blocks|key|3570757|text|No..|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|3570758|从事实来看|3570759|0|ordered-list-item|CODE|3570760|NULL指针不应指向有效对象。根据C11标准，第6.3.2.3节：
..。空指针，保证与指向任何对象或函数的指针不相等。|3570761|相应的手册页面没有提到dest+==+NULL的可能性，|3570762|不允许这样做。|3570763|entityMap|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/9894013/is-null-always-zero-in-c^0|0|3|0|0|0|1|0|1|0|0|0|4|0|B|C|0|0|3|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@$9|13|A|14|B|K]]|D|@$9|15|A|16|1|17]]|E|$]]|$1|L|3|M|5|J|7|18|8|@$9|19|A|1A|B|K]]|D|@]|E|$]]|$1|N|3|O|5|6|7|1B|8|@$9|1C|A|1D|B|K]]|D|@]|E|$]]|$1|P|3|Q|5|6|7|1E|8|@$9|1F|A|1G|B|C]]|D|@]|E|$]]|$1|R|3|-4|5|6|7|1H|8|@]|D|@]|E|$]]]|S|$I|$5|T|U|V|E|$W|X]]]]

No.

Judging from the facts that

<ol>
<li><a href="https://stackoverflow.com/questions/9894013/is-null-always-zero-in-c">The null pointer constant <code>NULL</code>'s value equals <code>0</code></a></li>
<li>A <code>NULL</code> pointer shall not point to a valid object. From the C11 standard, §6.3.2.3:

<blockquote>
 [...] a null pointer [,] is guaranteed to compare
 unequal to a pointer to any object or function.
</blockquote></li>
</ol>

and that the corresponding man page does not mention the possibility of <code>dest == NULL</code>,
 
it is not allowed to.

blocks|key|1743034|text|想想看。手册是怎么说的？它说它返回dest。因此，如果dest不是NULL，则返回值将不是NULL。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1743035|因为buff永远不会是NULL+(假设它是char+buff[100])，那么您可以肯定地说，不，它永远不会返回NULL。|1743036|如果是的话，它就会有个窃听器。|1743037|entityMap^0|H|4|R|4|X|4|19|4|0|2|4|B|4|L|E|1K|4|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|13|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|K|$]]

Think about it. What does the man page say? It says it returns <code>dest</code>. Therefore, if <code>dest</code> is not <code>NULL</code>, then the return value will not be <code>NULL</code>.

Since <code>buff</code> will never be <code>NULL</code> (assuming it's <code>char buff[100]</code>), then you can say with certainty that no, it will never return <code>NULL</code>.

If it does, it's got a bug.

blocks|key|3100961|text|在我看来，如果执行的代码不包含任何未定义的行为，那么给定的代码就不能返回NULL。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3100962|唯一可以返回NULL的情况是，目标是NULL。在这种情况下(最有可能是)，除非复制的字符数为0，否则执行将发生分段错误。|BOLD|3100963|在给定的例子中：|3100964|strncpy(buff,+"something",+9)|code-block|syntax|javascript|3100965|目标既不能是NULL+(因为它是char+buff[100])，也不能复制字符数为0，因为它被指定为9。|3100966|entityMap^0|10|4|0|6|4|I|4|U|5|0|0|0|6|4|G|E|0^^$0|@$1|2|3|4|5|6|7|T|8|@$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|H]]|D|@]|E|$]]|$1|I|3|J|5|6|7|13|8|@]|D|@]|E|$]]|$1|K|3|L|5|M|7|14|8|@]|D|@]|E|$N|O]]|$1|P|3|Q|5|6|7|15|8|@$9|16|A|17|B|C]|$9|18|A|19|B|C]]|D|@]|E|$]]|$1|R|3|-4|5|6|7|1A|8|@]|D|@]|E|$]]]|S|$]]

In my opinion if the executed code doesn't contain any undefined behavior, the given code cannot return <code>NULL</code>.

The only case when it can return <code>NULL</code> is if the destination is <code>NULL</code>. In such case (most likely) the execution will segfault unless the number of copied characters is 0.

In the given example:

<pre><code>strncpy(buff, "something", 9)
</code></pre>

Neither the destination can be <code>NULL</code> ( since it is <code>char buff[100]</code>), nor number of copied characters can be 0 since it is given as 9.

blocks|key|3101035|text|根据ISO/IEC+9899:2011标准§7.24.2.4/c4，strncpy__：function|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3101036|简要说明
1#包含+char+*strncpy(char+*限制s1，const+char+*限制s2，size_t+n)；
描述
2+strncpy函数将不超过n字符(>跟随空字符的字符未被复制)从s2指向的数组复制到
3如果s2指向的数组是一个短于n个字符的字符串，则将>+null字符追加到s1指向的数组中的副本中，直到所有n个字符都被写入为止。
返回
4+strncpy+函数返回s1.+s1的值。如果复制发生在重叠的对象之间，则行为是未定义的。|blockquote|BOLD|3101037|因此，strncpy返回0+(即NULL)而不引起未定义行为的唯一方法是，如果s1为NULL，而n为零。|3101038|LIVE+DEMO|3101039|entityMap|0|LINK|mutability|MUTABLE|url|http://coliru.stacked-crooked.com/a/5e6d46ffafab01f6^0|Y|7|0|0|4|1R|2|4X|2|52|7|5A|7|1W|7|29|1|2S|2|36|2|44|2|52|7|5I|2|0|3|7|C|1|G|4|13|2|16|4|1C|1|0|0|9|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Y|8|@$9|Z|A|10|B|I]|$9|11|A|12|B|I]|$9|13|A|14|B|I]|$9|15|A|16|B|I]|$9|17|A|18|B|I]|$9|19|A|1A|B|C]|$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]|$9|1J|A|1K|B|C]|$9|1L|A|1M|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|1N|8|@$9|1O|A|1P|B|C]|$9|1Q|A|1R|B|C]|$9|1S|A|1T|B|C]|$9|1U|A|1V|B|C]|$9|1W|A|1X|B|C]|$9|1Y|A|1Z|B|C]]|D|@]|E|$]]|$1|L|3|M|5|6|7|20|8|@]|D|@$9|21|A|22|1|23]]|E|$]]|$1|N|3|-4|5|6|7|24|8|@]|D|@]|E|$]]]|O|$P|$5|Q|R|S|E|$T|U]]]]

According to the ISO/IEC 9899:2011 standard §7.24.2.4/c4 The <code>strncpy</code>: function

<blockquote>
 Synopsis

<pre><code>1 #include &lt;string.h&gt;
 char *strncpy(char * restrict s1,
 const char * restrict s2,
 size_t n);
</code></pre>
 
 Description 
 
 2 The <code>strncpy</code> function copies not more than <code>n</code> characters (characters that > follow a null character are not copied) from the array pointed to by <code>s2</code> to 
 the array pointed to by
 
 3 If the array pointed to by <code>s2</code> is a string that is shorter than n characters, > null characters are appended to the copy in the array pointed to by <code>s1</code>, until 
 n characters in all have been written.
 
 Returns
 
 4 The <code>strncpy</code> function returns the value of s1.
 <code>s1</code>. If copying takes place between objects that overlap, the behavior is 
 undefined.
</blockquote>

As such, the only way for <code>strncpy</code> to return <code>0</code> (i.e., <code>NULL</code>) with out causing undefined behaviour is if <code>s1</code> is <code>NULL</code> and <code>n</code> is zero.

<a href="http://coliru.stacked-crooked.com/a/5e6d46ffafab01f6" rel="nofollow">LIVE DEMO</a>

blocks|key|3101083|text|是的，strncpy()可以返回NULL。只需在将mmap定义为指针的系统上，将地址为零的可写页定义为以零作为其数值：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3101084|#include+<sys/mman.h>
#include+<stdio.h>
#include+<strings.h>

int+main(+int+argc,+char+**argv+)
{
++++char+*p;
++++char+*buf+=+mmap(+(+void+*+)+0,+4096,
++++++++PROT_READ+%7C+PROT_WRITE,+MAP_PRIVATE+%7C+MAP_FIXED+%7C+MAP_ANON,+-1,+0+);
++++p+=+strncpy(+buf,+"string",+strlen(+"string"+)+);
++++printf(+"p:+%25p\n",+p+);
++++printf(+"NULL:+%25p\n",+NULL+);
++++return(+0+);
}|code-block|syntax|javascript|3101085|是的，这故意违反了C标准的要求，即NULL不能指向有效的对象，因此它是未定义的行为。|3101086|但可能会发生。|BOLD|3101087|entityMap^0|3|9|G|4|P|4|0|0|H|4|0|1|5|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Y|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Z|8|@$9|10|A|11|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|12|8|@$9|13|A|14|B|O]]|D|@]|E|$]]|$1|P|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|Q|$]]

Yes <code>strncpy()</code> can return <code>NULL</code>. Just <code>mmap</code> a writeable page at address zero on a system that defines <code>NULL</code> as a pointer with zero as its numeric value:

<pre><code>#include &lt;sys/mman.h&gt;
#include &lt;stdio.h&gt;
#include &lt;strings.h&gt;

int main( int argc, char **argv )
{
 char *p;
 char *buf = mmap( ( void * ) 0, 4096,
 PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_FIXED | MAP_ANON, -1, 0 );
 p = strncpy( buf, "string", strlen( "string" ) );
 printf( "p: %p\n", p );
 printf( "NULL: %p\n", NULL );
 return( 0 );
}
</code></pre>

Yes, this is intentionally violating the C standard requirement that <code>NULL</code> must not point to a valid object, so it's undefined behavior.

But it CAN happen.

The man page of <code>strncpy</code> states:
<blockquote>
char *strncpy(char *dest, const char *src, size_t n);
The strcpy() and strncpy() functions return a pointer to the destination string dest.
</blockquote>
Is it possible that <code>strncpy(buff, &quot;something&quot;, 9) == 0</code> be true if <code>char buff[100]</code>?
UPDATE
I also consider it as not very probable but it's part of a program that I should make to bufferoverflow and this condition stays on my way to achieve this.

Can strncpy return 0?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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strncpy的手册页声明：char *strncpy(char *dest，const *src，size_t n)；函数的作用是: strcpy()和strncpy()函数返回指向目标字符串dest的指针。如果strncpy(buff, "something", 9) == 0是char buff[100]，是否有可能是真的？更新我也认为这不太可能，但这是程序的一部分，我应该做缓冲区溢出，这个

问strncpy能返回0吗？
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