Why does the nginx source dock file contain < >, instead of " "

Question

When looking at the nginx source code, I see that header files are included using #include <ngx_core.h> #include <nginx.h> A similar format, an error will be reported in the IDE that the file cannot be found
These files are not in the system directory, why use "<>"

Answer 1

The following excerpt is from the original text of ISO/IEC 9899:1999:

# include <h-char-sequence> new-line
searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the < and > delimiters, and causes the replacement of that directive by the entire contents of the header.

How the places are specified or the header identified is implementation-defined.

Did you notice the last one, implementation-defined - no one stipulates that it is the system directory, and the default behavior of many compilers is not to only search the system directory. Even if it is the same compiler, the specific behavior may depend on the options you provide, not to mention the variety of C language compilers.