求助：使用malloc的C程序program.exe出现运行终止问题

Hey there, let's walk through exactly why your program is throwing that malloc-related crash and get it working properly. I see a few key issues here:

1. Insufficient Memory Allocation + Unsafe gets() Function #

You only allocated 1 byte of memory with malloc(sizeof(char)), but gets() reads the entire user-input string—no matter how long it is—right into that tiny buffer. This causes a buffer overflow, which corrupts the heap memory structure that malloc/free rely on. That's directly why you're seeing the "malloc has stopped working" error.

On top of that, gets() is a completely unsafe function (it's been deprecated since C11 for this exact reason) because it never checks the size of the buffer it's writing to.

2. Freeing the Wrong Memory Address #

In your while loop, you're doing *s++, which moves the pointer s forward through the string. By the time you call free(s), s is no longer pointing to the original address returned by malloc—you're trying to free a random memory location, which is an illegal operation and will crash your program.

Fixed Code with Explanations #

Here's a corrected version of your code, with comments explaining each change:

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

// Use standard int main() instead of void main() (complies with C standards)
int main() {
    // Allocate a sufficiently large buffer (1024 bytes) to hold user input
    char *s = (char*)malloc(1024 * sizeof(char));
    // Always check if malloc succeeded (it can fail if system is out of memory)
    if (s == NULL) {
        perror("Failed to allocate memory");
        return 1;
    }

    printf("Enter a string:\n");
    // Replace unsafe gets() with fgets()—it limits the number of bytes read
    fgets(s, 1024, stdin);

    // Remove the trailing newline character that fgets might capture
    size_t str_length = strlen(s);
    if (str_length > 0 && s[str_length - 1] == '\n') {
        s[str_length - 1] = '\0';
    }

    // Save the original malloc address so we can free it later
    char *original_ptr = s;

    while (*s) {
        // Use character constants ('A', 'Z') instead of raw ASCII codes for readability
        if (*s >= 'A' && *s <= 'Z')
            printf("%c", *s + 32);
        else if (*s >= 'a' && *s <= 'z')
            printf("%c", *s - 32);
        else
            printf("%c", *s);
        s++; // No need for *s++ here—we just need to move the pointer, not dereference it
    }
    printf("\n"); // Add a newline for clean output

    // Free the original memory address, not the modified pointer
    free(original_ptr);
    return 0;
}

Key Changes Made: #

• Standardized main(): Switched from void main() to int main() and added a return value—this is required by the C standard.
• Safe Memory Allocation: Allocated a 1024-byte buffer (adjust this if you need to handle longer strings) and added a check for malloc failure.
• Replaced gets(): Used fgets() to safely read input without buffer overflow risks.
• Preserved Original Pointer: Saved the initial malloc address to ensure we free the correct memory later.
• Improved Readability: Used character constants instead of raw ASCII values, making the code easier to understand.
• Cleaner Output: Added a final newline to make the transformed string look neater.

内容的提问来源于stack exchange，提问作者Hosein Shendabadi