1、include search path
By default, the preprocessor looks for header files included by the quoteform of the directive #include "file" first relative tothe directory of the current file, and then in a preconfigured listof standard system directories.For example, if /usr/include/sys/stat.h contains #include "types.h", GCC looks for types.h first in /usr/include/sys, then in its usual search path.
For the angle-bracket form #include , thepreprocessor’s default behavior is to look only in the standard systemdirectories. The exact search directory list depends on the targetsystem, how GCC is configured, and where it is installed. You canfind the default search directory list for your version of CPP byinvoking it with the -v option.
For example, on tx2
#include "..." search starts here:
#include search starts here:
/usr/lib/gcc/aarch64-linux-gnu/5/include
/usr/local/include
/usr/lib/gcc/aarch64-linux-gnu/5/include-fixed
/usr/include/aarch64-linux-gnu
/usr/include
End of search list.