c++ - _vsnprintf 和 _vsnprintf_s 格式字符串中的自由百分比字符

Question

格式字符串中带有游离“%”的非常令人惊讶的行为。

#include <iostream>
#include <stdarg.h>
#include <stdio.h>

void foo(const char *format, ...) {
  const size_t buffSize = 1024;
  char msg[buffSize];
  {
    va_list args;
    va_start(args, format);
    _vsnprintf(msg, buffSize, format, args);
    va_end(args);
    std::cout << msg << std::endl; // here you get a,b,,c,d,e
  }
  {
    va_list args;
    va_start(args, format);
    _vsnprintf_s(msg, buffSize, format, args); // and here you crash and burn
    va_end(args);
    std::cout << msg << std::endl;
  }
}

int _tmain(int argc, _TCHAR *argv[]) {
  foo("%s,b,%,c,d,%s", "a", "e");
  return 0;
}

这不是您期望得到的，尤其是当出现以下 is stated by Microsoft 时

If a percent sign is followed by a character that has no meaning as a format field, the character is copied to the output unchanged.

那么我们在这里看到了什么？错误？从何时起？它是否已针对 msvc12/13 修复？

编辑01:好吧，我看错了声明，让我们专注于崩溃

#include <iostream>
#include <stdarg.h>
#include <stdio.h>

void foo(const char *format, ...) {
  const size_t buffSize = 1024;
  char msg[buffSize];
  va_list args;
  va_start(args, format);
  _vsnprintf_s(msg, buffSize, format, args);
  va_end(args);
  std::cout << msg << std::endl;
}

int main(int argc, char *argv[]) {
  foo("b,%,c,d"); // crash here
  foo("%s,b,%,c,d,%s", "a", "e"); // there and everywhere
  return 0;
}

EDIT02:
从 Microsoft Connect 得到了答案

Thanks for contacting Microsoft about this issue. We've found that we left out important information about how the secure functions behave from the Format Specification Syntax page. The secure functions do additional validation of the format string, and invoke the invalid parameter handler if an unexpected character follows the initial % character. By default, this terminates the program with a Dr. Watson report. In Debug mode, it causes an assert first. More information is available in the Parameter Validation topic at https://msdn.microsoft.com/en-us/library/ksazx244.aspx . The Format Specification Syntax page is being updated with this information. Expect to see the updated page for Visual Studio 2013 in the next week or so.

Answer 1

这实际上是一个文档错误。 Format Specification Syntax 中应该会很快出现更正(等一两天)。话题。缺少的信息是格式化函数的安全版本(带有尾随 _s 的函数)对格式化字符串进行额外验证，并且它们拒绝百分号字符后跟意外字符。然后，在这种情况下，它们会调用无效参数处理程序，而非安全版本则不会。默认情况下，这将终止程序并调用 Dr. Watson 报告机制。

如果您尝试处理任意格式字符串问题，您可以通过调用 _set_invalid_parameter_handler 自行设置无效参数处理程序。捕获这样的情况，并防止崩溃。在 DEBUG 构建中，此格式字符串应导致无效参数断言。 Parameter Validation Security Features in the CRT 下的主题有更多信息。