blocks|key|2459280|text|这意味着它复制源字符串的终止空，但如果源字符串不适合目标，则不添加终止空。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2459281|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

That means that it copies the terminating null of the source string, but doesnt add a terminating null if the source string doesnt fit into the destination.

blocks|key|903463|text|字符串"foo"有3个字符%2B1个空终止符(以"foo\0"的形式存储)，总长度为4。如果使用n=3+(或更少)调用strncpy，它不会在目标字符串的末尾追加空终止符，而是只复制"foo"。由于缺少指示字符串结束的null终止符，试图打印结果字符串将导致未定义的行为。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|903464|您必须非常小心，或者传递大于最大源的n，或者自己添加空终止符。|903465|entityMap^0|3|5|M|7|1A|3|1L|7|2H|5|0|I|1|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$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|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|I|$]]

The string <code>"foo"</code> has 3 characters + 1 null-terminator (it's stored as <code>"foo\0"</code>) giving a total length of 4. If you call <code>strncpy</code> with <code>n=3</code> (or fewer) it won't append a null-terminator to the end of the target string, but will copy only <code>"foo"</code>. Attempting to print the resulting string will result in undefined behaviour due to the lack of a null-terminator which signals the end of the string.

You have to be very careful of this and either pass <code>n</code> one greater than the maximum source or add the null-terminator yourself.

blocks|key|2459310|text|这意味着，例如，如果源字符串是20个字符加上一个空终止符，并且strncpy指定的字符少于21个，那么目标字符串将不会追加一个空字符。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2459311|这是因为它的工作方式：strncpy保证它将精确地写入N个字节，其中N是传入的长度值。|2459312|如果源字符串(sans空字节)的长度小于该长度，它将使用空值填充目标区域。如果它等于或更大，则不会向目标添加空值。|2459313|这意味着它在技术上可能不是你得到的C字符串。这可以通过如下代码来解决：|2459314|char+d[11];++++++++++//+Have+enough+room+for+string+and+null.
strncpy+(d,+s,+10);++//+Copy+up+to+10+bytes+of+string,+null+the+rest.
d[10]+=+'\0';++++++++//+Then+append+null+manually+in+case+s+was+too+long.|code-block|syntax|javascript|2459315|分配11个字节(数组索引0..+10)，最多复制10+(索引0.9)，然后将第11字节(索引10)设置为null。|2459316|下面是一个图表，显示了用strncpy+(d,+s,+10)将各种大小的字符串写入10个字符区域的三种可能性，其中.表示空字节：|2459317|s++++++++++++++d
-------------++----------
Hello.+++++++++Hello.....
Hello+Fred.++++Hello+Fred
Hello+George.++Hello+Geor|2459318|请注意，在第二和第三种情况下，不会写入空字节，因此，如果将d视为字符串，则可能会对结果感到失望。|2459319|entityMap^0|V|7|0|B|7|0|0|0|0|0|C|I|1L|1|0|0|T|1|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|13|8|@$9|14|A|15|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|16|8|@]|D|@]|E|$]]|$1|J|3|K|5|6|7|17|8|@]|D|@]|E|$]]|$1|L|3|M|5|N|7|18|8|@]|D|@]|E|$O|P]]|$1|Q|3|R|5|6|7|19|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|1A|8|@$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]]|D|@]|E|$]]|$1|U|3|V|5|N|7|1F|8|@]|D|@]|E|$O|P]]|$1|W|3|X|5|6|7|1G|8|@$9|1H|A|1I|B|C]]|D|@]|E|$]]|$1|Y|3|-4|5|6|7|1J|8|@]|D|@]|E|$]]]|Z|$]]

It means that, for example, if your source string is 20 characters plus a null terminator and your <code>strncpy</code> specifies less than 21 characters, the target string will not have a null appended to it.

It's because of the way it works: <code>strncpy</code> guarantees that it will write exactly N bytes where N is the length value passed in.

If the length of the source string (sans null byte) is less than that, it will pad the destination area with nulls. If it's equal or greater, you won't get a null added to the destination.

That means it may not technically be a C string that you get. This can be solved with code like:

<pre><code>char d[11]; // Have enough room for string and null.
strncpy (d, s, 10); // Copy up to 10 bytes of string, null the rest.
d[10] = '\0'; // Then append null manually in case s was too long.
</code></pre>

You allocate 11 bytes (array indexes 0..10), copy up to 10 (indexes 0..9) then set the 11th (index 10) to null.

Here's a diagram showing the three possibilities for writing various-sized strings to a 10-character area with <code>strncpy (d, s, 10)</code> where <code>.</code> represents a null byte:

<pre><code>s d
------------- ----------
Hello. Hello.....
Hello Fred. Hello Fred
Hello George. Hello Geor
</code></pre>

Note that in the second and third case, no null byte is written so, if you treat <code>d</code> as a string, you're likely to be disappointed in the outcome.

blocks|key|2100258|text|这意味着只有num字节将从源缓冲区复制到目标缓冲区；因此，如果源字符串长度(如果大于或等于num+)，则终止空字节不会被复制，结果将不会有空终止字节，这是危险的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2100259|建议改用斯特拉比。|2100260|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Strlcpy^0|6|3|19|3|0|4|4|0|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@]|D|@$9|V|A|W|1|X]]|E|$]]|$1|H|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|I|$J|$5|K|L|M|E|$N|O]]]]

It means that only <code>num</code> bytes will be copied from the source buffer to the destination buffer; so if the source string length if upper or equal to <code>num</code>, the terminating NULL byte will not be copied, and the result will not have a NULL terminating byte, which is dangerous.

It is recommended to use <a href="http://en.wikipedia.org/wiki/Strlcpy" rel="nofollow">strlcpy</a> instead.

blocks|key|903551|text|在作为char数组存储的C字符串中，它们是以空结尾的，这意味着它们在末尾附加了一个额外的0，这标志着字符串的结束，以后可以用来计算字符串的长度。因此，字符串"hello"在内存中如下所示：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|903552|char+hello[]+=+{'h',+'e',+'l',+'l',+'o',+0};|code-block|syntax|javascript|903553|通常，当您复制一个字符串时，null字符也应该被复制。因此，字符串缓冲区所需的内存是它的长度%2B1(例如(strlen(hello)+%2B+1)+*+sizeof(char))。|903554|函数strncpy只允许在提供的缓冲区中复制尽可能多的字符。如果您提供的缓冲区不够大，无法容纳额外的null，则不会添加该缓冲区。或者，如果字符串被剪切，它将不会以空结尾。|903555|char+hello[]+=+"hello";+//+5+characters,+6+bytes+long
char+hel[3];
strncpy(hel,+hello,+3);+//+hel+is+{'h',+'e',+'l'}|903556|在调用strncpy之后，您应该始终小心，因为结果可能不是有效的C字符串。如果字符串不是空终止的，那么就不可能知道它的长度，并且大多数字符串操作函数都会失败，或者会做一些意想不到的事情。|903557|entityMap^0|3|4|18|1|26|7|0|0|E|4|1F|Y|0|2|7|1E|4|0|0|3|7|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|17|8|@$9|18|A|19|B|C]|$9|1A|A|1B|B|C]]|D|@]|E|$]]|$1|O|3|P|5|H|7|1C|8|@]|D|@]|E|$I|J]]|$1|Q|3|R|5|6|7|1D|8|@$9|1E|A|1F|B|C]]|D|@]|E|$]]|$1|S|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]]|T|$]]

In C strings stored as arrays of <code>char</code>'s and they are null-terminated, which means they have an extra <code>0</code> appended at the end, which marks the end of the string and could be used later to figure out the string's length. So the string <code>"hello"</code> looks like this in memory:

<pre><code>char hello[] = {'h', 'e', 'l', 'l', 'o', 0};
</code></pre>

Normally, when you copy a string, the <code>null</code> character should be copied as well. So the memory needed for the string buffer is its length + 1 (e.g. <code>(strlen(hello) + 1) * sizeof(char)</code>). 

Function <code>strncpy</code> allows you to copy only as many characters as it's possible to fit in the provided buffer. In case the buffer you provided is not big enough to hold that extra <code>null</code>, it won't be added. Or if the string is cut, it won't be null-terminated. 

<pre><code>char hello[] = "hello"; // 5 characters, 6 bytes long
char hel[3];
strncpy(hel, hello, 3); // hel is {'h', 'e', 'l'}
</code></pre>

You should be always careful after calling <code>strncpy</code>, since the result might not be a valid C string. If the string is not null-terminated it's impossible to know its length and most of the string manipulation functions would fail or would do something unexpected.

blocks|key|2459429|text|strncpy()的语义，即使在上面的C%2B%2B引用中进行了精确的解释，也被广泛误解了。该函数的行为违反直觉，容易出错。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2459430|为了避免在使用时或在开发过程中进一步出现问题，当维护人员会误读代码并添加更多的子代码错误时，有一个简单的解决方案：永远不会使用这个函数。|BOLD|2459431|您可以在布鲁斯·道森的这篇文章中了解有关此问题的更多详细信息。|2459432|要回答您的问题:如果源字符串大于作为第三个参数传递的大小(通常对应于目标缓冲区的大小)，则函数将将大小字符复制到目标，其中将不存在空字节。然后调用strlen(destination);将调用未定义的行为，因为它将尝试在数组结束后读取，直到找到空终止符为止。正是这种特定的行为使得strncpy如此容易出错。|2459433|entityMap|0|LINK|mutability|MUTABLE|url|https://randomascii.wordpress.com/2013/04/03/stop-using-strncpy-already/^0|0|9|0|1L|A|0|4|B|0|0|21|K|3W|7|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|H]]|D|@]|E|$]]|$1|I|3|J|5|6|7|10|8|@]|D|@$9|11|A|12|1|13]]|E|$]]|$1|K|3|L|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

The semantics of <code>strncpy()</code>, even when precisely explained as they are in the C++ reference above, are widely misunderstood. The behavior of this function is counterintuitive and error prone.

To avoid problems when using it or further down the development process, when the maintainer will misread the code and add more subtile bugs, there is a simple solution: NEVER EVER USE THIS FUNCTION.

You can read further details about this in <a href="https://randomascii.wordpress.com/2013/04/03/stop-using-strncpy-already/" rel="nofollow noreferrer">this article by Bruce Dawson</a>.

To answer your question: if the source string is longer than the size passed as a third argument (usually corresponding to the size of the destination buffer), the function will copy size characters to the destination and no null byte will be present among these. Calling <code>strlen(destination);</code> will then invoke undefined behavior because it will attempt to read beyond the end of the array until it finds a null terminator. This specific behavior is what makes <code>strncpy</code> so error prone.

At the following regarding <code>strncpy</code>: <a href="http://www.cplusplus.com/reference/clibrary/cstring/strncpy/" rel="noreferrer">http://www.cplusplus.com/reference/clibrary/cstring/strncpy/</a>, it mentions the following:

<blockquote>
 No null-character is implicitly appended to the end of destination, so destination will only be null-terminated if the length of the C string in source is less than num.
</blockquote>

What is meant by this sentence?

strncpy documentation question

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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在关于strncpy：的以下部分中，它提到了以下内容：没有空字符被隐式地追加到目的地的末尾，因此只有当源中的C字符串长度小于num时，才会以空终止。这句话是什么意思？

问strncpy文档问题
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