是的snprintf（）总是空终止？ [英] Is snprintf() ALWAYS null terminating?

问题描述

总是snprintf的空终止目标缓冲区？

Is snprintf always null terminating the destination buffer?

在换句话说，这是足够了：

In other words, is this sufficient:

char dst[10];

snprintf(dst, sizeof (dst), "blah %s", somestr);

或做你必须做的就是这样，如果somestr足够长？

or do you have to do like this, if somestr is long enough?

char dst[10];

somestr[sizeof (dst) - 1] = '\0';
snprintf(dst, sizeof (dst) - 1, "blah %s", somestr);

我无论在什么标准说什么一些流行的libc可能会做这是不规范的行为感兴趣。

I am interested both in what the standard says and what some popular libc might do which is not standard behavior.

推荐答案

至于其他的答案建立：它的应该的：

As the other answers establish: It should:

的snprintf ......结果写入字符串缓冲区。 （......）
  将与空字符终止，除非buf_size为零。

snprintf ... Writes the results to a character string buffer. (...) will be terminated with a null character, unless buf_size is zero.

所以你要照顾的是，你没有一个零大小的缓冲区传递给它，因为（显然）就不能写一个零无门。

So all you have to take care is that you don't pass an zero-size buffer to it, because (obviously) it cannot write a zero to "nowhere".

然而，提防，微软的库<罢工>的没有的调用的函数的snprintf 而是历史的只有的已调用的函数 _snprintf （注意前导下划线），它的不追加的终止空。这里的文档（VS 2012，VS〜2013）：

However, beware that Microsoft's library does not have a function called snprintf but instead historically only had a function called _snprintf (note leading underscore) which does not append a terminating null. Here's the docs (VS 2012, ~~ VS 2013):

http://msdn.microsoft。 COM / EN-US /库/ 2ts7cx93％28V = vs.110％29.aspx

让LEN是格式化的数据串的长度（不包括
  终止空）。 LEN和数量都在为_snprintf字节宽
  字符_snwprintf。

Return Value

Let len be the length of the formatted data string (not including the terminating null). len and count are in bytes for _snprintf, wide characters for _snwprintf.

  

• 如果LEN＆LT;算，然后len个字符被存储在缓冲区中，一个
    空终止符附加，并返回LEN。

• If len < count, then len characters are stored in buffer, a null-terminator is appended, and len is returned.

如果len =计数，然后len个字符存储在缓冲区中，否
  空终止符附加，并返回LEN。

If len = count, then len characters are stored in buffer, no null-terminator is appended, and len is returned.

如果LEN>计数，然后计算字符存储在缓冲区中，否
  空终止符附加，并返回一个负值。

If len > count, then count characters are stored in buffer, no null-terminator is appended, and a negative value is returned.

（...）

的Visual Studio 2015年（ VC14）显然是推出了符合的snprintf 的功能，但是传统的一个具有领先的下划线和的不的空终止的行为仍然存在：

Visual Studio 2015 (VC14) apparently introduced the conforming snprintf function, but the legacy one with the leading underscore and the non null-terminating behavior is still there:

在 的snprintf 截断功能时的输出len为更大
  大于或等于算，通过放置一个空终止
  缓冲[计数1] 。 （...）

有关的所有功能的其他的比的snprintf ，如果len =计数，LEN
  字符存储在缓冲区的没有空终止附加的，
  （...）

For all functions other than snprintf, if len = count, len characters are stored in buffer, no null-terminator is appended, (...)

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