What is the difference between #include <filename> and #include "filename"?

Question

In the C and C++ programming languages, what is the difference between using angle brackets and using quotes in an include statement, as follows?

1. #include <filename>
2. #include "filename"

Answer 1

In practice, the difference is in the location where the preprocessor searches for the included file.

For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.

For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.

A more complete description is available in the GCC documentation on search paths.

Answer 2

The only way to know is to read your implementation's documentation.

In the C standard, section 6.10.2, paragraphs 2 to 4 state:

• A preprocessing directive of the form

  #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.

• A preprocessing directive of the form

  #include "q-char-sequence" new-line 

  causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the " delimiters. The named source file is searched for in an implementation-defined manner. If this search is not supported, or if the search fails, the directive is reprocessed as if it read

  #include <h-char-sequence> new-line 

  with the identical contained sequence (including > characters, if any) from the original directive.

• A preprocessing directive of the form

  #include pp-tokens new-line 

  (that does not match one of the two previous forms) is permitted. The preprocessing tokens after include in the directive are processed just as in normal text. (Each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens.) The directive resulting after all replacements shall match one of the two previous forms. The method by which a sequence of preprocessing tokens between a < and a > preprocessing token pair or a pair of " characters is combined into a single header name preprocessing token is implementation-defined.

Definitions:

• h-char: any member of the source character set except the new-line character and >

• q-char: any member of the source character set except the new-line character and "