# do { ... } while (0)
In C programming language,`do { ... } while (0)` is a loop which does a single iteration. A compiler will optimize it away completely, so the syntax produces no actual machine code.


Most commonly, it's used to make [[function-macro|function-macros]].

Example:
```c
/*
 * - clearing psr.i is implicitly serialized (visible by next insn)
 * - setting psr.i requires data serialization
 * - we need a stop-bit before reading PSR because we sometimes
 *   write a floating-point register right before reading the PSR
 *   and that writes to PSR.mfl
 */
#define __local_irq_save(x)         \
do {                    \
    ia64_stop();                \
    (x) = ia64_getreg(_IA64_REG_PSR);   \
    ia64_stop();                \
    ia64_rsm(IA64_PSR_I);           \
} while (0)

```

## Links

 - https://stackoverflow.com/questions/2381300/what-does-do-while-0-do-exactly-in-kernel-code