0;
6 while(1)
7 {
8 sleep(3);
9 printf("time: %d\n", ++i);
10 }
