原问题是clang failed with error 1，不过这个不要紧了，原因见下。
- 更新：这个错误和company-c-header没有关系，不过这个插件并没有起作用，设置如下：
(custom-set-variables
'(company-backends
(quote
(company-bbdb company-nxml company-css company-eclim company-semantic company-clang company-xcode company-cmake company-capf company-files
(company-dabbrev-code company-gtags company-etags company-keywords)
company-oddmuse company-dabbrev company-c-headers))))
- 更新2：有没有这样一种插件，不需要打tags，里文件里有一行
#include <iostream>，就可以选择打开iostream这个文件，不需要找声明定义这些，只需要打开文件就行了。
這個庫調用 libclang 嗎？如果是 libclang ，1 代表：
https://github.com/llvm-mirror/clang/blob/master/include/clang-c/CXErrorCode.h#L41
/**
* \brief Error codes returned by libclang routines.
*
* Zero (\c CXError_Success) is the only error code indicating success. Other
* error codes, including not yet assigned non-zero values, indicate errors.
*/
enum CXErrorCode {
.....
/**
* \brief A generic error code, no further details are available.
*
* Errors of this kind can get their own specific error codes in future
* libclang versions.
*/
CXError_Failure = 1,
並沒有更多信息。更多 diagnostics 要用 CINDEX_LINKAGE unsigned clang_getNumDiagnosticsInSet(CXDiagnosticSet Diags); 等取出字串形式的 diagnostics
等一下，第一次用，才知道有个clang-error buffer
Sat Jan 13 16:54:16 2018
clang failed with error 1:
/usr/bin/clang -fsyntax-only -Xclang -code-completion-macros -x c++ -std=c++11 -Xclang -code-completion-at=-:2:9 -
<stdin>:2:9: error: expected "FILENAME" or <FILENAME>
#include<U+0000>
^
1 error generated.