c - 请解释这个结果。 printf ("%c", 'abcd' )

Question

#include <stdio.h>

int main()
{
    printf("%c\n", 'abcd');
    printf("%p\n", 'abcd');
    printf("%c\n", 0x61626364);
    printf("%c\n", 0x61626363);
    printf("%c\n", 0x61626365);
    return 0;
}

我想问这一行:printf("%c\n", 'abcd');
在这一行中，结果是 'd' 但是，我不明白为什么会出现 'd'。
我试图寻找其他内存。在这种情况下，我发现其他内存都是字母。
请解释为什么结果是'd'以及为什么其他内存都是字母。
谢谢。

Answer 1

'abcd' 是一个多字符常量，它的值是实现定义的。

C11 §6.4.4.4 Character constants section 10

An integer character constant has type int. The value of an integer character constant containing a single character that maps to a single-byte execution character is the numerical value of the representation of the mapped character interpreted as an integer. The value of an integer character constant containing more than one character (e.g., 'ab'), or containing a character or escape sequence that does not map to a single-byte execution character, is implementation-defined. If an integer character constant contains a single character or escape sequence, its value is the one that results when an object with type char whose value is that of the single character or escape sequence is converted to type int.

一个常见的实现给 'abcd' 一个值 'a' * 256 * 256 * 256 + 'b' * 256 * 256 + 'c' * 256 + 'd' (1633837924)，您可以通过使用 "%d" 打印它来检查其在您的实现中的值。虽然 C 合法，但在实践中很少使用。