C语言运行时获取用户输入字符串长度的问题求助

Hey there! Let's break down the issues in your code and get it working properly—you're close, but there are a few critical fixes needed first.

First: Fix the Uninitialized Input Pointer #

Your char *input; is just a pointer that doesn't point to any valid memory yet. When you try to use scanf with it, you'll get a crash (undefined behavior) because there's no space allocated to store the user's input.

Two simple fixes here:

• Use a fixed-size character array (great for beginners): char input[256]; (pick a size that makes sense for your use case)
• Or dynamically allocate memory (more flexible, but requires cleanup): char *input = malloc(256 * sizeof(char)); (don't forget to free(input); later!)

Second: Fix the scanf Call #

Your scanf("%s\n", input); has an extra \n in the format string—this will cause scanf to wait for additional input after the string, which isn't what you want. Use scanf("%s", input); instead. Note that %s stops reading at the first whitespace character; if you need to read strings with spaces, use fgets(input, sizeof(input), stdin); instead (just remember to trim the trailing newline that fgets adds).

Third: Calculate the String Length #

To get the length of the input string at runtime, use the strlen() function from the <string.h> standard library. Just add #include <string.h> at the top of your code, then set:

int n = strlen(input);

strlen() returns the number of characters in the string excluding the null terminator (\0), which is exactly what you need for your loop counts.

Thread Handling Notes #

Since you're creating and joining threads, you'll need:

• To include the <pthread.h> header
• An array to store thread IDs (since you can't track n threads without it): pthread_t threads[n];
• To compile with the -lpthread flag (e.g., gcc your_code.c -o your_program -lpthread)

Full Modified Code Example #

#include <stdio.h>
#include <string.h>
#include <pthread.h>

// Use a fixed-size array for simplicity
char input[256];

void *thread_function(void *arg) {
    // Example thread logic—replace with your code
    int index = *(int *)arg;
    printf("Thread %d handling character: %c\n", index, input[index]);
    return NULL;
}

int main(){
    printf("Enter the string : ");
    // Use scanf for whitespace-separated strings, or fgets for strings with spaces
    scanf("%s", input);
    
    // Get the string length
    int n = strlen(input);
    
    // Array to hold thread IDs
    pthread_t threads[n];
    int thread_args[n]; // To pass indices to threads safely

    // Create threads
    for (int i=0; i<n; i++){
        thread_args[i] = i;
        if (pthread_create(&threads[i], NULL, thread_function, &thread_args[i]) != 0) {
            perror("Failed to create thread");
            return 1;
        }
    }

    // Join threads
    for (int i=0; i<n; i++){
        if (pthread_join(threads[i], NULL) != 0) {
            perror("Failed to join thread");
            return 1;
        }
    }

    return 0;
}

A few extra notes on the thread code:

• We pass thread indices via a separate array (thread_args) because passing &i directly would cause all threads to see the same changing value of i.
• Added error checking for pthread_create and pthread_join to catch issues early.

内容的提问来源于stack exchange，提问作者Latey Laley