为什么这个小 C 程序会崩溃? [英] Why does this small C program crash?

问题描述

程序是:

#include <stdio.h>
#include <stdlib.h>
int main(void) {
    char *a="abc",*ptr;
    ptr=a;
    ptr++;
    *ptr='k';
    printf("%c",*ptr);
    return 0;
}

问题出在

*ptr='k';  

行，当我删除它时，程序正常工作.但我无法弄清楚原因.

line, when I remove it program works normally. But I can't figure out the reason.

推荐答案

问题是因为您试图将字符串文字 "abc" 更改为:

The problem is because you are trying to change the string literal "abc" with:

char *a="abc",*ptr;
ptr=a;                  // ptr points to the 'a'.
ptr++;                  // now it points to the 'b'.
*ptr='k';               // now you try to change the 'b' to a 'k'.

这是未定义的行为.该标准明确规定，您不得根据 C99 的 6.4.5 String literals 节更改字符串文字:

That's undefined behaviour. The standard explicitly states that you are not permitted to change string literals as per section 6.4.5 String literals of C99:

如果这些数组的元素具有适当的值，则不确定这些数组是否不同.如果程序试图修改这样的数组，则行为未定义.

It is unspecified whether these arrays are distinct provided their elements have the appropriate values. If the program attempts to modify such an array, the behavior is undefined.

如果你替换它会工作:

char *a="abc",*ptr;

与:

char a[]="abc",*ptr;

因为这会将字符串文字复制到可以安全修改的地方.

since that copies the string literal to a place that's safe to modify.

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