Source file inclusion

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C
 
C language
 
Preprocessor
 

Includes another source file into the current source file at the line immediately after the directive.

Syntax

#include < h-char-sequence > new-line (1)
#include " q-char-sequence " new-line (2)
#include pp-tokens new-line (3)
__has_include ( " q-char-sequence " )
__has_include ( < h-char-sequence > ) 		(4) (since C23)
__has_include ( string-literal )
__has_include ( < h-pp-tokens > ) 		(5) (since C23)
1) Searches for a header identified uniquely by h-char-sequence and replaces the directive by the entire contents of the header.
2) Searches for a source file identified by q-char-sequence and replaces the directive by the entire contents of the source file. It may fallback to (1) and treat q-char-sequence as a header identifier.
3) If neither (1) nor (2) is matched, pp-tokens will undergo macro replacement. The directive after replacement will be tried to match with (1) or (2) again.
4) Checks whether a header or source file is available for inclusion.
5) If (4) is not matched, h-pp-tokens will undergo macro replacement. The directive after replacement will be tried to match with (4) again.
new-line - The new-line character
h-char-sequence - A sequence of one or more h-chars, where the appearance of any of the following causes undefined behavior:
  • the character '
  • the character "
  • the character \
  • the character sequence //
  • the character sequence /*
h-char - Any member of the source character set except new-line and >
q-char-sequence - A sequence of one or more q-chars, where the appearance of any of the following causes undefined behavior:
  • the character '
  • the character \
  • the character sequence //
  • the character sequence /*
q-char - Any member of the source character set except new-line and "
pp-tokens - A sequence of one or more preprocessing tokens
string-literal - A string literal
h-pp-tokens - A sequence of one or more preprocessing tokens except >

Explanation

1) Searches for the file identified by h-char-sequence in implementation-defined manner. The intent of this syntax is to search for the files under control of the implementation. Typical implementations search only standard include directories. The standard C library is implicitly included in these standard include directories. The standard include directories usually can be controlled by the user through compiler options.
2) Searches for the file identified by q-char-sequence in implementation-defined manner. The intent of this syntax is to search for the files that are not controlled by the implementation. Typical implementations first search the directory where the current file resides and, only if the file is not found, search the standard include directories as with (1).
3) The preprocessing tokens after include in the directive are processed just as in normal text (i.e., 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 < and > preprocessing token pair or a pair of " characters is combined into a single header name preprocessing token is implementation-defined.
4) The header or source file identified by h-char-sequence or q-char-sequence is searched for as if that preprocessing token sequence were the pp-tokens in syntax (3), except that no further macro expansion is performed. If such a directive would not satisfy the syntactic requirements of an #include directive, the program is ill-formed. The __has_include expression evaluates to 1 if the search for the source file succeeds, and to ​0​ if the search fails.
5) This form is considered only if syntax (4) does not match, in which case the preprocessing tokens are processed just as in normal text.

In the case the file is not found, the program is ill-formed.

__has_include can be expanded in the expression of #if and #elif. It is treated as a defined macro by #ifdef, #ifndef, #elifdef, #elifndef and defined but cannot be used anywhere else.

(since C23)

Notes

Typical implementations search only standard include directories for syntax (1). The standard C library is implicitly included in these standard include directories. The standard include directories usually can be controlled by the user through compiler options.

The intent of syntax (2) is to search for the files that are not controlled by the implementation. Typical implementations first search the directory where the current file resides then falls back to (1).

When a file is included, it is processed by translation phases 1-4, which may include, recursively, expansion of the nested #include directives, up to an implementation-defined nesting limit. To avoid repeated inclusion of the same file and endless recursion when a file includes itself, perhaps transitively, header guards are commonly used: the entire header is wrapped in 

#ifndef FOO_H_INCLUDED /* any name uniquely mapped to file name */
#define FOO_H_INCLUDED
// contents of the file are here
#endif

Many compilers also implement the non-standard pragma #pragma once with similar effects: it disables processing of a file if the same file (where file identity is determined in OS-specific way) has already been included.

A __has_include result of 1 only means that a header or source file with the specified name exists. It does not mean that the header or source file, when included, would not cause an error or would contain anything useful.

Example

This section is incomplete
Reason: no example

References

  • C23 standard (ISO/IEC 9899:2024):
  • 6.4.7 Header names (p: 69)
  • 6.10.1 Conditional inclusion (p: 165-169)
  • 6.10.2 Source file inclusion (p: 169-170)
  • C17 standard (ISO/IEC 9899:2018):
  • 6.10.2 Source file inclusion (p: 119-120)
  • C11 standard (ISO/IEC 9899:2011):
  • 6.10.2 Source file inclusion (p: 164-166)
  • C99 standard (ISO/IEC 9899:1999):
  • 6.10.2 Source file inclusion (p: 149-151)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 3.8.2 Source file inclusion

See also

A list of C Standard Library header files
C++ documentation for Source file inclusion
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