为什么在以下代码中将结构传递给线程会导致分段错误？

Question

I've analyzed this multiple times over and cannot find what is causing it to segfault. Perhaps I'm just being dense, but I see no reason why this code shouldn't run. Might anyone be able to offer their insight?

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

typedef struct {
    int a;
    int b;
} struct1;

typedef struct {
    struct1 s1;
} struct2;

void* thread_activity(void* v)
{
    struct2 s2 = *((struct2*)v);
    printf("%d\n", s2.s1.a);
    return NULL;
}

int main(int argc, char* argv[])
{
    struct1 s1;
    s1.a = 10;
    s1.b = 20;

    struct2* s2;
    s2->s1 = s1;
    pthread_t tid;

    if(pthread_create(&tid, NULL, thread_activity, s2)==0) {
        printf("done\n");
    }
}

Answer 1

You didn't allocate memory for s2. Your program most likely crashed at s2->s1 = s1 before you even got to the pthread_create. Use a debugger such as gdb (Linux) or Visual Studio (Windows).

You say you "analyzed this multiple times over" ... whatever that consisted of, you should add to it checks that your pointers point to valid memory, that your functions are being called correctly, and that you're using your tools (eg, warning levels, debuggers) fully.

Answer 2

The other answers are correct that you have not allocated space for s2 . However, there is also a second bug - you don't wait for the second thread to finish before returning from main() . Returning from main() will deallocate any local function variables declared there, so if the other thread is accessing them, you can't do that until the other thread is done.

You need to do something like:

struct struct2 s2;
s2.s1 = s1;

pthread_t tid;

if (pthread_create(&tid, NULL, thread_activity, &s2) == 0)
{
    pthread_join(tid, NULL);
    printf("done\n");
}

Answer 3

struct2* s2;
s2->s1 = s1;

Undefined behavior! You are dereferencing an uninitialized pointer. Instead:

struct2 s2;
s2.s1 = s1;

Then pass it to pthread_create() as &s2 .

Answer 4

It's not a matter of multi-threading.

You need allocate memory for both s1 and s2 using the malloc function.