2.1 Include Syntax
Both user and system header files are included using the preprocessing directive ‘#include’. It has two variants:
#include <file>-
This variant is used for system header files. It searches for a file named file in a standard list of system directories. You can prepend directories to this list with the -I option (see Invocation). #include "file"-
This variant is used for header files of your own program. It searches for a file named file first in the directory containing the current file, then in the quote directories and then the same directories used for<file>. You can prepend directories to the list of quote directories with the -iquote option.
The argument of ‘#include’, whether delimited with quote marks or angle brackets, behaves like a string constant in that comments are not recognized, and macro names are not expanded. Thus, #include <x/*y> specifies inclusion of a system header file named x/*y.
However, if backslashes occur within file, they are considered ordinary text characters, not escape characters. None of the character escape sequences appropriate to string constants in C are processed. Thus, #include "x\n\\y" specifies a filename containing three backslashes. (Some systems interpret ‘\’ as a pathname separator. All of these also interpret ‘/’ the same way. It is most portable to use only ‘/’.)
It is an error if there is anything (other than comments) on the line after the file name.
摘抄自 https://gcc.gnu.org/onlinedocs/cpp/Include-Syntax.html#Include-Syntax
简单理解就是：
1. 尖括号告诉编译器去搜索系统目录，可使用 -I 选项添加其它搜索目录
2. 双引号告诉编译器先搜索源文件所在目录，其次搜索引号内的目录，最后搜索系统目录， -iquote 选项可添加其它目录到引号目录列表中。
原文地址：https://www.cnblogs.com/miaoxiong/p/10938136.html