<<split from https://cboard.cprogramming.com/showthread.php?t=40936>>
If I call this offsetof() macro in a function:
What should be the [type]s in the function prototype? That is, what are the types of the parameters of offsetof()?Code:int func([type] struct_name, [type] member) { ... x = offsetof(struct_name, member); ... }
> What should be the [type]s in the function prototype?
You can't.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
How are we supposed to know?
Post the real code.
"I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008
"the internet is a scary place to be thats why i dont use it much." - billet, 03/17/2010
offsetof is a macro, not a function, and the first argument is a struct or union, the second a member of said struct/union.
Eg.
Pedantisizm prevention: If we are usign C99, we probably should use %zd as the format specifier, as offsetof forms a size_t value, and that should use the z modifier to tell printf that it is of "size_t" size, rather than regular int.Code:struct blah { int x; int y; }; int main() { printf("offsetof(blah, y)= %d\n", offsetof(blah, y)); return 0; }
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Mats
Compilers can produce warnings - make the compiler programmers happy: Use them!
Please don't PM me for help - and no, I don't do help over instant messengers.
Just some messing about...
Code:#include <stddef.h> #include <stdio.h> struct T { int i; double d; char c; char s[10]; float f; }; int Toffset(int member) { switch ( member ) { case 0: return offsetof(struct T, i); case 1: return offsetof(struct T, d); case 2: return offsetof(struct T, c); case 3: return offsetof(struct T, s); case 4: return offsetof(struct T, f); default: break; } return 0; } int main(void) { int i; for ( i = 0; i < 6; ++i ) { printf("Toffset(%d) = %d\n", i, Toffset(i)); } return 0; } /* my output Toffset(0) = 0 Toffset(1) = 8 Toffset(2) = 16 Toffset(3) = 17 Toffset(4) = 28 Toffset(5) = 0 */
7. It is easier to write an incorrect program than understand a correct one.
40. There are two ways to write error-free programs; only the third one works.*
Pendant attack!Originally Posted by matsp
d is for signed integrals, u is for unsigned integrals, so the correct C99 format specifier for size_t would be %zu. In strict C89, it would be better to use %lu and cast to unsigned long instead of using %d.
Pedantisizm accepted. However, it is HIGHLY unlikely that the offset within a struct is bigger than 2^n-1 where n is the number of bits in size_t. So signed or unsigned will make no difference.
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Mats
Compilers can produce warnings - make the compiler programmers happy: Use them!
Please don't PM me for help - and no, I don't do help over instant messengers.
Agreed, it likely wouldn't make a difference.
LOL!!! I didn't even see that 'n' there.
"I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008
"the internet is a scary place to be thats why i dont use it much." - billet, 03/17/2010
no, I meant necklaces.
they'll choke you, they will.
edit: off to change my user title...
