# C do-while-0

**C do-while-0** is an idiom using `do { ... } while(0)` to create a single-iteration loop that allows `break` to exit cleanly. It's used in macros to ensure multiple statements execute as a unit, or to allow early exit without goto or nested conditionals.

Use do-while-0 in macros to wrap multiple statements safely, preventing issues when macros are used in conditionals or with semicolons.

## Example

This example shows do-while-0 fixing common macro pitfalls.

```c
// compile: gcc -o do_while do_while.c
// run: ./do_while
// description: do-while-0 macro pattern

#include <stdio.h>
#include <stdlib.h>

#define LOG_SAFE(msg) \
    do { \
        printf("LOG: %s\n", msg); \
        fflush(stdout); \
    } while(0)

#define VALIDATE(ptr) \
    do { \
        if ((ptr) == NULL) { \
            fprintf(stderr, "Validation failed\n"); \
            break; \
        } \
        printf("Validation passed\n"); \
    } while(0)

int main() {
    if (1)
        LOG_SAFE("Message");
    
    int *arr = malloc(100);
    VALIDATE(arr);
    
    free(arr);
    return 0;
}
```

## Common patterns

**Why do-while-0 is needed**:
- Wraps multiple statements into single unit
- Allows `break` for early exit without goto
- Handles trailing semicolon safely

**Semicolon handling**:
- User writes `MACRO;` with semicolon
- Macro ends with `while(0)`, not `while(0);`
- Extra semicolon is benign

**Compiler optimization**:
- Compiler removes the loop (condition always false)
- Zero runtime overhead
- Same as inline { } block

**When to use**:
- Multi-statement macros in conditionals
- Cleanup/error handling in macros
- Backward-compatible C code