gcc -l 选项相关

加了-ldl还是说找不相关的标识，看原文档才是正道.... 

what following is quoted from gcc manual

Search the library namedlibrary when linking. (The second alternative with

the library as a separate argument is only for POSIX compliance and is not

recommended.)

It makes a difference where in the command you write this option; the linker

searches and processes libraries and object files in the order they are speci-fied. Thus, ‘foo.o -lz bar.o’ searches library ‘z’ after file ‘foo.o’ but before

‘bar.o’. If ‘bar.o’ refers to functions in ‘z’, those functions may not be loaded.

The linker searches a standard list of directories for the library, which is actually

a file named ‘liblibrary.a’. The linker then uses this file as if it had been

specified precisely by name.

The directories searched include several standard system directories plus any

that you specify with ‘-L’.

Normally the files found this way are library files—archive files whose members

are object files. The linker handles an archive file by scanning through it for

members which define symbols that have so far been referenced but not defined.

But if the file that is found is an ordinary object file, it is linked in the usual

fashion. The only difference between using an ‘-l’ option and specifying a file

name is that ‘-l’ surrounds library with ‘lib’ and ‘.a’ and searches several

directories.