C语言函数接收双指针后传递给pthread线程函数的问题

Hey there, let's work through your problems step by step—this kind of pointer/thread confusion is super common when you're new to C, so you're not alone!

First: Fixing the "dereferencing pointer to incomplete type" Error #

This error happens because your process function doesn't have access to the full definition of struct mystruct. If you only declared struct mystruct; (a forward declaration) in the scope where process lives, the compiler knows it's a struct type but has no idea what's inside it—so it can't let you dereference pointers to it.

To fix this:

• Put the complete struct mystruct definition in a header file (like mystruct.h) that you include in every source file that uses the struct (both where testing is defined and where process is defined).
• Or, if everything is in one source file, make sure the struct definition comes before both testing and process functions.

Example of a complete struct definition:

// mystruct.h or top of your source file
typedef struct mystruct {
    // Add your struct members here (example fields)
    int thread_id;
    char message[50];
} mystruct;

Second: Fixing Thread Parameter Passing & Runtime Crashes #

Let's break down your pointer confusion here:
Your testing function takes a mystruct **str parameter. That means:

• str is a pointer to a pointer to a mystruct
• *str is a pointer to an actual mystruct instance (the one you want your threads to use)

When you tried passing &(*str) to pthread_create, that's equivalent to passing str itself (since &(*str) just gives you back the original pointer str). So you were passing a mystruct ** to the thread, but then trying to treat it as a different type in process—that's why you got runtime crashes.

Correct Approach: #

1. Pass the right pointer to the thread: If you want all threads to access the same mystruct instance, pass (void*)*str (the actual pointer to the struct) to pthread_create.
2. Convert back correctly in the thread function: Cast the void* argument back to mystruct * (not mystruct **).

Here's a corrected code example:

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

// Complete struct definition (visible to all functions)
typedef struct mystruct {
    int thread_id;
    char message[50];
} mystruct;

void *process(void *arg) {
    // Convert the void* back to mystruct* (matches what we passed)
    mystruct *value = (mystruct*)arg;
    
    // Now you can safely access struct members
    printf("Thread %d says: %s\n", value->thread_id, value->message);
    
    return NULL;
}

void testing(mystruct **str) {
    // First, make sure *str points to a valid mystruct instance (e.g., malloc'd)
    if (*str == NULL) {
        *str = malloc(sizeof(mystruct));
        if (*str == NULL) {
            perror("malloc failed");
            return;
        }
        // Initialize struct values
        (*str)->thread_id = 0;
        snprintf((*str)->message, sizeof((*str)->message), "Hello from shared struct");
    }

    pthread_t threads[5];
    for (int i = 0; i < 5; i++) {
        // Pass the pointer to the actual struct instance (*str)
        int ret = pthread_create(&threads[i], NULL, process, (void*)*str);
        if (ret != 0) {
            perror("Failed to create thread");
            // Cleanup if needed
            return;
        }
    }

    // Wait for all threads to finish (critical to avoid main exiting early)
    for (int i = 0; i < 5; i++) {
        pthread_join(threads[i], NULL);
    }

    // Don't forget to free the struct if you malloc'd it!
    free(*str);
    *str = NULL;
}

Why Your Previous Attempts Failed: #

• When you cast arg to mystruct ** in process, you were treating a mystruct * (or worse, a mystruct **) as a double pointer. Dereferencing that would try to read memory that's not a valid pointer, causing a crash.
• The line struct mystruct **value = *(mystruct *)arg was a type mismatch: you were trying to assign a mystruct instance (from dereferencing a wrong cast) to a mystruct ** pointer, which the compiler rejected once it couldn't see the full struct definition.

Key Takeaways #

1. Always ensure struct definitions are visible to any function that needs to access their members.
2. Match pointer types when passing to threads: What you pass as void* must match what you cast back to in the thread function.
3. Don't forget to join threads: If your main function exits before threads finish, they'll be terminated abruptly.

内容的提问来源于stack exchange，提问作者DunkRuZ