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# C Error Directive
The `#error` directive is a standard C preprocessor directive that instructs the implementation to produce a diagnostic message containing a specified sequence of preprocessor tokens. While the C standard strictly mandates only the generation of this diagnostic message, encountering this directive conventionally prevents the successful translation of the program.
## Syntax
```c theme={"dark"}
# error pp-tokens(opt)
```
* **`#`**: The preprocessing operator. It must be the first non-whitespace character on the logical source line.
* **`error`**: The identifier specifying the directive. Because `#` and `error` are separate preprocessing tokens, they may be separated by whitespace (e.g., `# error` is perfectly valid).
* **`pp-tokens`**: An optional sequence of preprocessor tokens that constitute the message. The sequence is terminated by a newline character.
## Mechanics and Behavior
**Tokenization and String Literals**
The message following the `error` identifier does not need to be enclosed in double quotes. The preprocessor consumes all characters up to the next newline as a sequence of raw preprocessor tokens. If string literals (`"..."`) are used, the quotes are treated as part of the token sequence and are preserved in the diagnostic output.
```c theme={"dark"}
/* Emits diagnostic: Fatal error: Architecture unsupported */
#error Fatal error: Architecture unsupported
/* Emits diagnostic: "Fatal error: Architecture unsupported" */
# error "Fatal error: Architecture unsupported"
```
**Macro Expansion**
The preprocessor does not perform macro expansion on the `pp-tokens` provided to the `#error` directive. The literal names of the tokens are emitted in the diagnostic message, regardless of whether they are defined as macros elsewhere in the translation unit.
```c theme={"dark"}
#define MAX_LIMIT 100
#error The limit is MAX_LIMIT
/* The diagnostic message will literally read "The limit is MAX_LIMIT", not "The limit is 100" */
```
**Standard Compliance and Translation**
According to the ISO/IEC 9899 C Standard (ยง6.10.5), a conforming implementation must produce a diagnostic message that includes the specified preprocessor tokens. The standard does not specify the destination of this diagnostic message (such as standard output, standard error, or a log file); the routing of the message is entirely implementation-defined.
Furthermore, while the directive is intended to indicate a fatal translation error, the standard does not strictly mandate that the compiler immediately halt and bypass all subsequent translation phases. A compiler may theoretically continue parsing to report multiple diagnostics, though the presence of the `#error` diagnostic ensures the translation process will ultimately fail to produce a valid executable.
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