当我将字符串文字分配给非常量指针时，为什么我的 C 编译器没有发出警告？

Question

以下代码可以很好地编译，例如 Xcode 11.3.1 中的默认设置：

#include <stdio.h>

int main(int argc, const char * argv[]) {
    char* thing = "123";
    thing[2] = '4';
    printf("%s\n", thing);
    return 0;
}

但是，在运行时，代码会在 thing[2] = '4' 上陷入 EXC_BAD_ACCESS。我假设这是因为代表 "123" 的字节的内存被编译到我的程序的二进制文件中的某个地方，在现代 processor/OS 上被标记为 code rather than data. (This answer 确认 - 更不用说有一个 leaq 0x4d(%rip), %rsi ; "123" 反汇编中的行，将指针传递给相对于 指令指针 !)

的地址

自修改代码时代以来，C 允许这样做只是历史产物吗？我注意到我也可以分配 void* x = main; 而不会抱怨我正在丢弃修饰符。

This answer 说：

According to the C99 rationale, there were people in the committee who wanted string literals to be modifiable, so the standard does not explicitly forbid it.

有没有我可以阅读的进一步讨论？更实际的是，有没有办法告诉 clang and/or gcc 标记此类分配（即使它们实际上并未被禁止） 并发出警告, 不编译为 C++?

Answer 1

与 C++ 相反在 C 中，字符串文字具有非常量字符数组的类型。

然而，根据 C 标准，任何修改字符串文字的尝试都会导致未定义的行为。

从历史上看，C 语言没有限定符 const。限定符 const 最早出现在 C++ 中。因此，为了向后兼容，C 中的字符串文字具有非常量字符数组的类型。

Answer 2

你有 -Wwrite-strings:

When compiling C, give string constants the type const char[length] so that copying the address of one into a non-const char * pointer produces a warning. These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make -Wall request these warnings.

https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Warning-Options.html

Answer 3

你引用的答案是没有引用的观点，坦率地说是胡说八道。在现代编译器中保持可编译性无非就是不破坏大量现有的遗留 C 代码。

然而，如果您设置了必要的警告级别或选项，许多编译器将发出警告。在 GCC 例如：

-Wwrite-strings

When compiling C, give string constants the type const char[length] so that copying the address of one into a non-const char* pointer produces a warning. These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make -Wall request these warnings.

When compiling C++, warn about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.

CLANG 也有 -Wwrite-strings，其中是 -Wwriteable-strings

的同义词

-Wwritable-strings

This diagnostic is enabled by default.

Also controls -Wdeprecated-writable-strings.

Diagnostic text:
warning: ISO C++11 does not allow conversion from string literal to A

C 编译的诊断文本不同 - 我只是引用手册。

在 -Wwrite-strings 的 GCC 中：

int main()
{
    char* x = "hello" ;
    return 0;
}

产生：

main.c:3:15: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]

CLANG 产生：

source_file.c:3:15: warning: initializing 'char *' with an expression of type 'const char [6]' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]

当我将字符串文字分配给非常量指针时，为什么我的 C 编译器没有发出警告？

Why doesn't my C compiler warn when I assign a string literal to a non-const pointer?

c

compiler-warnings

const-correctness