What does it mean when define a struct var without identifier in a structure in c language?
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1
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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 at 5:39
Azeem
2,85041023
asked Nov 20 at 4:49
Ring.Yee
214
add a comment |
up vote
1
down vote
favorite
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 at 5:39
Azeem
2,85041023
asked Nov 20 at 4:49
Ring.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 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 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 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 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 at 5:14
add a comment |
up vote
1
down vote
favorite
up vote
1
down vote
favorite
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 at 5:39
Azeem
2,85041023
asked Nov 20 at 4:49
Ring.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 at 5:39
Azeem
2,85041023
asked Nov 20 at 4:49
Ring.Yee
214
edited Nov 20 at 5:39
Azeem
2,85041023
asked Nov 20 at 4:49
Ring.Yee
214
edited Nov 20 at 5:39
Azeem
2,85041023
edited Nov 20 at 5:39
Azeem
2,85041023
edited Nov 20 at 5:39
Azeem
2,85041023
2,85041023
asked Nov 20 at 4:49
Ring.Yee
214
asked Nov 20 at 4:49
Ring.Yee
214
asked Nov 20 at 4:49
Ring.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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 at 5:14
add a comment |
1 Answer
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up vote
5
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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 at 5:02
Keith Thompson
189k25278466
1
-pedantic-errorsmakes GCC reject the code, so I'm not sure if it's actually allowed.
– HolyBlackCat
Nov 20 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 at 7:03
add a comment |
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1 Answer
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active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
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active
oldest
votes
up vote
5
down vote
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 at 5:02
Keith Thompson
189k25278466
1
-pedantic-errorsmakes GCC reject the code, so I'm not sure if it's actually allowed.
– HolyBlackCat
Nov 20 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 at 7:03
add a comment |
up vote
5
down vote
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 at 5:02
Keith Thompson
189k25278466
1
-pedantic-errorsmakes GCC reject the code, so I'm not sure if it's actually allowed.
– HolyBlackCat
Nov 20 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 at 7:03
add a comment |
up vote
5
down vote
up vote
5
down vote
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 at 5:02
Keith Thompson
189k25278466
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 at 5:02
Keith Thompson
189k25278466
edited Nov 20 at 7:03
answered Nov 20 at 5:02
Keith Thompson
189k25278466
answered Nov 20 at 5:02
Keith Thompson
189k25278466
answered Nov 20 at 5:02
Keith Thompson
189k25278466
189k25278466
1
-pedantic-errorsmakes GCC reject the code, so I'm not sure if it's actually allowed.
– HolyBlackCat
Nov 20 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 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 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 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 at 5:43
-pedantic-errors makes GCC reject the code, so I'm not sure if it's actually allowed.– HolyBlackCat
Nov 20 at 5:43
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 at 7:03
@HolyBlackCat: Good catch. I've updated my answer.
– Keith Thompson
Nov 20 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 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 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 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 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 at 5:14