do { ... } while (0) — what is it good for?

Question

I've been seeing that expression for over 10 years now. I've been trying to think what it's good for. Since I see it mostly in #defines, I assume it's good for inner scope variable declaration and for using breaks (instead of gotos.)

Is it good for anything else? Do you use it?

Answer 1

It's the only construct in C that you can use to #define a multistatement operation, put a semicolon after, and still use within an if statement. An example might help:

#define FOO(x) foo(x); bar(x)

if (condition)
    FOO(x);
else // syntax error here
    ...;

Even using braces doesn't help:

#define FOO(x) { foo(x); bar(x); }

Using this in an if statement would require that you omit the semicolon, which is counterintuitive:

if (condition)
    FOO(x)
else
    ...

If you define FOO like this:

#define FOO(x) do { foo(x); bar(x); } while (0)

then the following is syntactically correct:

if (condition)
    FOO(x);
else
    ....

Answer 2

It is a way to simplify error checking and avoid deep nested if's. For example:

do {
  // do something
  if (error) {
    break;
  }
  // do something else
  if (error) {
    break;
  }
  // etc..
} while (0);

Answer 3

It helps to group multiple statements into a single one so that a function-like macro can actually be used as a function. Suppose you have:

#define FOO(n)   foo(n);bar(n)

and you do:

void foobar(int n) {
  if (n)
     FOO(n);
}

then this expands to:

void foobar(int n) {
  if (n)
     foo(n);bar(n);
}

Notice that the second call bar(n) is not part of the if statement anymore.

Wrap both into do { } while(0), and you can also use the macro in an if statement.

Answer 4

It is interesting to note the following situation where the do {} while (0) loop won't work for you:

If you want a function-like macro that returns a value, then you will need a statement expression: ({stmt; stmt;}) instead of do {} while(0):

#include <stdio.h>

#define log_to_string1(str, fmt, arg...) \
    do { \
        sprintf(str, "%s: " fmt, "myprog", ##arg); \
    } while (0)

#define log_to_string2(str, fmt, arg...) \
    ({ \
        sprintf(str, "%s: " fmt, "myprog", ##arg); \
    })

int main() {
        char buf[1000];
        int n = 0;

        log_to_string1(buf, "%s\n", "No assignment, OK");

        n += log_to_string1(buf + n, "%s\n", "NOT OK: gcc: error: expected expression before 'do'");

        n += log_to_string2(buf + n, "%s\n", "This fixes it");
        n += log_to_string2(buf + n, "%s\n", "Assignment worked!");
        printf("%s", buf);
        return 0;
}

Answer 5

Generically, do/while is good for any sort of loop construct where one must execute the loop at least once. It is possible to emulate this sort of looping through either a straight while or even a for loop, but often the result is a little less elegant. I'll admit that specific applications of this pattern are fairly rare, but they do exist. One which springs to mind is a menu-based console application:

do {
    char c = read_input();

    process_input(c);
} while (c != 'Q');