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

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"

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.

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 "

The sequence of characters between < and > uniquely refer to a header, which isn't necessarily a file. Implementations are pretty much free to use the character sequence as they wish. (Mostly, however, just treat it as a file name and do a search in the include path, as the other posts state.)

If the #include "file" form is used, the implementation first looks for a file of the given name, if supported. If not (supported), or if the search fails, the implementation behaves as though the other (#include <file>) form was used.

Also, a third form exists and is used when the #include directive doesn't match either of the forms above. In this form, some basic preprocessing (such as macro expansion) is done on the "operands" of the #include directive, and the result is expected to match one of the two other forms.

Some good answers here make references to the C standard but forgot the POSIX standard, especially the specific behavior of the c99 (e.g. C compiler) command.

According to The Open Group Base Specifications Issue 7,

-I directory

Change the algorithm for searching for headers whose names are not absolute pathnames to look in the directory named by the directory pathname before looking in the usual places. Thus, headers whose names are enclosed in double-quotes ( "" ) shall be searched for first in the directory of the file with the #include line, then in directories named in -I options, and last in the usual places. For headers whose names are enclosed in angle brackets ( "<>" ), the header shall be searched for only in directories named in -I options and then in the usual places. Directories named in -I options shall be searched in the order specified. Implementations shall support at least ten instances of this option in a single c99 command invocation.

So, in a POSIX compliant environment, with a POSIX compliant C compiler, #include "file.h" is likely going to search for ./file.h first, where . is the directory where is the file with the #include statement, while #include <file.h>, is likely going to search for /usr/include/file.h first, where /usr/include is your system defined usual places for headers (it's seems not defined by POSIX).