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This shows you the differences between two versions of the page.

--- c-do-while-0 [August 22, 2026 at 16:03] – Ivan Janevski
+++ c-do-while-0 [August 26, 2026 at 11:41] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 # C do-while-0
-**[do { ... } while (0)](https://stackoverflow.com/questions/2381300/what-does-do-while-0-do-exactly-in-kernel-code)** is a C idiom that executes a block exactly once while appearing syntactically like a loop. Compilers optimize it away completely, producing no extra machine code. It's commonly used in [[c-function-macro|function macros]] to safely wrap multiple statements and enforce semicolon syntax.
+**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 enable safe statement grouping and prevent control flow issues.
+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 wrapping multiple statements in a function macro.
+This example shows do-while-0 fixing common macro pitfalls.
 ```c
-// compile: gcc -o dowhile dowhile.c
+// compile: gcc -o do_while do_while.c
-// run: ./dowhile
+// run: ./do_while
-// description: do-while-0 enables safe multi-statement function macros
+// description: do-while-0 macro pattern
 #include <stdio.h>
+#include <stdlib.h>
-#define LOG_ERROR(fmt, ...) do { \
+#define LOG_SAFE(msg) \
-    fprintf(stderr, "[ERROR] " fmt "\n", ##__VA_ARGS__); \
+    do { \
-    perror("System error"); \
+        printf("LOG: %s\n", msg); \
-} while (0)
+        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) {
+    if (1)
-        LOG_ERROR("Failed to open file");
+        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