使用动态分配内存后所有值均相同的问题原因排查

Let's break down the root cause of this issue and how to fix it in a straightforward way:

The Core Problem: Pointer Assignment Instead of String Copying #

Looking at your code, here's the critical mistake that's causing duplicate outputs:

list.words[list.length - 1] = (char*)malloc(sizeof(char) * 30);
list.words[list.length - 1] = str;

You're doing two things here:

1. Allocating a new block of memory for each word (great start!)
2. Immediately overwriting that new pointer with the address of the local array str (this breaks everything)

Why This Causes Duplicate Outputs #

• str is a fixed-size array declared in main(). Every time you call fgets(str, 30, stdin), you're writing the new word into the exact same chunk of memory.
• When you assign list.words[...] = str, you're storing the address of str in every element of your words array. So every entry in list.words points to the same memory location.
• By the end of the loop, str contains the last word you entered. Since all pointers in list.words point to str, printing them all will show that last word repeatedly.

Bonus Issue: Memory Leak #

The malloc call for each word is wasted—you allocate memory but never use it (since you overwrite the pointer). This creates a memory leak because that allocated memory is never freed.

How to Fix the Code #

Step 1: Copy the String Instead of Assigning the Pointer #

Replace the pointer assignment with strcpy to copy the contents of str into the allocated memory. You'll need to include <string.h> for this function.

Step 2 (Optional): Trim the Newline Character #

fgets includes the newline character (\n) when the user presses Enter. To remove it, you can find the newline and replace it with a null terminator for cleaner output.

Fixed Code Example #

#include <stdio.h>
#include <stdlib.h>
#include <string.h> // Add this for strcpy

struct my_struct {
    char** words;
    int length;
};

int main(void) {
    struct my_struct list = {NULL, 0};
    int choice = 1, i = 0;
    char str[30];

    printf("Enter 0 to exit or any other number to continue: ");
    scanf("%d", &choice);

    while (choice) {
        list.length += 1;
        // Reallocate the words array (check for failure!)
        list.words = realloc(list.words, sizeof(char*) * list.length);
        if (list.words == NULL) {
            fprintf(stderr, "Memory allocation failed\n");
            return 1;
        }

        // Allocate memory for the new word (check for failure!)
        list.words[list.length - 1] = malloc(sizeof(char) * 30);
        if (list.words[list.length -1] == NULL) {
            fprintf(stderr, "Memory allocation failed\n");
            return 1;
        }

        printf("Enter a word: ");
        getchar(); // Clear the newline left by scanf
        fgets(str, 30, stdin);

        // Trim the newline character if present
        size_t len = strlen(str);
        if (len > 0 && str[len-1] == '\n') {
            str[len-1] = '\0';
        }

        // Copy the string into the allocated memory
        strcpy(list.words[list.length - 1], str);

        printf("Enter 0 to exit or any other number to continue: ");
        scanf("%d", &choice);
    }

    printf("Values:\n");
    for (i = 0; i < list.length; i++) {
        printf("%s\n", list.words[i]); // Clean up output with newlines
        free(list.words[i]); // Free each word's memory to avoid leaks
    }
    free(list.words); // Free the main words array

    getchar();
    getchar();
    return 0;
}

Key Fixes Explained #

• Used strcpy to copy the content of str into the allocated memory block for each word, so each entry points to its own unique data.
• Added checks for malloc/realloc failure to prevent crashes from out-of-memory scenarios.
• Trimmed the newline from str so your output doesn't have extra line breaks.
• Freed each individual word's memory before freeing the words array to eliminate memory leaks.

内容的提问来源于stack exchange，提问作者Omer