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The #undef directive is a C preprocessor command that removes a previously established macro definition, effectively unbinding an identifier from its associated replacement token sequence. Once undefined, the preprocessor ceases to expand subsequent occurrences of that identifier within the translation unit.

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Syntax

#undef identifier
  • identifier: The exact name of the macro to be removed.

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Preprocessor Mechanics

  • Lexical Scope: The removal of the macro takes effect immediately following the #undef directive. Any code parsed prior to the #undef directive retains the expanded macro values. The identifier remains undefined until the end of the translation unit unless explicitly redefined using another #define directive.
  • Function-like Macros: When undefining a function-like macro, only the base identifier is provided. The parameter list must be omitted.
  • Idempotency (No-op behavior): Applying #undef to an identifier that is not currently defined—either because it was never defined or has already been undefined—is strictly valid in standard C. The preprocessor treats this as a safe no-op and will not generate a compilation error or warning.
  • Standard Restrictions: The C standard dictates that standard predefined macros (such as __FILE__, __LINE__, __DATE__, __TIME__, and __STDC__) and the defined operator must not be the subject of an #undef directive. Attempting to undefine these reserved identifiers results in undefined behavior.
  • Phase of Translation: #undef operates during Phase 4 of the C translation process (preprocessing). It manipulates the preprocessor’s internal symbol table before the compiler performs syntactic or semantic analysis.

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Syntax Visualization

#define BUFFER_SIZE 1024
#define COMPUTE_MAX(x, y) ((x) > (y) ? (x) : (y))

/* Preprocessor expands these normally */
int current_buffer = BUFFER_SIZE; 
int max_val = COMPUTE_MAX(10, 20);

/* Remove the object-like macro */
#undef BUFFER_SIZE

/* Remove the function-like macro (parameters omitted) */
#undef COMPUTE_MAX

/* Safe no-op: Undefining an identifier that does not exist */
#undef NON_EXISTENT_MACRO

/* 
 * ILLEGAL: The C standard prohibits undefining predefined macros 
 * or the 'defined' operator.
 * #undef __FILE__
 * #undef defined
 */

/* 
 * Subsequent use of BUFFER_SIZE or COMPUTE_MAX here will not be expanded 
 * by the preprocessor, resulting in standard compiler errors for 
 * undeclared identifiers.
 */
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