c++ - 从哪个版本的 Visual Studio 开始，vsnprintf moSTLy 符合标准？

Question

根据Microsoft's documentation for vsnprintf ，该函数是 C(++) 运行时库的一部分，至少从 Visual Studio 2003 版开始。

int vsnprintf( char *buffer,        // Storage location for output
               size_t count,        // Maximum number of characters to write
               const char *format,  // Format specification
               va_list argptr )     // Pointer to list of other arguments

我在问:vsnprintf 是哪个版本的 Visual Studio x86 和 x64 的捆绑 C(++) RTL 实现符合 C99 标准 (ISO/IEC 9899:1999)，假设

• #define _CRT_SECURE_NO_WARNINGS在 #include <stdio.h> 之前执行，这是 Visual Studio RTL 的现代版本所必需的；
• 如果 count大于零，则 buffer是指向(至少)count 的指针可写字符；
• 格式不是 NULL并符合 Microsoft's Format Specification syntax适用于特定版本的 RTL；
• count 的值并且要生成的字符数都足够小以适合类型 int ;

并且我们希望一致性包括(除了标称输入的基本功能之外)这些要求(由标准规范 snprintf 暗示，vsnprintf 引用):

1. 在上述假设下不会产生未定义的行为(包括调用 Microsoft's invalid parameter handler)；
2. buffer==NULL 时返回要写入的长度(不包括终止空字符)和 count==0 ，从而允许飞行前确定输出的长度；
3. 在 buffer!=NULL 时，始终使用终止空字符填充输出字符串和 count>0并且返回的结果是非负的，包括由于小的 count 而截断的输出.

注意 comment : 我愿意承认缺少restrict限定符仍然在允许大部分符合标准的情况下。

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文档中关于 (3.) 的一致性模棱两可；据我所知，与 Visual Studio Community 2015 捆绑在一起的实现很好，但并非全部都是。

If there is room at the end (that is, if the number of characters to write is less than count), the buffer will be null-terminated.

文档中还有明确暗示 vsnprintf 的措辞。当 buffer==NULL 时 (1.) 和 (2.) 不符合 C99 标准和 count==0 ;但是文档的这些部分似乎是错误的:

if the number of characters to write is greater than count, these functions return -1 indicating that output has been truncated.

If buffer or format is NULL, or if count is less than or equal to zero, these functions invoke the invalid parameter handler

---

测试代码:

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdarg.h>

int f( char *buffer,
       size_t count,
       const char *format,
       ...
     )
{
    va_list vArgs;
    int vRes;
    va_start(vArgs, format);
    vRes = vsnprintf( buffer, count, format, vArgs);
    va_end(vArgs);
    return vRes;
}

int main(void)
{
    char vBuf[6];
    int j, count;
#ifdef _MSC_VER
    printf("_MSC_VER = %ld\n",(long)(_MSC_VER));
#else
    printf("_MSC_VER is undefined\n");
#endif
    printf("f(NULL,0,\"%%d\",777):%3d\n", f(NULL,0,"%d",777));
    for(count=0 ;count<=sizeof(vBuf); ++count)
    {
        for(j=0; j<sizeof(vBuf)-1; ++j)
            vBuf[j] = '!';
        vBuf[j] = 0;
        j =  f(vBuf,count,"%d",777);
        printf("f(vBuf,%d,\"%%d\",777):%3d  vBuf: \"%s\"\n",count,j,vBuf);
    }
    return 0;
}

在我安装的 Visual Studio Community 2015 下给予

_MSC_VER = 1900
f(NULL,0,"%d",777):  3
f(vBuf,0,"%d",777):  3  vBuf: "!!!!!"
f(vBuf,1,"%d",777):  3  vBuf: ""
f(vBuf,2,"%d",777):  3  vBuf: "7"
f(vBuf,3,"%d",777):  3  vBuf: "77"
f(vBuf,4,"%d",777):  3  vBuf: "777"
f(vBuf,5,"%d",777):  3  vBuf: "777"
f(vBuf,6,"%d",777):  3  vBuf: "777"

并且在 Visual Studio 2008 的某些安装下(我相信 SP1 + PSDK 7.1)

_MSC_VER = 1500
f(NULL,0,"%d",777):  3
f(vBuf,0,"%d",777): -1  vBuf: "!!!!!"
f(vBuf,1,"%d",777): -1  vBuf: "7!!!!"
f(vBuf,2,"%d",777): -1  vBuf: "77!!!"
f(vBuf,3,"%d",777):  3  vBuf: "777!!"
f(vBuf,4,"%d",777):  3  vBuf: "777"
f(vBuf,5,"%d",777):  3  vBuf: "777"
f(vBuf,6,"%d",777):  3  vBuf: "777"

请注意缺少终止空字符，特别是对于 count==3 ，即使输出是正数。

Answer 1

您现在提到的页面给出了答案:

Beginning with the UCRT in Visual Studio 2015 and Windows 10, vsnprintf is no longer identical to _vsnprintf. The vsnprintf function complies with the C99 standard; _vnsprintf is retained for backward compatibility with older Visual Studio code.

而且您的输出与 _vsnprintf 一致:

Both _vsnprintf and _vsnwprintf functions return the number of characters written if the number of characters to write is less than or equal to count; if the number of characters to write is greater than count, these functions return -1 indicating that output has been truncated.