将“ int_least8_t”施放到`char中时如何发出警告？

Question

我正在构建一个弦库来支持ASCII和UTF8。
我为t_ascii和t_utf8创建两个Typedef。 ASCII可以安全地阅读为UTF8，但是UTF8不可能被读为ASCII。
当从t_utf8隐含地施放到T_ASCII时，我是否有任何方法可以发出警告，但是当隐含地施放t_ascii t_utf8 < t_utf8 /代码>？

理想情况下，我希望发出这些警告（并且只有这些警告）：

#include <stdint.h>

typedef char           t_ascii;
typedef uint_least8_t  t_utf8;

int main()
{
    t_ascii const* asciistr = "Hello world"; // Ok
    t_utf8 const*   utf8str = "你好世界";    // Ok

    asciistr = utf8str; // Warning: utf8 to ascii is not safe
    utf8str = asciistr; // Ok: ascii to utf8 is safe

    t_ascii asciichar = 'A';
    t_utf8   utf8char = 'B';

    asciichar = utf8char; // Warning: utf8 to ascii is not safe
    utf8char = asciichar; // Ok: ascii to utf8 is safe
}

当前，当使用-wall构建时（甚至使用-funsigned-char）进行构建时，我会收到这些警告：

gcc main.c -Wall -Wextra                          
main.c: In function ‘main’:
main.c:10:35: warning: pointer targets in initialization of ‘const t_utf8 *’ {aka ‘const unsigned char *’} from ‘char *’ differ in signedness [-Wpointer-sign]
   10 |         t_utf8 const*   utf8str = "你好世界";    // Ok
      |                                   ^~~~~~~~~~
main.c:12:18: warning: pointer targets in assignment from ‘const t_utf8 *’ {aka ‘const unsigned char *’} to ‘const t_ascii *’ {aka ‘const char *’} differ in signedness [-Wpointer-sign]
   12 |         asciistr = utf8str; // Warning: utf8 to ascii is not safe
      |                  ^
main.c:16:17: warning: pointer targets in assignment from ‘const t_ascii *’ {aka ‘const char *’} to ‘const t_utf8 *’ {aka ‘const unsigned char *’} differ in signedness [-Wpointer-sign]
   16 |         utf8str = asciistr; // Ok: ascii to utf8 is safe
      |                 ^

Answer 1

用-wall编译。始终使用-wall编译。

<user>@squall:~/src/p1$ gcc -Wall -c test2.c
test2.c: In function ‘main’:
test2.c:9:31: warning: pointer targets in initialization of ‘const t_utf8 *’ {aka ‘const signed char *’} from ‘char *’ differ in signedness [-Wpointer-sign]
    9 |     t_utf8  const*  utf8str = "你好世界";
      |                               ^~~~~~~~~~~~~~
test2.c:11:13: warning: pointer targets in assignment from ‘const t_ascii *’ {aka ‘const char *’} to ‘const t_utf8 *’ {aka ‘const signed char *’} differ in signedness [-Wpointer-sign]
   11 |     utf8str = asciistr; // Ok: ascii to utf8 is safe
      |             ^
test2.c:12:14: warning: pointer targets in assignment from ‘const t_utf8 *’ {aka ‘const signed char *’} to ‘const t_ascii *’ {aka ‘const char *’} differ in signedness [-Wpointer-sign]
   12 |     asciistr = utf8str; // Should issue warning: utf8 to ascii is not safe
      |              ^

您希望它可以安全地从t_ascii从t_utf8施放，但这根本不是。签名有所不同。

警告并不是关于有效UTF8有时不是有效的ASCII的事实 - 编译器对此一无所知。警告是关于标志的。

如果您想要无符号char，请使用-funsigned-char编译。但是随后都会发出任何警告。

（顺便说一句，如果您认为该类型int_least8_t将能够保留多键char/完整的UTF8 CodePoint编码 - 它不会。单个汇编单元中的UTF8_T 将具有完全相同的大小。）

Answer 2

只需用标准C编译器编译它即可。 建议初学者学习哪些编译器选项C？

结果：结果：

<source>: In function 'main':
<source>:9:31: error: pointer targets in initialization of 'const t_utf8 *' {aka 'const unsigned char *'} from 'char *' differ in signedness [-Wpointer-sign]
    9 |     t_utf8 const*   utf8str = "你好世界";    // Ok
      |                               ^~~~~~~~~~
<source>:11:14: error: pointer targets in assignment from 'const t_utf8 *' {aka 'const unsigned char *'} to 'const t_ascii *' {aka 'const char *'} differ in signedness [-Wpointer-sign]
   11 |     asciistr = utf8str; // Warning: utf8 to ascii is not safe
      |              ^
<source>:12:13: error: pointer targets in assignment from 'const t_ascii *' {aka 'const char *'} to 'const t_utf8 *' {aka 'const unsigned char *'} differ in signedness [-Wpointer-sign]
   12 |     utf8str = asciistr; // Ok: ascii to utf8 is safe
      |             ^

---

，但不是当隐含地将t_ascii施加到t_utf8？

时

不，您不能在标准C中拥有它，因为它是无效的指针转换。您可以用明确的演员表使编译器沉默，但是如果这样做，您正在调用不确定的行为。

---

除此之外，您可以使用c11 _generic找出哪种类型uint_least8_t归结为：

#include <stdint.h>
#include <stdio.h>

#define what_type(obj) printf("%s is same as %s\n", #obj, \
  _Generic ((obj),                                        \
            char: "char",                                 \
            unsigned char: "unsigned char",               \
            signed char: "signed char") );
  

int main (void)
{
    typedef char           t_ascii;
    typedef uint_least8_t  t_utf8;

    t_ascii ascii;
    t_utf8  utf8;

    what_type(ascii);
    what_type(utf8);
}

在GCC X86 Linux上输出：

ascii is same as char
utf8 is same as unsigned char