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Background I can see the need to use { } when implementing a function-like macro such as this one: #define HCF(code) fprintf(stderr, "halt and catch fire"); exit(code); Because if we use the f...
#1: Post edited
by
Lundin
·
2023-11-09T13:57:37Z (5 months ago)
- **Background**
I can see the need to use `{}` when implementing a function-like macro such as this one:- #define HCF(code) fprintf(stderr, "halt and catch fire"); exit(code);
- Because if we use the following calling code
- bool bad = false;
- if(bad)
- HCF(0xDEADBEEF);
- printf("good");
- Then it will expand to
- bool bad = false;
- if(bad)
- fprintf(stderr, "halt and catch fire");
- exit(0xDEADBEEF);
- printf("good");
- which was not the intention - it will now exit with an error code even though the program is working as intended.
- So the correct way would be to write the macro like this instead:
- #define HCF(code) { fprintf(stderr, "halt and catch fire"); exit(code); }
- ---
- **Question**
But I've come across macros that instead of just plain braces `{ }` use this:- #define HCF(code) do { fprintf(stderr, "halt and catch fire"); \
- exit(code); } while(0)
This seems similar to `{}` but more obscure. What's the purpose of this `do { } while(0)` and why should it be used instead of `{ }`?
- **Background**
- I can see the need to use `{ }` when implementing a function-like macro such as this one:
- #define HCF(code) fprintf(stderr, "halt and catch fire"); exit(code);
- Because if we use the following calling code
- bool bad = false;
- if(bad)
- HCF(0xDEADBEEF);
- printf("good");
- Then it will expand to
- bool bad = false;
- if(bad)
- fprintf(stderr, "halt and catch fire");
- exit(0xDEADBEEF);
- printf("good");
- which was not the intention - it will now exit with an error code even though the program is working as intended.
- So the correct way would be to write the macro like this instead:
- #define HCF(code) { fprintf(stderr, "halt and catch fire"); exit(code); }
- ---
- **Question**
- But I've come across macros that instead of just plain braces `{ }` uses this:
- #define HCF(code) do { fprintf(stderr, "halt and catch fire"); \
- exit(code); } while(0)
- This seems similar to `{ }` but more obscure. What's the purpose of this `do { } while(0)` and why should it be used instead of `{ }`?