c - 是 snprintf(NULL,0,...);行为规范？

Question

在 Linux 上，它返回要打印的字符数。

这是标准化行为吗？

Answer 1

是的。

来自 7.21.6.5 snprintf 函数，N1570(C11 草案):

The snprintf function is equivalent to fprintf, except that the output is written into an array (specified by argument s) rather than to a stream. If n is zero, nothing is written, and s may be a null pointer. Otherwise, output characters beyond the n-1st are discarded rather than being written to the array, and a null character is written at the end of the characters actually written into the array. If copying takes place between objects that overlap, the behavior is undefined.

这是一种查找未知数据长度的有用方法，您可以先找到所需的长度，然后再分配准确的内存量。一个典型的用例是:

char *p;

int len = snprintf(0, 0, "%s %s some_long_string_here_", str1, str2);

p = malloc(len + 1);

snprintf(p, len + 1, "%s %s some_long_string_here", str1, str2);