c&时间问题 [英] c & time problem
问题描述
我在理解这段代码的工作原理时遇到了问题。
据我所知，ctime function返回一个字符串。主要是
返回string分配给一个整数（16位）并在MAIN中用
a字符串格式打印。
有人会解释这是如何工作和/或纠正我的理解吗？ />
#include< stdio.h>
#include< time.h>
//获取系统日期和时间
int main（无效）
{
int i;
i = mytim（）;
printf（D / T =％s，i）;
}
mytim（）
{
time_t now;
if（（now = time（NULL））==（time_t） - 1）
puts（"失败：无法获得时间）;
else
printf（" Date& time：％s"，ctime（& now））;
//下列任何一项都同样有效
return（ctime（& now））;
}
RVO写道：
我有一个问题的理解为什么这段代码有效。
As我理解为ctime function返回一个字符串。主要是
返回string被赋值为一个整数（16位），并在MAIN中以字符串格式打印
。
为什么假设16位int？函数名称是main，而不是MAIN。
有人会详细解释这是如何工作的和/或纠正我的理解吗？
#include< stdio.h>
#include< time.h>
//获取系统日期和时间
int main（无效）
{
int i;
i = mytim（）;
printf（" D / T =％s"，i）;
}
mytim（）
不要依赖隐式int返回类型。
{
time_t now;
if（（now = time（NULL））==（time_t） - 1）
puts（FAIL：无法获得时间）;
else
printf（" Date& time：％s"，ctime（& now））;
//以下任何一项都同样有效
返回（ctime（& now））;
}
您是否注意到所有编译器警告当你编译这个时，会发出什么？
未定义行为的经典案例，sizeof char *碰巧等于
sizeof int 。
-
Ian Collins。
Ian Collins写道：
< blockquote class =post_quotes>
> RVO写道：
>>
我有一个问题理解为什么这个代码有效。
据我所知，ctime function返回一个字符串。主要是
返回的字符串分配给一个整数（16位）并在MAIN中打印一个字符串格式。
为什么假设16位int？函数名称是main，而不是MAIN。
1）因为它是。
2）MAIN强调。
>
>>有人会解释这是如何工作和/或纠正我的<理解？
#include< stdio.h>
#include< time.h>
//获取系统日期和时间
int main （无效）
{
int i;
i = mytim（）;
printf（" D / T =％s"，i）;
}
mytim（）
不要依赖隐式的int返回类型。
3）从C获得代码网络专家。
>
>> {
time_t now;
if（（now = time（NULL））==（time_t） - 1）
puts（FAIL：Unable get time）;
printf（ 日期&时间：％s，ctime（& now））;
//以下任何一项都能很好地工作
return（ctime（& now））;
}
你是否注意到编译它时发出的所有编译器警告？
4）就我所知，警告与此无关。
>未定义行为的经典案例，sizeof char *碰巧等于
sizeof int。
5）但是如果C是是如此独特为什么它没有用
整数标记％s的使用？
RVO写道：
Ian Collins写道：
> RVO写道：
>>
我在理解这段代码的工作原理时遇到了问题。
正如我所理解的那样，ctime function返回一个字符串。主要是
返回的字符串分配给一个整数（16位）并在MAIN中用字符串格式打印。
为什么假设16位int？函数名称是main，而不是MAIN。
1）因为它是。
什么？
>>
不要依赖隐式int返回类型。
3）从C获得代码网络专家。
一个可以避免的。
> ;>
你是否注意到编译它时发出的所有编译器警告？
4）据我所知，警告与此无关。
/ tmp / xc：在函数main中：
/ tmp / xc：7：警告：隐式声明函数`mytim''
/ tmp / xc：8：警告：格式参数不是指针（arg 2）
/ tmp / xc：9：警告：控制到达无效功能的结束
/ tmp / xc：在顶级：
/ tmp / xc：12：警告：返回类型默认为int'
/ tmp / xc：在函数mytim中：
/ tmp / xc：19：警告：返回从指针中生成整数而没有强制转换
哪一个？第8行的警告看起来很明显。也许你没有在符合模式下调用你的编译器。
>经典案例对于未定义的行为，sizeof char *恰好等于
sizeof int。
5）但是如果C是是如此独特为什么它没有用
整数标记％s的用法？
gcc做了，见上文。
-
Ian Collins。
Hi,
I am having a problem understanding why this code works.
As I understand it the "ctime" function returns a string. In main the
returned "string" is assigned to an integer (16 bit) and is printed with
a string format in MAIN.
Would someone kindly explain how this works and/or correct my understanding?
#include <stdio.h>
#include <time.h>
// Gets System Date and Time
int main(void)
{
int i;
i = mytim();
printf("D/T= %s", i);
}
mytim()
{
time_t now;
if( (now = time(NULL)) == (time_t) - 1 )
puts("FAIL: Unable to get time");
else
printf("Date & time: %s", ctime(&now));
// Either of following works equally well
return (ctime(&now));
}
RVO wrote:Hi,
I am having a problem understanding why this code works.
As I understand it the "ctime" function returns a string. In main the
returned "string" is assigned to an integer (16 bit) and is printed with
a string format in MAIN.Why do you assume 16 bit int? The function name is main, not MAIN.
Would someone kindly explain how this works and/or correct my
understanding?
#include <stdio.h>
#include <time.h>
// Gets System Date and Time
int main(void)
{
int i;
i = mytim();
printf("D/T= %s", i);
}
mytim()Don''t rely on implicit int return type.
{
time_t now;
if( (now = time(NULL)) == (time_t) - 1 )
puts("FAIL: Unable to get time");
else
printf("Date & time: %s", ctime(&now));
// Either of following works equally well
return (ctime(&now));
}Did you take note of all of the compiler warnings emitted when you
compiled this?
A classic case of undefined behaviour, sizeof char* happens to equal
sizeof int.
--
Ian Collins.
Ian Collins wrote:
>RVO wrote:
>>Hi,
I am having a problem understanding why this code works.
As I understand it the "ctime" function returns a string. In main the
returned "string" is assigned to an integer (16 bit) and is printed with
a string format in MAIN.
Why do you assume 16 bit int? The function name is main, not MAIN.
1) because it is.
2) MAIN for emphasis.
>
>>Would someone kindly explain how this works and/or correct my
understanding?
#include <stdio.h>
#include <time.h>
// Gets System Date and Time
int main(void)
{
int i;
i = mytim();
printf("D/T= %s", i);
}
mytim()
Don''t rely on implicit int return type.
3) Got code from "C" expert on web.
>
>>{
time_t now;
if( (now = time(NULL)) == (time_t) - 1 )
puts("FAIL: Unable to get time");
else
printf("Date & time: %s", ctime(&now));
// Either of following works equally well
return (ctime(&now));
}
Did you take note of all of the compiler warnings emitted when you
compiled this?
4) the warning was not related to this as far as I can tell.
>A classic case of undefined behaviour, sizeof char* happens to equal
sizeof int.
5) But if "C" is so unique why does it not flag the %s usage with the
integer?
RVO wrote:Ian Collins wrote:
>RVO wrote:
>>Hi,
I am having a problem understanding why this code works.
As I understand it the "ctime" function returns a string. In main the
returned "string" is assigned to an integer (16 bit) and is printed with
a string format in MAIN.
Why do you assume 16 bit int? The function name is main, not MAIN.
1) because it is.
On what?
>>
Don''t rely on implicit int return type.
3) Got code from "C" expert on web.
One to avoid then.
>>
Did you take note of all of the compiler warnings emitted when you
compiled this?
4) the warning was not related to this as far as I can tell.
/tmp/x.c: In function `main'':
/tmp/x.c:7: warning: implicit declaration of function `mytim''
/tmp/x.c:8: warning: format argument is not a pointer (arg 2)
/tmp/x.c:9: warning: control reaches end of non-void function
/tmp/x.c: At top level:
/tmp/x.c:12: warning: return type defaults to `int''
/tmp/x.c: In function `mytim'':
/tmp/x.c:19: warning: return makes integer from pointer without a cast
Which one? The warning for line 8 looks plain enough. Perhaps you
didn''t invoke your compiler in conforming mode.
>A classic case of undefined behaviour, sizeof char* happens to equal
sizeof int.
5) But if "C" is so unique why does it not flag the %s usage with the
integer?gcc did, see above.
--
Ian Collins.
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