C字符串指针函数strdel

Question

有人可以解释一下为什么我会得到"分段错误......"以及如何在这段代码上修复它？

#include<stdio.h>

int str_length(char *s) {
    int length = 0, i;
    for(i = 0; *s; i++) {
        s++;
    }
    return i;
}

char *strdel(char *s, int pos, int n) {
    int i;
    char *p, str[] = "";
    p = str;
    for(i = 0; i < str_length(s) - n + 1; i++)  {
        if(i >= pos) {
            *(p + i) = *(s + i + n);
        }
        else {
            *(p + i) = *(s + i);
        }
    }
    s = str;
    return s;
}

int main() {
    char *str = "abcdef";
    printf("str_lengh: %d\n", str_length(str));
    printf("strdel: %s\n", strdel(str, 1, 2));
    return 0;
}

我得到这个输出:

str_lengh: 6
strdel: adef
Segmentation fault (core dumped)

另外,有没有更好的方法来创建一个函数:char*strdel(char*s,int pos,int n); 删除位置pos的n个字符比我做的那个？

Answer 1

我想你在这里写的都是...

char *strdel(char *s, int pos, int n) {
    int i;
    char *p, str[] = "";
    p = str; // p points to str which is "" and is on the stack with length 0.
    for(i = 0; i < str_length(s) - n + 1; i++)  {
        if(i >= pos) {
            *(p + i) = *(s + i + n); // now you are writing onto the stack past p
        }
        else {
            *(p + i) = *(s + i);// now you are writing onto the stack past p
        }
    }
    s = str; // now s points to space on stack
    return s; // now you return a pointer to the stack which is about to disapear 
}

无论何时写入过去的p,通常都会遇到未定义的行为.UB您正在写入尚未在堆上或堆栈上分配的空间.

您可以编写仅适用于s的strdel版本.如果我理解正确的strdel,那就是这样:(粗略地,未经测试!,需要检查pos和n的边界)

char *strdel(char *s, int pos, int n) {
    char *dst = s + pos, *src = s + pos + n;
    while(*src) {
        *dst++ = *src++;
    }
    *dst = 0;
    return s;
}