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What does it mean when define a struct var without identifier in a structure in c language?
In C language, what is the meaning of struct Animal; in line # 6?
Is it legal in C89 or C99 or C11?
struct Animal {
char *name;
int age;
};
struct Cat {
struct Animal; // line 6
int category;
};
Thanks!
c structure
edited Nov 20 '18 at 5:39
Azeem
2,86041023
asked Nov 20 '18 at 4:49
Ring.YeeRing.Yee
214
add a comment |
In C language, what is the meaning of struct Animal; in line # 6?
Is it legal in C89 or C99 or C11?
struct Animal {
char *name;
int age;
};
struct Cat {
struct Animal; // line 6
int category;
};
Thanks!
c structure
edited Nov 20 '18 at 5:39
Azeem
2,86041023
asked Nov 20 '18 at 4:49
Ring.YeeRing.Yee
214
I think it says "there is a typestruct Animal", and that's all. It doesn't do anything useful because line 1 already said that. When I compile it, I getx.c:9:18: error: declaration does not declare anything [-Werror]—struct Animal;which is what I'd expect. It would be a warning if I didn't compile with-Werror.
– Jonathan Leffler
Nov 20 '18 at 4:54
I've tried the code in VS2015 with a .c file, and it compiled ok, and output: sizeof(struct Animal): 8 sizeof(struct Cat): 12 which looks like there is an "struct Animal" entity in struct cat. So I'm not sure is it legal in C89 or C99?
– Ring.Yee
Nov 20 '18 at 5:01
It's dubious, but legal. I have GCC set to very fussy, and any warnings it produces are treated as errors.
– Jonathan Leffler
Nov 20 '18 at 5:02
In clang, it looks like meaning nothing, I've tried the code in xcode9, and it outputs: sizeof(struct Animal): 16 sizeof(struct Cat): 4
– Ring.Yee
Nov 20 '18 at 5:08
Working on a Mac, 64-bit compilation, and suppressing the-Werroroption, I get size of Animal as 16 and size of Cat as 4, which is exactly what I'd expect. Your result on VS2015 is curious — I find it hard to explain how you got that result.
– Jonathan Leffler
Nov 20 '18 at 5:14
add a comment |
In C language, what is the meaning of struct Animal; in line # 6?
Is it legal in C89 or C99 or C11?
struct Animal {
char *name;
int age;
};
struct Cat {
struct Animal; // line 6
int category;
};
Thanks!
c structure
edited Nov 20 '18 at 5:39
Azeem
2,86041023
asked Nov 20 '18 at 4:49
Ring.YeeRing.Yee
214
In C language, what is the meaning of struct Animal; in line # 6?
Is it legal in C89 or C99 or C11?
struct Animal {
char *name;
int age;
};
struct Cat {
struct Animal; // line 6
int category;
};
Thanks!
c structure
c structure
edited Nov 20 '18 at 5:39
Azeem
2,86041023
asked Nov 20 '18 at 4:49
Ring.YeeRing.Yee
214
edited Nov 20 '18 at 5:39
Azeem
2,86041023
asked Nov 20 '18 at 4:49
Ring.YeeRing.Yee
214
edited Nov 20 '18 at 5:39
Azeem
2,86041023
edited Nov 20 '18 at 5:39
Azeem
2,86041023
edited Nov 20 '18 at 5:39
Azeem
2,86041023
2,86041023
asked Nov 20 '18 at 4:49
Ring.YeeRing.Yee
214
asked Nov 20 '18 at 4:49
Ring.YeeRing.Yee
214
asked Nov 20 '18 at 4:49
Ring.YeeRing.Yee
214
214
I think it says "there is a typestruct Animal", and that's all. It doesn't do anything useful because line 1 already said that. When I compile it, I getx.c:9:18: error: declaration does not declare anything [-Werror]—struct Animal;which is what I'd expect. It would be a warning if I didn't compile with-Werror.
– Jonathan Leffler
Nov 20 '18 at 4:54
I've tried the code in VS2015 with a .c file, and it compiled ok, and output: sizeof(struct Animal): 8 sizeof(struct Cat): 12 which looks like there is an "struct Animal" entity in struct cat. So I'm not sure is it legal in C89 or C99?
– Ring.Yee
Nov 20 '18 at 5:01
It's dubious, but legal. I have GCC set to very fussy, and any warnings it produces are treated as errors.
– Jonathan Leffler
Nov 20 '18 at 5:02
In clang, it looks like meaning nothing, I've tried the code in xcode9, and it outputs: sizeof(struct Animal): 16 sizeof(struct Cat): 4
– Ring.Yee
Nov 20 '18 at 5:08
Working on a Mac, 64-bit compilation, and suppressing the-Werroroption, I get size of Animal as 16 and size of Cat as 4, which is exactly what I'd expect. Your result on VS2015 is curious — I find it hard to explain how you got that result.
– Jonathan Leffler
Nov 20 '18 at 5:14
add a comment |
I think it says "there is a typestruct Animal", and that's all. It doesn't do anything useful because line 1 already said that. When I compile it, I getx.c:9:18: error: declaration does not declare anything [-Werror]—struct Animal;which is what I'd expect. It would be a warning if I didn't compile with-Werror.
– Jonathan Leffler
Nov 20 '18 at 4:54
I've tried the code in VS2015 with a .c file, and it compiled ok, and output: sizeof(struct Animal): 8 sizeof(struct Cat): 12 which looks like there is an "struct Animal" entity in struct cat. So I'm not sure is it legal in C89 or C99?
– Ring.Yee
Nov 20 '18 at 5:01
It's dubious, but legal. I have GCC set to very fussy, and any warnings it produces are treated as errors.
– Jonathan Leffler
Nov 20 '18 at 5:02
In clang, it looks like meaning nothing, I've tried the code in xcode9, and it outputs: sizeof(struct Animal): 16 sizeof(struct Cat): 4
– Ring.Yee
Nov 20 '18 at 5:08
Working on a Mac, 64-bit compilation, and suppressing the-Werroroption, I get size of Animal as 16 and size of Cat as 4, which is exactly what I'd expect. Your result on VS2015 is curious — I find it hard to explain how you got that result.
– Jonathan Leffler
Nov 20 '18 at 5:14
I think it says "there is a type
struct Animal", and that's all. It doesn't do anything useful because line 1 already said that. When I compile it, I get x.c:9:18: error: declaration does not declare anything [-Werror] — struct Animal; which is what I'd expect. It would be a warning if I didn't compile with -Werror.– Jonathan Leffler
Nov 20 '18 at 4:54
I think it says "there is a type
struct Animal", and that's all. It doesn't do anything useful because line 1 already said that. When I compile it, I get x.c:9:18: error: declaration does not declare anything [-Werror] — struct Animal; which is what I'd expect. It would be a warning if I didn't compile with -Werror.– Jonathan Leffler
Nov 20 '18 at 4:54
I've tried the code in VS2015 with a .c file, and it compiled ok, and output: sizeof(struct Animal): 8 sizeof(struct Cat): 12 which looks like there is an "struct Animal" entity in struct cat. So I'm not sure is it legal in C89 or C99?
– Ring.Yee
Nov 20 '18 at 5:01
I've tried the code in VS2015 with a .c file, and it compiled ok, and output: sizeof(struct Animal): 8 sizeof(struct Cat): 12 which looks like there is an "struct Animal" entity in struct cat. So I'm not sure is it legal in C89 or C99?
– Ring.Yee
Nov 20 '18 at 5:01
It's dubious, but legal. I have GCC set to very fussy, and any warnings it produces are treated as errors.
– Jonathan Leffler
Nov 20 '18 at 5:02
It's dubious, but legal. I have GCC set to very fussy, and any warnings it produces are treated as errors.
– Jonathan Leffler
Nov 20 '18 at 5:02
In clang, it looks like meaning nothing, I've tried the code in xcode9, and it outputs: sizeof(struct Animal): 16 sizeof(struct Cat): 4
– Ring.Yee
Nov 20 '18 at 5:08
In clang, it looks like meaning nothing, I've tried the code in xcode9, and it outputs: sizeof(struct Animal): 16 sizeof(struct Cat): 4
– Ring.Yee
Nov 20 '18 at 5:08
Working on a Mac, 64-bit compilation, and suppressing the
-Werror option, I get size of Animal as 16 and size of Cat as 4, which is exactly what I'd expect. Your result on VS2015 is curious — I find it hard to explain how you got that result.– Jonathan Leffler
Nov 20 '18 at 5:14
Working on a Mac, 64-bit compilation, and suppressing the
-Werror option, I get size of Animal as 16 and size of Cat as 4, which is exactly what I'd expect. Your result on VS2015 is curious — I find it hard to explain how you got that result.– Jonathan Leffler
Nov 20 '18 at 5:14
add a comment |
1 Answer
1
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oldest
votes
It doesn't mean much of anything -- and in fact it's a constraint violation.
The language grammar allows
struct Animal;
in place of a member declaration inside a struct or union declaration. But it violates a language rule (N1570 6.7.2.1 paragraph 2):
A struct-declaration that does not declare an anonymous structure or
anonymous union shall contain a struct-declarator-list.
This is a "constraint", which means that violating it requires a diagnostic. (A non-fatal warning message qualifies as a diagnostic.)
If you had written:
struct Cat {
struct Animal foo;
int category;
};
then the foo would be a "declarator". The constraint I quoted above means you're not allowed to omit it. gcc and clang both warn about this by default ("declaration does not declare anything") and reject it with -Wpedantic-errors.
(Anonymous structs and unions were added to the language in C11, and are discussed here, but the code in your question is not an anonymous struct.)
answered Nov 20 '18 at 5:02
Keith ThompsonKeith Thompson
190k25283473
1
-pedantic-errorsmakes GCC reject the code, so I'm not sure if it's actually allowed.
– HolyBlackCat
Nov 20 '18 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 '18 at 7:03
add a comment |
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1 Answer
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It doesn't mean much of anything -- and in fact it's a constraint violation.
The language grammar allows
struct Animal;
in place of a member declaration inside a struct or union declaration. But it violates a language rule (N1570 6.7.2.1 paragraph 2):
A struct-declaration that does not declare an anonymous structure or
anonymous union shall contain a struct-declarator-list.
This is a "constraint", which means that violating it requires a diagnostic. (A non-fatal warning message qualifies as a diagnostic.)
If you had written:
struct Cat {
struct Animal foo;
int category;
};
then the foo would be a "declarator". The constraint I quoted above means you're not allowed to omit it. gcc and clang both warn about this by default ("declaration does not declare anything") and reject it with -Wpedantic-errors.
(Anonymous structs and unions were added to the language in C11, and are discussed here, but the code in your question is not an anonymous struct.)
answered Nov 20 '18 at 5:02
Keith ThompsonKeith Thompson
190k25283473
1
-pedantic-errorsmakes GCC reject the code, so I'm not sure if it's actually allowed.
– HolyBlackCat
Nov 20 '18 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 '18 at 7:03
add a comment |
It doesn't mean much of anything -- and in fact it's a constraint violation.
The language grammar allows
struct Animal;
in place of a member declaration inside a struct or union declaration. But it violates a language rule (N1570 6.7.2.1 paragraph 2):
A struct-declaration that does not declare an anonymous structure or
anonymous union shall contain a struct-declarator-list.
This is a "constraint", which means that violating it requires a diagnostic. (A non-fatal warning message qualifies as a diagnostic.)
If you had written:
struct Cat {
struct Animal foo;
int category;
};
then the foo would be a "declarator". The constraint I quoted above means you're not allowed to omit it. gcc and clang both warn about this by default ("declaration does not declare anything") and reject it with -Wpedantic-errors.
(Anonymous structs and unions were added to the language in C11, and are discussed here, but the code in your question is not an anonymous struct.)
answered Nov 20 '18 at 5:02
Keith ThompsonKeith Thompson
190k25283473
1
-pedantic-errorsmakes GCC reject the code, so I'm not sure if it's actually allowed.
– HolyBlackCat
Nov 20 '18 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 '18 at 7:03
add a comment |
It doesn't mean much of anything -- and in fact it's a constraint violation.
The language grammar allows
struct Animal;
in place of a member declaration inside a struct or union declaration. But it violates a language rule (N1570 6.7.2.1 paragraph 2):
A struct-declaration that does not declare an anonymous structure or
anonymous union shall contain a struct-declarator-list.
This is a "constraint", which means that violating it requires a diagnostic. (A non-fatal warning message qualifies as a diagnostic.)
If you had written:
struct Cat {
struct Animal foo;
int category;
};
then the foo would be a "declarator". The constraint I quoted above means you're not allowed to omit it. gcc and clang both warn about this by default ("declaration does not declare anything") and reject it with -Wpedantic-errors.
(Anonymous structs and unions were added to the language in C11, and are discussed here, but the code in your question is not an anonymous struct.)
answered Nov 20 '18 at 5:02
Keith ThompsonKeith Thompson
190k25283473
It doesn't mean much of anything -- and in fact it's a constraint violation.
The language grammar allows
struct Animal;
in place of a member declaration inside a struct or union declaration. But it violates a language rule (N1570 6.7.2.1 paragraph 2):
A struct-declaration that does not declare an anonymous structure or
anonymous union shall contain a struct-declarator-list.
This is a "constraint", which means that violating it requires a diagnostic. (A non-fatal warning message qualifies as a diagnostic.)
If you had written:
struct Cat {
struct Animal foo;
int category;
};
then the foo would be a "declarator". The constraint I quoted above means you're not allowed to omit it. gcc and clang both warn about this by default ("declaration does not declare anything") and reject it with -Wpedantic-errors.
(Anonymous structs and unions were added to the language in C11, and are discussed here, but the code in your question is not an anonymous struct.)
answered Nov 20 '18 at 5:02
Keith ThompsonKeith Thompson
190k25283473
edited Nov 20 '18 at 7:03
answered Nov 20 '18 at 5:02
Keith ThompsonKeith Thompson
190k25283473
answered Nov 20 '18 at 5:02
Keith ThompsonKeith Thompson
190k25283473
answered Nov 20 '18 at 5:02
Keith ThompsonKeith Thompson
190k25283473
190k25283473
1
-pedantic-errorsmakes GCC reject the code, so I'm not sure if it's actually allowed.
– HolyBlackCat
Nov 20 '18 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 '18 at 7:03
add a comment |
1
-pedantic-errorsmakes GCC reject the code, so I'm not sure if it's actually allowed.
– HolyBlackCat
Nov 20 '18 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 '18 at 7:03
1
1
-pedantic-errors makes GCC reject the code, so I'm not sure if it's actually allowed.– HolyBlackCat
Nov 20 '18 at 5:43
-pedantic-errors makes GCC reject the code, so I'm not sure if it's actually allowed.– HolyBlackCat
Nov 20 '18 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 '18 at 7:03
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 '18 at 7:03
add a comment |
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I think it says "there is a type
struct Animal", and that's all. It doesn't do anything useful because line 1 already said that. When I compile it, I getx.c:9:18: error: declaration does not declare anything [-Werror]—struct Animal;which is what I'd expect. It would be a warning if I didn't compile with-Werror.– Jonathan Leffler
Nov 20 '18 at 4:54
I've tried the code in VS2015 with a .c file, and it compiled ok, and output: sizeof(struct Animal): 8 sizeof(struct Cat): 12 which looks like there is an "struct Animal" entity in struct cat. So I'm not sure is it legal in C89 or C99?
– Ring.Yee
Nov 20 '18 at 5:01
It's dubious, but legal. I have GCC set to very fussy, and any warnings it produces are treated as errors.
– Jonathan Leffler
Nov 20 '18 at 5:02
In clang, it looks like meaning nothing, I've tried the code in xcode9, and it outputs: sizeof(struct Animal): 16 sizeof(struct Cat): 4
– Ring.Yee
Nov 20 '18 at 5:08
Working on a Mac, 64-bit compilation, and suppressing the
-Werroroption, I get size of Animal as 16 and size of Cat as 4, which is exactly what I'd expect. Your result on VS2015 is curious — I find it hard to explain how you got that result.– Jonathan Leffler
Nov 20 '18 at 5:14