Why `do { ...; exit(...); } while (0)` in C?

Question

As a C newbie I'm having trouble understanding the following code:

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                           } while (0)

I gathered that the reason this function is #defined is to override an existing function, but what is the point of a do ... while(0) loop with an unconditional exit() statement? Is it not possible to write this without the loop construct?

Nice question, I've wondered the same! +1 – Kiril Kirov Mar 21 '13 at 14:37 — Kiril Kirov, Mar 21 '13 at 14:37
Too bad the title duplicate detector didn't pick up this... – l0b0 Mar 21 '13 at 14:42 — l0b0, Mar 21 '13 at 14:42

Benoit · Answer 1 · 2013-03-21T14:41:54.143

9

Many duplicates here I think.

The do...while(0) trick enables you to use errExit in various contexts without breaking anything:

if(x) errExit(msg);
else return 1;

is translated to:

if(x) do { ...; ...; } while(0);
else return 1;

If you omit the do...while(0) part, then you could not reliably add a semicolon for example.

edited Mar 21 '13 at 14:41

answered Mar 21 '13 at 14:35

Benoit

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Why would plain old braces not do the job? – Joe Mar 21 '13 at 14:37
Nice! +1. That's really clever and tricky. – Kiril Kirov Mar 21 '13 at 14:37
1

Ah, so the whole idea is to keep the resulting code to a single statement as seen from the surrounding code? Neat! – l0b0 Mar 21 '13 at 14:39
@Joe Read the complete answer here http://stackoverflow.com/a/154138/1488917 – asheeshr Mar 21 '13 at 14:40
1

@Joe: No. Because of semicolon. As said by AshRj the answer provided elsewhere is thorough. – Benoit Mar 21 '13 at 14:40

score 2 · Answer 2 · answered Mar 21 '13 at 14:35

2

Assume the macro didn't have the do { ... } while(0) loop, just the 2 statements inside. Now, what if I were to write

if( foo() )
    errExit("foo!" );

My conditional exit has become a non-conditional exit.

answered Mar 21 '13 at 14:35

Praetorian

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score 1 · Answer 3 · answered Mar 21 '13 at 14:36

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The do { ... } while(0) construct is common, and usually considered best practice, for macro functions of multiple statements, such as this. It allows use as a single statement, so there are no surprises.

answered Mar 21 '13 at 14:36

Kevin

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Why `do { ...; exit(...); } while (0)` in C?

3 Answers3