vsprintf和vsnprintf [-Wformat-nonliteral]警告

问题描述

我有这块代码

static void err_doit(int errnoflag, int level, const char *fmt, va_list ap)
{
    int     errno_save;
    unsigned long n;
char    buf[MAXLINE];
    errno_save = errno;    
    #ifdef  HAVE_VSNPRINTF
vsnprintf(buf, sizeof(buf), fmt, ap);   /* this is safe */
    #else
vsprintf(buf ,fmt, ap);         /* this is not safe */
    #endif
n = strlen(buf);
if (errnoflag)
    snprintf(buf + n, sizeof(buf) - n, ": %s", strerror(errno_save));
strcat(buf, "\n");

if (daemon_proc) {
    syslog(level,"%s", buf);
} else {
    fflush(stdout);     
    fputs(buf, stderr);
    fflush(stderr);
}
return;
}

当我编译它（Clang 5.0.0 with -Weverything）警告：

when i compile it (Clang 5.0.0 with -Weverything) i obtain those warnings:

Building C object lib/netutils/CMakeFiles/netutils.dir/error.c.o
/Users/User/Desktop/project.cmake/lib/netutils/error.c:98:16: warning: format string is        not a string literal [-Wformat-nonliteral]
    vsprintf(buf ,fmt, ap);                 /* this is not safe */
                  ^~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include/secure/_stdio.h:66:57: note: expanded from
  macro 'vsprintf'__builtin___vsprintf_chk (str, 0, __darwin_obsz(str), format, ap)
                                                    ^

同样的事情发生在这个其他函数vsnprintf（buf，sizeof（buf），fmt，ap）;

the same thing happens with this other function vsnprintf(buf, sizeof(buf), fmt, ap);

如何解决此警告？

感谢

推荐答案

显然，解决方案是告诉Clang vsnprintf 在实现 printf 函数的行为的函数内调用， $ c> vsnprintf 。您可以使用属性）执行此操作：

Apparently the solution is to tell Clang that your vsnprintf is called within a function that implements the behaviour of the printf family of functions by, well, calling vsnprintf. You do this with an attribute, as described here:

__attribute__((__format__ (__printf__, 3, 0)))
static void err_doit(int errnoflag, int level, const char *fmt, va_list ap)
{
    ...
}

格式字符串是否为调用函数上的文字。因为 err_doit 已经有一个 va_list ，你应该指定调用它的函数的格式。第二个数字，这里是0，应该是可变参数 ... 的参数索引。请参见。

This checks whether the format string is a literal on the calling function. As err_doit takes already a va_list, you should specify the format on the functions that call it, too. The second number, which is 0 here, should then be the argument index of the variadic argument, .... See also this discussion.

函数属性是gcc的非标准扩展，也是为Clang实现的。如果你想保持代码编译器的独立性，你应该将属性包装在一个宏中，隐藏它为不知道属性的编译器。

Function attributes are a non-standard extension of gcc, which is also implemented for Clang. If you want to keep the code compiler independent, you should wrap the attributes in a macro that hides it for compilers that don't know attributes.

（免责声明：I couldn 'cl检查这个，但它适用于gcc。编辑：修正错误参数索引在例子中）

(Disclaimer: I couldn't check this with Clang, but it works for gcc. fixed wrong argument index in example)

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