为什么变量在块结束后丢失其值？

Question

我编写了一个代码来测试子字符串操作。

测试.c

#include "compiler_expression.h"
#include <stdio.h>

int main()
{
    char *s1 = "(Hello) World";
    int l = 0;
    printf("%s\n", substr_limits(s1, '(', ')', &l));
    return 0;
}

编译器表达式.h

#include <string.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
// strcpy(s1, s2) Copies s2 into s1
// strcat(s1, s2) Concatenates s2 onto the end of s1
// strlen(s1) Returns the length of s1
// strcmp(s1, s2) Returns 0 if s1 and s2 are the same; less than 0 if s1<s2; greater 
// than 0 if s1>s2
// strchr(s1, ch) Returns a pointer to the first occurrence of ch in s1
// strstr(s1, s2) Returns a pointer to the first occurrence of s2 in s1

char * substr(char *str, int begin, int end)
{
    if (begin<0 || end>strlen(str))
    {
        printf("\nError: arguments (begin or end) is out of index\n");
        exit(1);
    }
    char *res = malloc(end-begin);
    for (int i=begin; i<end; i++)
    {
        res[i-begin] = str[i];
    }
    return res;
}
char *substr_limits(char *str, char begin_char, char end_char, int *no_limit)
{
    *no_limit = 1;
    if (begin_char==end_char)
    {
        printf("%s\n", "::begin char is equal to end char");
        int begin_limit = 0;
        int end_limit = 0;
        int limit_num = 0;
        for (int i=0;i<strlen(str);i++)
        {
            printf("%s\n", "::begin first loop");
            if (str[i]!=begin_char) continue;
            printf("%s\n", "::limit char found");
            if (limit_num==1)
            {
                printf("%s\n", "::limit_num==1");
                end_limit = i;
                char *res = malloc(end_limit-begin_limit-1);
                for (int j=begin_limit+1;j<end_limit;j++)
                {
                    printf("%s%c\n", "::char==", str[j]);
                    res[j-begin_limit] = str[j];
                    printf("%s%c\n", "::res_char==", res[j-begin_limit]);
                }
                printf("%s%s\n", "::result==", res);
                return res;
            }
            *no_limit = 0;
            limit_num = 1;
            begin_limit = i;
        }
        if (limit_num!=0) {
            printf("\nError: The limits are not justified\n");
            exit(1);
        }
        if (*no_limit==1) {
            return str;
        }
    }
    else
    {
        int begin_limit = 0;
        int end_limit = 0;
        int limit_num = 0;
        for (int i=0;i<strlen(str);i++)
        {
            if (str[i]!=begin_char&&str[i]!=end_char) continue;
            *no_limit = 0;
            if (str[i]==begin_char)
            {
                limit_num += 1;
                if (limit_num==1) begin_limit = i;
            }
            else if (str[i]==end_char)
            {
                limit_num -= 1;
                if (limit_num==0)
                {
                    end_limit = i;
                    char *res = malloc(end_limit-begin_limit);
                    for (i=begin_limit+1;i<end_limit;i++)
                    {
                        res[i-begin_limit] = str[i];
                    }
                    return res;
                }
            }
        }
        if (limit_num!=0) {
            printf("\nError: The limits are not justified\n");
            exit(1);
        }
        if (*no_limit==1) {
            return str;
        }
    }
}

在函数 (substr_limits) 处，有一个问题：当条件 (begin_char==end_char) 为真并且执行完所有代码后，我注意到在执行之前的最后一次操作 (return res;) 代码以正确的方式运行并将所有指定的字符从 (str) 获取到 (res) 变量中，但是在结束 (for 循环) 块之后，我注意到 (res) 的值是空字符串，这使得返回的值也是空字符串，我正在尝试通过编写此代码来解决问题：

            char *res = malloc(end_limit-begin_limit-1);
            for (int j=begin_limit+1;j<end_limit;j++)
            {
                printf("%s%c\n", "::char==", str[j]);
                res[j-begin_limit] = str[j];
                printf("%s%c\n", "::res_char==", res[j-begin_limit]);
                if (j==end_limit-1) {
                    printf("%s%s\n", "::result==", res);
                    return res;
                }
            }

但是问题依然存在！

Answer 1

错误 #1。 取而代之的是：

char *res = malloc(end-begin);
for (int i=begin; i<end; i++)
{
    res[i-begin] = str[i];
}

这个：

char *res = malloc(end-begin+1);  //+1 for null char
for (int i=begin; i<end; i++)
{
    res[i-begin] = str[i];
}
res[end-begin] = '\0'; // null terminate the string that gets returned

我没有看到substr被调用，但我看到此字符串复制模式的其他变体在您的限制函数中缺少空字符。

当我继续查看代码时，还会有更多内容......