库错误还是我的错？ [英] Library bug or my fault?
问题描述
请查看邮件末尾列出的示例代码。这是使用ARM / Linux交叉编译器编译并在ARM9目标上运行的
。我认为这个问题适用于这个组，因为来自不同供应商的两个交叉编译器
导致相同的错误。所以我猜这不是供应商<br />
具体。如果我的猜测是对的，那就意味着代码本身可能会出现问题，但我无法弄清楚它在哪里。
问题是，第38行复制3个字节，从p2开始，到
p1，但紧接着的memcmp（第42行）显示memcpy是
做得不好。我确信memcpy没有完成这项工作，因为如果我提供我自己的memcpy实现，如下所示，错误将会消失。
消失。
void memcpy（void * dest，void * src，size_t n）
{
uint8_t * d =（uint8_t *）dest;
uint8_t * s =（uint8_t *）src;
for（size_t i = 0; i< n; ++ i）
* d ++ = * s ++;
}
您能否查看代码以及运行结果和
告诉我有什么不对吗？谢谢。
-------------------------------------最小样本
------------------------------------------ --------
1 #include< stdio.h>
2 #include< string>
3＃ include< stdint.h>
4 #include< assert.h>
5
6 struct Foo {
7 uint8_t x;
8 uint8_t y;
9 uint8_t z;
10 uint8_t m [3];
11};
12
13 struct Bar
14 {
15 uint8_t m [3 ];
16};
17
18 void pr（const char * title，const void * block，size_t n）
19 {
20 printf（"％s\ n"，title）;
21
22 uint8_t * p =（uint8_t *）block;
23 for（size_t i = 0; i< n; ++ i）
24 printf（" 0x％02x" ;，* p ++）;
25
26 printf（" \ n"）;
27}
28
29 v oid cp（const Foo * foo）
30 {
31酒吧酒吧;
32
33 Bar * p1 =& bar;
34 Bar * p2 =（Bar *）（foo-> m）;
35 pr（" before：p1：" ，p1,3;;
36 pr（之前：p2：，p2,3 ;;
37
38 memcpy （p1，p2,3）;
39 pr（" after：p1："，p1,3）;
40 pr（" after：p2： "，p2,3 ;;
41
42 if（memcmp（p1，p2,3）！= 0）
43 printf （QUOT;！ cp是错的\ n;
44}
45
46 int main（）
47 {
48 Foo foo;
49 foo.x = 1;
50 foo.y = 2;
51 foo.z = 3;
52 foo.m [0] = 0x40;
53 foo.m [1] = 0x19;
54 foo.m [2] = 0x21;
55
56 cp（& foo）;
57 cp2（& foo）;
58返回0;
59}
----------------- ------------------------------------------------- <下面是ARM920T板上的运行输出：
之前：p1：
0xfc 0x01 0x12
之前：p2：
0x40 0x19 0x21
之后：p1：
0x40 0x01 0x02
之后：p2：
0x40 0x19 0x21
!!! cp错了
Steven Woody写道：
...
问题是，第38行复制3个字节，从
p2开始，再到p1，但紧随其后的memcmp（第42行）
显示memcpy做得不好。
这是前面的线路表示问题。
------ -------------------------------最小样本
-------- ------------------------------------------
1 #include< stdio.h>
2 #include< string>
3 #include< stdint.h>
4 #include< assert.h>
5
6 struct Foo {
7 uint8_t x;
8 uint8_t y;
9 uint8_t z;
10 uint8_t m [3];
11};
12
13 struct bar
14 {
15 uint8_t m [3];
16};
这些是完全不同的结构。
29 void cp（const Foo * foo）
30 {
31吧台酒吧;
32
33 Bar * p1 =& bar;
34 Bar * p2 =（Bar *）（foo-> m）;
事实上你有一个强制转换警告（或者更可能是因为
错误）而导致不兼容类型的问题。 Bar不是一个数组，
它是一个包含数组的结构。
标准允许转换（如果foo-> m对齐
正确），但它没有更多说明。只有你转换它
返回它必须比较相等。
35 pr（" before：p1："，p1,3 ）;
36 pr（之前：p2：，p2,3;）
38 memcpy（p1，p2 ，3）;
如果你真的想这样做，那么以下内容应该有效...
memcpy（bar-> m，foo-> m，sizeof foo-> m）;
如果失败了，你可以担心。
你有其他的选择，比如把一个Bar结构放在foo里面。
你可以使用Bar指针和简单的赋值。
但从根本上说，你应该复制喜欢。
< snip>
-
Peter
在文章< be ********************************** @ s50g2000hsb。 googlegroups.com>，
Steven Woody< na ******** @ gmail.comwrote：
> 1 #include< stdio.h>
2 #include< string>
应该是
#include< string.h>
你需要< string.h为memcpy原型。
问候，
Ike
Steven Woody写道：
请查看邮件末尾列出的示例代码。
它似乎是用一些非C语言编写的。你的意思是
而不是发布到C ++？看看这个版本（注意更改，更多
可能是可取的，但我只解决可能是你的问题）工作
给你。显然可能存在问题（例如，
你确定访问''bar''的未初始化内容是安全的吗？）。
＃ include< stdio.h>
#include< string.h / * mha：was''string''* /
#include< stdint.h>
struct Foo
{
uint8_t x;
uint8_t y;
uint8_t z;
uint8_t m [3];
};
struct Bar
{
uint8_t m [3];
};
void pr（const char * title，const void * block，size_t n）
{
printf（"％s \ n"，title）;
uint8_t * p =（uint8_t *）block;
for（size_t i = 0; i< n; ++ i）
printf（" 0x％02x"， * p ++）;
printf（" \ n"）;
}
void cp（ const struct Foo / * mha：添加了所需的''struct''* / * foo）
{
struc t酒吧; / * mha：在这里添加了''struct''&
以下* /
struct Bar * p1 =& bar;
struct Bar * p2 =（struct Bar *）（foo-> m）;
pr（" before：p1："，p1,3）;
pr（之前：p2：，p2,3 ;;
memcpy（p1，p2,3）;
pr（&after; after ：p1："，p1,3 ;;
pr（" after：p2："，p2,3）;
if（memcmp （p1，p2,3）！= 0）
printf（!!! cp是错的\ n）;
}
int main（）
{
struct Foo foo; / * mha：添加需要''struct''* /
foo.x = 1;
foo.y = 2;
foo .z = 3;
foo.m [0] = 0x40;
foo.m [1] = 0x19;
foo.m [2] = 0x21;
cp（& foo）;
返回0;
}
之前：p1：
0x20 0x20 0x20
之前：p2：
0x40 0x19 0x21
之后：p1：
0x40 0x19 0x21
之后：p2：
0x40 0x19 0x21
当然输出与你的不同。
以下是ARM920T板上的运行输出：
之前：p1：
0xfc 0x01 0x12
之前：p2：
0x40 0x19 0x21
之后：p1：
0x40 0x01 0x02
之后：p2：
0x40 0x19 0x21
Hi,
Please check the sample code listed in the end of the message. It was
compiled using ARM/Linux cross-compiler and run on an ARM9 target. I
think the problem applies to this group because two cross-compiler
from different vendor result same error. So I guess it is not vendor
specific. If my guess is right, then it means the code itself may get
problem, but I can not figure out where it is.
The problem is, the line 38 which copy 3 bytes, starting from p2, to
p1, but the immediately followed memcmp (line 42) shows the memcpy was
not well done. I am sure the memcpy did not do the job, since if I
provide my own memcpy implemention as below, the error will go
disappear.
void memcpy(void *dest, void *src, size_t n)
{
uint8_t *d = (uint8_t*)dest;
uint8_t *s = (uint8_t*)src;
for (size_t i = 0; i < n; ++i)
*d++ = *s++;
}
Could you please check the code as well as the running result and
tell me what wrong with it? Thanks.
------------------------------------- the minimum sample
--------------------------------------------------
1 #include <stdio.h>
2 #include <string>
3 #include <stdint.h>
4 #include <assert.h>
5
6 struct Foo {
7 uint8_t x;
8 uint8_t y;
9 uint8_t z;
10 uint8_t m[3];
11 };
12
13 struct Bar
14 {
15 uint8_t m[3];
16 };
17
18 void pr(const char *title, const void *block, size_t n)
19 {
20 printf("%s\n", title);
21
22 uint8_t *p = (uint8_t*)block;
23 for (size_t i = 0; i < n; ++i)
24 printf("0x%02x ", *p++);
25
26 printf("\n");
27 }
28
29 void cp(const Foo *foo)
30 {
31 Bar bar;
32
33 Bar *p1 = &bar;
34 Bar *p2 = (Bar*)(foo->m);
35 pr("before: p1:", p1, 3);
36 pr("before: p2:", p2, 3);
37
38 memcpy(p1, p2, 3);
39 pr("after: p1:", p1, 3);
40 pr("after: p2:", p2, 3);
41
42 if (memcmp(p1, p2, 3) != 0)
43 printf("!!! cp is wrong\n");
44 }
45
46 int main()
47 {
48 Foo foo;
49 foo.x = 1;
50 foo.y = 2;
51 foo.z = 3;
52 foo.m[0] = 0x40;
53 foo.m[1] = 0x19;
54 foo.m[2] = 0x21;
55
56 cp(&foo);
57 cp2(&foo);
58 return 0;
59 }
------------------------------------------------------------------
Below is the running output on an ARM920T board:
before: p1:
0xfc 0x01 0x12
before: p2:
0x40 0x19 0x21
after: p1:
0x40 0x01 0x02
after: p2:
0x40 0x19 0x21
!!! cp is wrong
Steven Woody wrote:...
The problem is, the line 38 which copy 3 bytes, starting from
p2, to p1, but the immediately followed memcmp (line 42)
shows the memcpy was not well done.It''s the lines before that signal a problem.
------------------------------------- the minimum sample
--------------------------------------------------
1 #include <stdio.h>
2 #include <string>
3 #include <stdint.h>
4 #include <assert.h>
5
6 struct Foo {
7 uint8_t x;
8 uint8_t y;
9 uint8_t z;
10 uint8_t m[3];
11 };
12
13 struct Bar
14 {
15 uint8_t m[3];
16 };These are completely different structs.
29 void cp(const Foo *foo)
30 {
31 Bar bar;
32
33 Bar *p1 = &bar;
34 Bar *p2 = (Bar*)(foo->m);The fact you have a cast to silence a warning (or more likely
error) on incompatible types is the problem. Bar is not an array,
it is a struct containing an array.
The standard allows the conversion (if foo->m is aligned
properly), but it says nothing more. Only if you convert it
back must it compare equal.
35 pr("before: p1:", p1, 3);
36 pr("before: p2:", p2, 3);
37
38 memcpy(p1, p2, 3);If you really wanted to do this, then the following should work...
memcpy(bar->m, foo->m, sizeof foo->m);
If that fails, you have cuase for concern.
You have other options, like putting a Bar struct inside foo.
The you can use Bar pointers and simple assignment.
But fundamentally, you should be copying like to like.
<snip>
--
Peter
In article <be**********************************@s50g2000hsb. googlegroups.com>,
Steven Woody <na********@gmail.comwrote:>1 #include <stdio.h>
2 #include <string>That should be
#include <string.h>
You need <string.hfor the memcpy prototype.
Regards,
Ike
Steven Woody wrote:Hi,
Please check the sample code listed in the end of the message.It seems to be written in some non-C but C-like language. Did you mean
to post to C++ instead? See if this version (notice the changes, more
may be advisable, but I address only what might be your problem) works
for you. There are clearly possible problems remaining (for example,
are you sure that accessing the uninitialized contents of ''bar'' is safe?).
#include <stdio.h>
#include <string.h /* mha: was ''string'' */
#include <stdint.h>
struct Foo
{
uint8_t x;
uint8_t y;
uint8_t z;
uint8_t m[3];
};
struct Bar
{
uint8_t m[3];
};
void pr(const char *title, const void *block, size_t n)
{
printf("%s\n", title);
uint8_t *p = (uint8_t *) block;
for (size_t i = 0; i < n; ++i)
printf("0x%02x ", *p++);
printf("\n");
}
void cp(const struct Foo /* mha: added needed ''struct'' */ *foo)
{
struct Bar bar; /* mha: added needed ''struct'' here &
below */
struct Bar *p1 = &bar;
struct Bar *p2 = (struct Bar *) (foo->m);
pr("before: p1:", p1, 3);
pr("before: p2:", p2, 3);
memcpy(p1, p2, 3);
pr("after: p1:", p1, 3);
pr("after: p2:", p2, 3);
if (memcmp(p1, p2, 3) != 0)
printf("!!! cp is wrong\n");
}
int main()
{
struct Foo foo; /* mha: added need ''struct'' */
foo.x = 1;
foo.y = 2;
foo.z = 3;
foo.m[0] = 0x40;
foo.m[1] = 0x19;
foo.m[2] = 0x21;
cp(&foo);
return 0;
}
before: p1:
0x20 0x20 0x20
before: p2:
0x40 0x19 0x21
after: p1:
0x40 0x19 0x21
after: p2:
0x40 0x19 0x21
Certainly the output is different from yours.
Below is the running output on an ARM920T board:
before: p1:
0xfc 0x01 0x12
before: p2:
0x40 0x19 0x21
after: p1:
0x40 0x01 0x02
after: p2:
0x40 0x19 0x21
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