标准内存分配器对齐问题... [英] standard memory allocator alignment issue...
问题描述
有多少C编译器提供扩展，允许执行以下hack的标准
？
__________________________________________________ __________________
#include< stdio.h>
typedef union aligner_types_u aligner_types;
typedef struct aligner_offset_s aligner_offset;
union aligner_types_u {
char char_;
短s_l;
int i_;
long l_;
double d_;
long double ld_;
float f_;
void * p_;
char（* fp0_）（char）;
long double（* fp1_）（char，long double）;
union aligner_types * uap_;
/ * long long ll_; * /
/ * [...] * /
};
struct aligner_offset_s {
char offset ;
aligner_types类型;
};
#define ALIGN_MAX（\
sizeof（aligner_offset）sizeof（ aligner_types）\
？sizeof（aligner_offset） - sizeof（aligner_types）\
：sizeof（aligner_types）\
）
int main（）{
printf（" ALIGN_MAX ==％d\\\
n，nnit enter to exit ... \ n"，ALIGN_MAX）;
getchar（）;
返回0;
}
__________________________________________________ __________________
谢谢...
BTW，ALIGN_MAX宏是必需的，因为仅使用sizeof（aligner_types）
是不够的..有多少人在运行平台
（ALIGN_MAX == 8）是真的？
Chris Thomasson < cr ***** @ comcast.netwrote in message
news：5u ************************* *****@comcast.com。 ..
有多少C编译器提供扩展，允许执行以下hack的标准
实现？
[...]
标准在各种版本的便携式意义上
给出供应商C编译器......
：^ o
Chris Thomasson写道：
有多少C编译器提供扩展，允许执行以下hack的标准
实现？
__________________________________________________ __________________
#include< stdio.h>
typedef union aligner_types_u aligner_types;
typedef struct aligner_offset_s aligner_offset;
union aligner_types_u {
char char_;
short s_l;
int i_;
long l_;
double d_;
long double ld_;
float f_;
void * p_ ;
char（* fp0_）（char）;
long double（* fp1_）（char，long double）;
union aligner_types * uap_;
/ * long long ll_; * /
/ * [...] * /
};
struct aligner_offset_s {
char offset;
aligner_types类型;
};
#define ALIGN_MAX（\
sizeof（aligner_offset）sizeof（aligner_types）\
？sizeof（aligner_offset） - sizeof（aligner_types）\
：sizeof（aligner_types）\
）
int main（）{
printf（" ALIGN_MAX ==％d\\\
\\\
hit enter to exit。 .. \ n"，ALIGN_MAX）;
getchar（）;
返回0;
}
__________________________________________________ __________________
谢谢...
BTW，因为使用了sizeof（aligner_types）所以需要ALIGN_MAX宏/>
单独是不够的...有多少人在运行平台
（ALIGN_MAX == 8）是真的？
观察＃1：否则是不可能的。分支
的ALIGN_MAX''扩展将被评估，所以你可能只需要把'42''放在那里。
观察＃2：ALIGN_MAX计算结构中的字节数
，减去一个'char''元素，减去
填充前的填充字节数，减去*之后的数字
填充字节*。我从来没有见过一个
编译器，其最终用语不为零，但是......
观察＃3：在某些平台上程序会说
" ALIGN_MAX == 0"，因为这是printf（）调用中未定义行为的可能结果之一
。 />
观察＃4：我不完全确定，但我认为
"扩展名"你寻求的是< stddef.h>中的偏移量。
-
Eric Sosman
es ***** @ ieee-dot-org.inva 盖子
" ; Eric Sosman < es ***** @ ieee-dot-org.invalidwrote in message
news：0f ********************* *********@comcast.com。 ..
Chris Thomasson写道：
>有多少C编译器提供允许的扩展标准
实施以下黑客攻击？
_________________________________________________ ___________________
[...]
> _________________________________________________ ___________________
[...]
< blockquote class =post_quotes>
> BTW，ALIGN_MAX宏是必需的，因为单独使用sizeof（aligner_types）
是不够的......有多少人运行平台
（ALIGN_MAX == 8）是真的吗？
观察＃1：否则是不可能的。分支
的ALIGN_MAX''扩展将被评估，所以你可能只需要把'42''放在那里。
观察＃2：ALIGN_MAX计算结构中的字节数
，减去`char'的一个在联合之前减去
填充字节数，减去*之后的填充字节数*
。我从来没有见过一个
编译器，其最终用语非零，但是......
观察＃3：在某些平台上，程序会说
ALIGN_MAX == 0，因为这是可能的结果之一
的未定义行为printf（）调用。
完全同意你所说的一切。至于printf，至少我应该将它boned格式化为无符号整数：％u。 ; ^（
观察＃4：我不完全确定，但我认为你寻求的
扩展名是< stddef.h>中的offsetof宏。
你得到了它：
__________________________________________________ _________________
＃ include< stdio.h>
#include< stddef.h>
typedef union aligner_types_u aligner_types;
typedef struct aligner_offset_s aligner_offset;
union aligner_types_u {
char char_; short s_l; int i_; long l_;
double d_; long double ld_; float f _;
union aligner_types * uap_;
void * p_; char（* fp0_）（char）;
long double（* fp1_）（char，long双倍）;
/ * long long ll _; * /
/ * [...] * /
};
struct aligner_offset_s {
char offset;
aligner_types类型;
};
#define ALIGN_MAX offsetof（aligne r_offset，types）
int main（）{
printf（" ALIGN_MAX ==％u\\\
n \\ nnit enter enter to exit ... \ n" ，ALIGN_MAX）;
getchar（）;
返回0;
}
__________________________________________________ _________________
我试图想出一些便携式的东西。 hack可以尝试
确定多个不同的b $ b编译器中整数类型的最大对齐。
How many C compilers provide extensions which allow for a standard
implementation of the following hack?
__________________________________________________ __________________
#include <stdio.h>
typedef union aligner_types_u aligner_types;
typedef struct aligner_offset_s aligner_offset;
union aligner_types_u {
char char_;
short s_l;
int i_;
long l_;
double d_;
long double ld_;
float f_;
void *p_;
char (*fp0_) (char);
long double (*fp1_) (char, long double);
union aligner_types* uap_;
/* long long ll_; */
/* [...] */
};
struct aligner_offset_s {
char offset;
aligner_types types;
};
#define ALIGN_MAX ( \
sizeof(aligner_offset) sizeof(aligner_types) \
? sizeof(aligner_offset) - sizeof(aligner_types) \
: sizeof(aligner_types) \
)
int main() {
printf("ALIGN_MAX == %d\n\nhit enter to exit...\n", ALIGN_MAX);
getchar();
return 0;
}
__________________________________________________ __________________
Thanks...
BTW, the ALIGN_MAX macro is needed because using a sizeof(aligner_types)
alone is not sufficient... How many people are running platforms where
(ALIGN_MAX == 8) is true?
"Chris Thomasson" <cr*****@comcast.netwrote in message
news:5u******************************@comcast.com. ..How many C compilers provide extensions which allow for a standard
implementation of the following hack?[...]
Standard in the sense of being portable within the various versions of a
given vendors C compiler...
:^o
Chris Thomasson wrote:How many C compilers provide extensions which allow for a standard
implementation of the following hack?
__________________________________________________ __________________
#include <stdio.h>
typedef union aligner_types_u aligner_types;
typedef struct aligner_offset_s aligner_offset;
union aligner_types_u {
char char_;
short s_l;
int i_;
long l_;
double d_;
long double ld_;
float f_;
void *p_;
char (*fp0_) (char);
long double (*fp1_) (char, long double);
union aligner_types* uap_;
/* long long ll_; */
/* [...] */
};
struct aligner_offset_s {
char offset;
aligner_types types;
};
#define ALIGN_MAX ( \
sizeof(aligner_offset) sizeof(aligner_types) \
? sizeof(aligner_offset) - sizeof(aligner_types) \
: sizeof(aligner_types) \
)
int main() {
printf("ALIGN_MAX == %d\n\nhit enter to exit...\n", ALIGN_MAX);
getchar();
return 0;
}
__________________________________________________ __________________
Thanks...
BTW, the ALIGN_MAX macro is needed because using a sizeof(aligner_types)
alone is not sufficient... How many people are running platforms where
(ALIGN_MAX == 8) is true?Observation #1: It is impossible that the "else" branch
of ALIGN_MAX'' expansion will be evaluated, so you might as
well just put `42'' there.
Observation #2: ALIGN_MAX computes the number of bytes
in the struct, minus one for the `char'' element, minus the
number of padding bytes before the union, minus the number
of padding bytes *after* the union. I''ve never seen a
compiler where that final term would be non-zero, but ...
Observation #3: On some platforms the program will say
"ALIGN_MAX == 0", because that''s one of the likely outcomes
of the undefined behavior in the printf() call.
Observation #4: I''m not entirely sure, but I think the
"extension" you seek is the offsetof macro in <stddef.h>.
--
Eric Sosman
es*****@ieee-dot-org.invalid
"Eric Sosman" <es*****@ieee-dot-org.invalidwrote in message
news:0f******************************@comcast.com. ..Chris Thomasson wrote:>How many C compilers provide extensions which allow for a standard
implementation of the following hack?
_________________________________________________ ___________________
[...]
>_________________________________________________ ___________________
[...]
>BTW, the ALIGN_MAX macro is needed because using a sizeof(aligner_types)
alone is not sufficient... How many people are running platforms where
(ALIGN_MAX == 8) is true?
Observation #1: It is impossible that the "else" branch
of ALIGN_MAX'' expansion will be evaluated, so you might as
well just put `42'' there.
Observation #2: ALIGN_MAX computes the number of bytes
in the struct, minus one for the `char'' element, minus the
number of padding bytes before the union, minus the number
of padding bytes *after* the union. I''ve never seen a
compiler where that final term would be non-zero, but ...
Observation #3: On some platforms the program will say
"ALIGN_MAX == 0", because that''s one of the likely outcomes
of the undefined behavior in the printf() call.Totally agree with everything you said. As for printf, at least I should
have it formatted for an unsigned integer: %u. ;^(
Observation #4: I''m not entirely sure, but I think the
"extension" you seek is the offsetof macro in <stddef.h>.You got it:
__________________________________________________ _________________
#include <stdio.h>
#include <stddef.h>
typedef union aligner_types_u aligner_types;
typedef struct aligner_offset_s aligner_offset;
union aligner_types_u {
char char_; short s_l; int i_; long l_;
double d_; long double ld_; float f_;
union aligner_types* uap_;
void *p_; char (*fp0_) (char);
long double (*fp1_) (char, long double);
/* long long ll_; */
/* [...] */
};
struct aligner_offset_s {
char offset;
aligner_types types;
};
#define ALIGN_MAX offsetof(aligner_offset, types)
int main() {
printf("ALIGN_MAX == %u\n\nhit enter to exit...\n", ALIGN_MAX);
getchar();
return 0;
}
__________________________________________________ _________________
I am trying to come up with somewhat "portable" hack that can attempt to
determine maximum alignment for integral types across a number of different
compilers.
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