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What is the difference between #include and #include "file" ?

When writing your C program, you can include files in two ways. The first way is to surround the file you
want to include with the angled brackets < and >. This method of inclusion tells the preprocessor to look for
the file in the predefined default location. This predefined default location is often an INCLUDE environment
variable that denotes the path to your include files. For instance, given the INCLUDE variable

INCLUDE=C:\COMPILER\INCLUDE;S:\SOURCE\HEADERS;

using the  #include  version of file inclusion, the compiler first checks the C:\COMPILER\INCLUDE
directory for the specified file. If the file is not found there, the compiler then checks the
S:\SOURCE\HEADERS directory. If the file is still not found, the preprocessor checks the current
directory.

The second way to include files is to surround the file you want to include with double quotation marks. This
method of inclusion tells the preprocessor to look for the file in the current directory first, then look for it
in the predefined locations you have set up. Using the  #include "file" version of file inclusion and applying
it to the preceding example, the preprocessor first checks the current directory for the specified file. If the
file is not found in the current directory, the C:\COMPILER\INCLUDE directory is searched. If the file
is still not found, the preprocessor checks the S:\SOURCE\HEADERS directory.

The #include <file> method of file inclusion is often used to include standard headers such as stdio.h or
stdlib.h. This is because these headers are rarely (if ever) modified, and they should always be read from your
compiler's standard include file directory. 

The #include "file" method of file inclusion is often used to
include nonstandard header files that you have created for use in your program. This is because these headers
are often modified in the current directory, and you will want the preprocessor to use your newly modified
version of the header rather than the older, unmodified version.