如何在mingw-w64 gcc 7.1中无需警告地打印size_t？

Posted 2021-04-08

我在nuwen.net上使用minGW的mingw-w64（x64）分支。这是来自7.1版本的gcc：

gcc --version
gcc (GCC) 7.1.0

我正在编译这个程序：

#include <stdio.h>

int main(void)
{
    size_t a = 100;
    printf("a=%lu
",a);
    printf("a=%llu
",a);
    printf("a=%zu
",a);
    printf("a=%I64u
",a);
}

有警告和c11标准：

gcc -Wall -Wextra -Wpedantic -std=c11 test_size_t.c

我收到这些警告：

   test_size_t.c: In function 'main':
    test_size_t.c:6:14: warning: format '%lu' expects argument of type 'long unsigned int', but argument 2 has type 'size_t {aka long long unsigned int}' [-Wformat=]
      printf("a=%lu
",a);
                ~~^
                %I64u
    test_size_t.c:6:14: warning: format '%lu' expects argument of type 'long unsigned int', but argument 2 has type 'size_t {aka long long unsigned int}' [-Wformat=]
      printf("a=%lu
",a);
                ~~^
                %I64u
    test_size_t.c:7:14: warning: unknown conversion type character 'l' in format [-Wformat=]
      printf("a=%llu
",a);
                  ^
    test_size_t.c:7:9: warning: too many arguments for format [-Wformat-extra-args]
      printf("a=%llu
",a);
             ^~~~~~~~~~
    test_size_t.c:7:14: warning: unknown conversion type character 'l' in format [-Wformat=]
      printf("a=%llu
",a);
                  ^
    test_size_t.c:7:9: warning: too many arguments for format [-Wformat-extra-args]
      printf("a=%llu
",a);
             ^~~~~~~~~~
    test_size_t.c:8:13: warning: unknown conversion type character 'z' in format [-Wformat=]
      printf("a=%zu
",a);
                 ^
    test_size_t.c:8:9: warning: too many arguments for format [-Wformat-extra-args]
      printf("a=%zu
",a);
             ^~~~~~~~~
    test_size_t.c:8:13: warning: unknown conversion type character 'z' in format [-Wformat=]
      printf("a=%zu
",a);
                 ^
    test_size_t.c:8:9: warning: too many arguments for format [-Wformat-extra-args]
      printf("a=%zu
",a);
             ^~~~~~~~~
    test_size_t.c:9:9: warning: ISO C does not support the 'I64' ms_printf length modifier [-Wformat=]
      printf("a=%I64u
",a);
         ^~~~~~~~~~~
    test_size_t.c:9:9: warning: ISO C does not support the 'I64' ms_printf length modifier [-Wformat=]

我想在没有警告的情况下printf一个size_t但在这种情况下不知道正确的格式说明符。

答案

问题不是编译器而是C库。 MinGW使用微软的“Visual C Runtime”（msvcrt），它只符合c89，并且不支持z格式说明符。

以下是使用MinGW时可以安全打印size_t的方法：

#include <inttypes.h>
#include <stdio.h>

#ifdef _WIN32
#  ifdef _WIN64
#    define PRI_SIZET PRIu64
#  else
#    define PRI_SIZET PRIu32
#  endif
#else
#  define PRI_SIZET "zu"
#endif

int main(void)
{
    size_t mySize = 24;

    printf("%" PRI_SIZET "
", mySize);
}

在win64上，您将收到此代码的警告，因为PRIu64扩展为msvcrt特定的I64u格式说明符。但是你可以用GCC旗帜-Wno-pedantic-ms-format来平息这个警告。

---

请注意，你需要一个类似的技巧long long（这里使用32位和64位窗口上的PRIu64）因为msvcrt也不知道ll。

---

编辑：正如@ M.M在评论中所指出的，您可以将MinGW提供的替代stdio函数与#define __USE_MINGW_ANSI_STDIO 1相关联。如果我可以解决msvcrt的特点，我宁愿不链接额外的代码，但这当然是品味的问题。

另一答案

注释中提到的替代解决方案是在__USE_MINGW_ANSI_STDIO编译器开关中抛出：

#define __USE_MINGW_ANSI_STDIO 1

#include <stdio.h>

int main(void)
{
    size_t a = 100;
    printf("a=%lu
",a);
    printf("a=%llu
",a);
    printf("a=%zu
",a);
    printf("a=%I64u
",a);
}

这使代码按预期编译，gcc现在提供适当的警告：

warning: format '%lu' expects argument of type 'long unsigned int', but argument 2 has type 'size_t' [-Wformat=]  
warning: ISO C does not support the 'I' printf flag [-Wformat=]  
warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'size_t' [-Wformat=]