> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# C Undef Directive
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.
## Syntax
```c theme={"dark"}
#undef identifier
```
* **`identifier`**: The exact name of the macro to be removed.
## 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.
## Syntax Visualization
```c theme={"dark"}
#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.
*/
```
Tired of Poor C Skills? Fix That With Deep Grasping!
Learn More
