在Visual Studio中使用C语言，如何在未知字符串长度且不使用realloc或预先设置最大长度的情况下从用户处获取字符串

Hey there, let's work through this problem. Since we can't predefine a fixed-size array, can't use realloc, and have no clue how long the input will be, a linked-list based approach is the perfect workaround. Here's a step-by-step breakdown with code that works smoothly in Visual Studio:

Core Idea #

Instead of trying to guess the input length upfront, we'll store each incoming character in a linked list node first. Once the user finishes inputting, we'll count how many characters we've collected, allocate exactly enough memory for the final string, then copy all characters over and clean up the linked list.

Step-by-Step Implementation #

• Define a linked list node structure
  Each node holds a single character and a pointer to the next node, letting us build a chain of input characters without preallocating a big block of memory.

• Read characters one by one
  Use getchar() to loop through user input until we hit a newline (normal end of input) or EOF (triggered by Ctrl+Z in Visual Studio). Add each valid character to the end of the linked list.

• Calculate total length and allocate exact memory
  Traverse the linked list to count how many characters we have, then use malloc to allocate a block of memory sized length + 1 (the extra byte is for the null terminator \0).

• Copy linked list data to the new string
  Traverse the list again to copy each character into the newly allocated memory, then add the null terminator to make it a valid C string.

• Clean up allocated memory
  Don't forget to free all linked list nodes and the final string to avoid memory leaks.

Full Code Example #

#include <stdio.h>
#include <stdlib.h>

// Linked list node to store individual characters
typedef struct Node {
    char c;
    struct Node* next;
} Node;

int main() {
    Node* head = NULL;
    Node* tail = NULL;
    char input_char;
    int char_count = 0;
    char* final_string = NULL;

    printf("Enter your string (press Enter to finish, Ctrl+Z to cancel):\n");

    // Read characters and build the linked list
    while ((input_char = getchar()) != '\n' && input_char != EOF) {
        Node* new_node = (Node*)malloc(sizeof(Node));
        if (!new_node) {
            fprintf(stderr, "Failed to allocate memory for node!\n");
            // Clean up existing nodes before exiting
            Node* temp;
            while (head) {
                temp = head;
                head = head->next;
                free(temp);
            }
            return 1;
        }
        new_node->c = input_char;
        new_node->next = NULL;

        if (!head) {
            head = new_node;
            tail = new_node;
        } else {
            tail->next = new_node;
            tail = new_node;
        }
        char_count++;
    }

    // Handle case where no input was entered
    if (char_count == 0) {
        printf("No characters were entered.\n");
    } else {
        // Allocate exact memory for the final string
        final_string = (char*)malloc(char_count + 1);
        if (!final_string) {
            fprintf(stderr, "Failed to allocate memory for string!\n");
            // Clean up linked list
            Node* temp;
            while (head) {
                temp = head;
                head = head->next;
                free(temp);
            }
            return 1;
        }

        // Copy characters from linked list to final string
        Node* current = head;
        int index = 0;
        while (current) {
            final_string[index++] = current->c;
            current = current->next;
        }
        final_string[char_count] = '\0'; // Add null terminator

        // Output the result
        printf("Your input string: %s\n", final_string);
    }

    // Clean up linked list memory
    Node* temp;
    while (head) {
        temp = head;
        head = head->next;
        free(temp);
    }

    // Clean up final string memory
    if (final_string) {
        free(final_string);
    }

    return 0;
}

Notes for Visual Studio Users #

• To trigger EOF during input, press Ctrl+Z (you may need to press Enter after it for the program to recognize it).
• Always check if malloc returns a null pointer—this prevents crashes if the system runs out of memory.
• Memory cleanup is crucial here—leaving linked list nodes or the final string allocated can cause issues in long-running programs.

内容的提问来源于stack exchange，提问作者Robi