6.10.1 — Conditional inclusion

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6.10.1 — Conditional inclusion

Constraints

• (1) The expression that controls conditional inclusion shall be an integer constant expression except that: it shall not contain a cast; identifiers (including those lexically identical to keywords) are interpreted as described below;140) and it may contain unary operator expressions of the form
  defined identifier
  or
  defined ( identifier )
  which evaluate to 1 if the identifier is currently defined as a macro name (that is, if it is

Semantics

• (2) Preprocessing directives of the forms
• (3) Prior to evaluation, macro invocations in the list of preprocessing tokens that will become the controlling constant expression are replaced (except for those macro names modified by the defined unary operator), just as in normal text. If the token defined is generated as a result of this replacement process or use of the defined unary operator does not match one of the two specified forms prior to macro replacement, the behavior is undefined. After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers are replaced with the pp-number 0, and then each preprocessing token is converted into a token. The resulting tokens compose the controlling constant expression which is evaluated according to the rules of 6.6, except that all signed integer types and all unsigned integer types act as if they have the same representation as, respectively, the types intmax_t and uintmax_t defined in the header <stdint.h>. This includes interpreting character constants, which may involve converting escape sequences into execution character set members. Whether the numeric value for these character constants matches the value obtained when an identical character constant occurs in an expression (other than within a #if or #elif directive) is implementation-defined.141) Also, whether a single-character character constant may have a negative value is implementation-defined.
• (4) Preprocessing directives of the forms
• (5) Each directive’s condition is checked in order. If it evaluates to false (zero), the group that it controls is skipped: directives are processed only through the name that determines the directive in order to keep track of the level of nested conditionals; the rest of the directives’ preprocessing tokens are ignored, as are the other preprocessing tokens in the group. Only the first group whose control condition evaluates to true (nonzero) is processed. If none of the conditions evaluates to true, and there is a #else directive, the group controlled by the #else is processed; lacking a #else directive, all the groups until the #endif are skipped.142)

140) Because the controlling constant expression is evaluated during translation phase 4, all identifiers either are or are not macro names — there simply are no keywords, enumeration constants, etc.
predefined or if it has been the subject of a #define preprocessing directive without an intervening #undef directive with the same subject identifier), 0 if it is not.
# if constant-expression new-line groupopt
# elif constant-expression new-line groupopt
check whether the controlling constant expression evaluates to nonzero.
# ifdef identifier new-line groupopt
# ifndef identifier new-line groupopt
check whether the identifier is or is not currently defined as a macro name. Their conditions are equivalent to #if defined identifier and #if !defined identifier respectively.

141) Thus, the constant expression in the following #if directive and if statement is not guaranteed to evaluate to the same value in these two contexts.
#if 'z' - 'a' == 25
if ('z' - 'a' == 25)