其实是tags文件有问题，确切说是生成tags文件时，使用的命令行参数有问题。
我最开始这样：ctags -R -f ~/.vim/tags/c 。很好笑吧，我就为它折腾了半个下午。
是在google上看了一篇帖子，改成：ctags -R -f ~/.vim/tags/c --C-kinds=+p --fields=+aS --extra=+q就好了。
原帖答案贴过来（是他自问自答）：
I found the problem.
My tags file is NOT Ok as I thought.
Incorrect version generated with:
ctags -R --C-kinds=+p --field=+S
This generates this tags file:
-bash-3.2$ cat tags
!_TAG_FILE_FORMAT 2 /extended format; --format=1 will not append ;" to lines/
!_TAG_FILE_SORTED 1 /0=unsorted, 1=sorted, 2=foldcase/
!_TAG_PROGRAM_AUTHOR Darren Hiebert /[hidden email]/
!_TAG_PROGRAM_NAME Exuberant Ctags //
!_TAG_PROGRAM_URL http://ctags.sourceforge.net /official site/
!_TAG_PROGRAM_VERSION 5.8 //
a f1.h /^ int a; $/;" m struct:mystruct
b f1.h /^ int b;$/;" m struct:mystruct
main f1.c /^void main (void)$/;" f signature:(void)
myType f1.h /^} myType;$/;" t typeref:struct:mystruct
mystruct f1.h /^typedef struct mystruct {$/;" s
-bash-3.2$
tags that works for this particular example were generated with:
ctags -R --C-kinds=+p --fields=+aS --extra=+q
-bash-3.2$ cat tags
!_TAG_FILE_FORMAT 2 /extended format; --format=1 will not append ;" to lines/
!_TAG_FILE_SORTED 1 /0=unsorted, 1=sorted, 2=foldcase/
!_TAG_PROGRAM_AUTHOR Darren Hiebert /[hidden email]/
!_TAG_PROGRAM_NAME Exuberant Ctags //
!_TAG_PROGRAM_URL http://ctags.sourceforge.net /official site/
!_TAG_PROGRAM_VERSION 5.8 //
a f1.h /^ int a; $/;" m struct:mystruct access:public
b f1.h /^ int b;$/;" m struct:mystruct access:public
main f1.c /^void main (void)$/;" f signature:(void)
myType f1.h /^} myType;$/;" t typeref:struct:mystruct
mystruct f1.h /^typedef struct mystruct {$/;" s
mystruct::a f1.h /^ int a; $/;" m struct:mystruct access:public
mystruct::b f1.h /^ int b;$/;" m struct:mystruct access:public
-bash-3.2$
The help file in omniCppComplete plugin (in .vim/doc/ folder) calls for:
ctags -R --c++-kinds=+p --fields=+iaS --extra=+q .
I had assumed that since I am not using C++ (simply C), that the +ia in --fields and +q in --extra would be irrelevant.
Thanks