Mixing
How does __attribute__((packed)) for a field affect struct which contains this field?
If I have a field in my struct which is packed, why my whole structure is becoming packed?
Example:
#include <stdio.h>
struct foo {
int a;
} __attribute__((packed));
struct bar {
char b;
struct foo bla;
char a;
};
int main() {
printf("%ldn", sizeof(struct bar));
return 0;
}
https://ideone.com/bjoZHB
Sizeof of bar struct is 6, but it should be 12, because it should be aligned.
c gcc
asked Jan 2 at 17:44
Sukhanov NiсkolaySukhanov Niсkolay
6971617
add a comment |
If I have a field in my struct which is packed, why my whole structure is becoming packed?
Example:
#include <stdio.h>
struct foo {
int a;
} __attribute__((packed));
struct bar {
char b;
struct foo bla;
char a;
};
int main() {
printf("%ldn", sizeof(struct bar));
return 0;
}
https://ideone.com/bjoZHB
Sizeof of bar struct is 6, but it should be 12, because it should be aligned.
c gcc
asked Jan 2 at 17:44
Sukhanov NiсkolaySukhanov Niсkolay
6971617
add a comment |
If I have a field in my struct which is packed, why my whole structure is becoming packed?
Example:
#include <stdio.h>
struct foo {
int a;
} __attribute__((packed));
struct bar {
char b;
struct foo bla;
char a;
};
int main() {
printf("%ldn", sizeof(struct bar));
return 0;
}
https://ideone.com/bjoZHB
Sizeof of bar struct is 6, but it should be 12, because it should be aligned.
c gcc
asked Jan 2 at 17:44
Sukhanov NiсkolaySukhanov Niсkolay
6971617
If I have a field in my struct which is packed, why my whole structure is becoming packed?
Example:
#include <stdio.h>
struct foo {
int a;
} __attribute__((packed));
struct bar {
char b;
struct foo bla;
char a;
};
int main() {
printf("%ldn", sizeof(struct bar));
return 0;
}
https://ideone.com/bjoZHB
Sizeof of bar struct is 6, but it should be 12, because it should be aligned.
c gcc
c gcc
asked Jan 2 at 17:44
Sukhanov NiсkolaySukhanov Niсkolay
6971617
asked Jan 2 at 17:44
Sukhanov NiсkolaySukhanov Niсkolay
6971617
asked Jan 2 at 17:44
Sukhanov NiсkolaySukhanov Niсkolay
6971617
asked Jan 2 at 17:44
Sukhanov NiсkolaySukhanov Niсkolay
6971617
asked Jan 2 at 17:44
Sukhanov NiсkolaySukhanov Niсkolay
6971617
6971617
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
it seems because __attribute__((packed)) means use the minimum memory for structure, it also means that it can ignore alignment for siding members when it is in another structure. Check following structure:
struct bar {
char b;
__attribute__((packed)) int bla;
char a;
};
When you check size for this structure, it will be 6. This happens because it ignores member alignment for 2 side members(a and b here). But this structure:
struct bar {
char b;
__attribute__((packed)) int bla;
char a;
int c;
};
has size of 12, because it is aligned c on 4 bytes boundary. In your case, if you use aligned attribute too at same time, it works as you expect:
struct bar {
char b;
__attribute__((aligned (4), packed)) int bla;
char a;
};
This structure size is 12.
Update:
I only found this in GCC's aligned section of attributes. I think it is related to what I mentioned here:
The aligned attribute can only increase the alignment; but you can
decrease it by specifying packed as well
.Just remember that if you want to keep child structure packed but main structure aligned, you need to use 2 attributes in 2 different declarations. For example following structure has size of 12:
struct foo {
char b;
int a;
} __attribute__((packed));
struct bar {
char b;
__attribute__((aligned(4))) struct foo bla;
char a;
};
but if you use aligned() in declaration of foo as __attribute__((aligned (4), packed)), size will be 16. This happens because foo gets aligned too, and it will not be useful in case of packing.
answered Jan 2 at 18:00
AfshinAfshin
3,1811627
Where can I read about "means that it can ignore alignment when it is in another structure" I didn't find it in the gcc docs or anywhere. :(
– Sukhanov Niсkolay
Jan 2 at 18:03
@SukhanovNiсkolay it had a "it seems" at start of sentence too. but I found 1 sentence about it in gcc manual and I add it to reply.
– Afshin
Jan 2 at 18:05
Yeah, I saw the same cite, then it seems it is an ISO C requirement. :) UPDATE: Sorry, wrong cite, I saw this: "ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct"
– Sukhanov Niсkolay
Jan 2 at 18:10
add a comment |
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1 Answer
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it seems because __attribute__((packed)) means use the minimum memory for structure, it also means that it can ignore alignment for siding members when it is in another structure. Check following structure:
struct bar {
char b;
__attribute__((packed)) int bla;
char a;
};
When you check size for this structure, it will be 6. This happens because it ignores member alignment for 2 side members(a and b here). But this structure:
struct bar {
char b;
__attribute__((packed)) int bla;
char a;
int c;
};
has size of 12, because it is aligned c on 4 bytes boundary. In your case, if you use aligned attribute too at same time, it works as you expect:
struct bar {
char b;
__attribute__((aligned (4), packed)) int bla;
char a;
};
This structure size is 12.
Update:
I only found this in GCC's aligned section of attributes. I think it is related to what I mentioned here:
The aligned attribute can only increase the alignment; but you can
decrease it by specifying packed as well
.Just remember that if you want to keep child structure packed but main structure aligned, you need to use 2 attributes in 2 different declarations. For example following structure has size of 12:
struct foo {
char b;
int a;
} __attribute__((packed));
struct bar {
char b;
__attribute__((aligned(4))) struct foo bla;
char a;
};
but if you use aligned() in declaration of foo as __attribute__((aligned (4), packed)), size will be 16. This happens because foo gets aligned too, and it will not be useful in case of packing.
answered Jan 2 at 18:00
AfshinAfshin
3,1811627
Where can I read about "means that it can ignore alignment when it is in another structure" I didn't find it in the gcc docs or anywhere. :(
– Sukhanov Niсkolay
Jan 2 at 18:03
@SukhanovNiсkolay it had a "it seems" at start of sentence too. but I found 1 sentence about it in gcc manual and I add it to reply.
– Afshin
Jan 2 at 18:05
Yeah, I saw the same cite, then it seems it is an ISO C requirement. :) UPDATE: Sorry, wrong cite, I saw this: "ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct"
– Sukhanov Niсkolay
Jan 2 at 18:10
add a comment |
it seems because __attribute__((packed)) means use the minimum memory for structure, it also means that it can ignore alignment for siding members when it is in another structure. Check following structure:
struct bar {
char b;
__attribute__((packed)) int bla;
char a;
};
When you check size for this structure, it will be 6. This happens because it ignores member alignment for 2 side members(a and b here). But this structure:
struct bar {
char b;
__attribute__((packed)) int bla;
char a;
int c;
};
has size of 12, because it is aligned c on 4 bytes boundary. In your case, if you use aligned attribute too at same time, it works as you expect:
struct bar {
char b;
__attribute__((aligned (4), packed)) int bla;
char a;
};
This structure size is 12.
Update:
I only found this in GCC's aligned section of attributes. I think it is related to what I mentioned here:
The aligned attribute can only increase the alignment; but you can
decrease it by specifying packed as well
.Just remember that if you want to keep child structure packed but main structure aligned, you need to use 2 attributes in 2 different declarations. For example following structure has size of 12:
struct foo {
char b;
int a;
} __attribute__((packed));
struct bar {
char b;
__attribute__((aligned(4))) struct foo bla;
char a;
};
but if you use aligned() in declaration of foo as __attribute__((aligned (4), packed)), size will be 16. This happens because foo gets aligned too, and it will not be useful in case of packing.
answered Jan 2 at 18:00
AfshinAfshin
3,1811627
Where can I read about "means that it can ignore alignment when it is in another structure" I didn't find it in the gcc docs or anywhere. :(
– Sukhanov Niсkolay
Jan 2 at 18:03
@SukhanovNiсkolay it had a "it seems" at start of sentence too. but I found 1 sentence about it in gcc manual and I add it to reply.
– Afshin
Jan 2 at 18:05
Yeah, I saw the same cite, then it seems it is an ISO C requirement. :) UPDATE: Sorry, wrong cite, I saw this: "ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct"
– Sukhanov Niсkolay
Jan 2 at 18:10
add a comment |
it seems because __attribute__((packed)) means use the minimum memory for structure, it also means that it can ignore alignment for siding members when it is in another structure. Check following structure:
struct bar {
char b;
__attribute__((packed)) int bla;
char a;
};
When you check size for this structure, it will be 6. This happens because it ignores member alignment for 2 side members(a and b here). But this structure:
struct bar {
char b;
__attribute__((packed)) int bla;
char a;
int c;
};
has size of 12, because it is aligned c on 4 bytes boundary. In your case, if you use aligned attribute too at same time, it works as you expect:
struct bar {
char b;
__attribute__((aligned (4), packed)) int bla;
char a;
};
This structure size is 12.
Update:
I only found this in GCC's aligned section of attributes. I think it is related to what I mentioned here:
The aligned attribute can only increase the alignment; but you can
decrease it by specifying packed as well
.Just remember that if you want to keep child structure packed but main structure aligned, you need to use 2 attributes in 2 different declarations. For example following structure has size of 12:
struct foo {
char b;
int a;
} __attribute__((packed));
struct bar {
char b;
__attribute__((aligned(4))) struct foo bla;
char a;
};
but if you use aligned() in declaration of foo as __attribute__((aligned (4), packed)), size will be 16. This happens because foo gets aligned too, and it will not be useful in case of packing.
answered Jan 2 at 18:00
AfshinAfshin
3,1811627
it seems because __attribute__((packed)) means use the minimum memory for structure, it also means that it can ignore alignment for siding members when it is in another structure. Check following structure:
struct bar {
char b;
__attribute__((packed)) int bla;
char a;
};
When you check size for this structure, it will be 6. This happens because it ignores member alignment for 2 side members(a and b here). But this structure:
struct bar {
char b;
__attribute__((packed)) int bla;
char a;
int c;
};
has size of 12, because it is aligned c on 4 bytes boundary. In your case, if you use aligned attribute too at same time, it works as you expect:
struct bar {
char b;
__attribute__((aligned (4), packed)) int bla;
char a;
};
This structure size is 12.
Update:
I only found this in GCC's aligned section of attributes. I think it is related to what I mentioned here:
The aligned attribute can only increase the alignment; but you can
decrease it by specifying packed as well
.Just remember that if you want to keep child structure packed but main structure aligned, you need to use 2 attributes in 2 different declarations. For example following structure has size of 12:
struct foo {
char b;
int a;
} __attribute__((packed));
struct bar {
char b;
__attribute__((aligned(4))) struct foo bla;
char a;
};
but if you use aligned() in declaration of foo as __attribute__((aligned (4), packed)), size will be 16. This happens because foo gets aligned too, and it will not be useful in case of packing.
answered Jan 2 at 18:00
AfshinAfshin
3,1811627
edited Jan 2 at 18:22
answered Jan 2 at 18:00
AfshinAfshin
3,1811627
answered Jan 2 at 18:00
AfshinAfshin
3,1811627
answered Jan 2 at 18:00
AfshinAfshin
3,1811627
3,1811627
Where can I read about "means that it can ignore alignment when it is in another structure" I didn't find it in the gcc docs or anywhere. :(
– Sukhanov Niсkolay
Jan 2 at 18:03
@SukhanovNiсkolay it had a "it seems" at start of sentence too. but I found 1 sentence about it in gcc manual and I add it to reply.
– Afshin
Jan 2 at 18:05
Yeah, I saw the same cite, then it seems it is an ISO C requirement. :) UPDATE: Sorry, wrong cite, I saw this: "ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct"
– Sukhanov Niсkolay
Jan 2 at 18:10
add a comment |
Where can I read about "means that it can ignore alignment when it is in another structure" I didn't find it in the gcc docs or anywhere. :(
– Sukhanov Niсkolay
Jan 2 at 18:03
@SukhanovNiсkolay it had a "it seems" at start of sentence too. but I found 1 sentence about it in gcc manual and I add it to reply.
– Afshin
Jan 2 at 18:05
Yeah, I saw the same cite, then it seems it is an ISO C requirement. :) UPDATE: Sorry, wrong cite, I saw this: "ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct"
– Sukhanov Niсkolay
Jan 2 at 18:10
Where can I read about "means that it can ignore alignment when it is in another structure" I didn't find it in the gcc docs or anywhere. :(
– Sukhanov Niсkolay
Jan 2 at 18:03
Where can I read about "means that it can ignore alignment when it is in another structure" I didn't find it in the gcc docs or anywhere. :(
– Sukhanov Niсkolay
Jan 2 at 18:03
@SukhanovNiсkolay it had a "it seems" at start of sentence too. but I found 1 sentence about it in gcc manual and I add it to reply.
– Afshin
Jan 2 at 18:05
@SukhanovNiсkolay it had a "it seems" at start of sentence too. but I found 1 sentence about it in gcc manual and I add it to reply.
– Afshin
Jan 2 at 18:05
Yeah, I saw the same cite, then it seems it is an ISO C requirement. :) UPDATE: Sorry, wrong cite, I saw this: "ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct"
– Sukhanov Niсkolay
Jan 2 at 18:10
Yeah, I saw the same cite, then it seems it is an ISO C requirement. :) UPDATE: Sorry, wrong cite, I saw this: "ISO C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the struct"
– Sukhanov Niсkolay
Jan 2 at 18:10
add a comment |
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