I see the issue. It seems that `current_thread` is not properly initialized, which is leading to a segmentation fault in `mythread_exit`.

Let's fix the code. I'll provide a modified version with the necessary corrections.

```c
#define _GNU_SOURCE
#include "mythread.h"
#include <stdio.h>
#include <stdlib.h>
#include <sched.h>
#include <setjmp.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
#include <sys/mman.h>

#define MAX_THREADS 10 // Define the maximum number of threads
#define STACK_SIZE (1 << 20)  // 1MB stack size, adjust as needed

int NUM_ALIVE_THREADS = 1;
int THREAD_COMPLETED = 0;
int current_thread = 0;

mythread_t* threads[MAX_THREADS];  // Array to hold thread pointers

void* routine_wrapper(void *arg){
    mythread_t* t = (mythread_t *)arg;
    setjmp(t->env); // Save the thread's context

    t->res = t->start_routine(t->arg);
    t->completed = 1;
    mythread_exit();
    if (__sync_fetch_and_add(&THREAD_COMPLETED, 1) == NUM_ALIVE_THREADS - 1) {
        // All threads completed, resume the main thread
        __sync_synchronize();
        longjmp(t->env, 1); // Jump back to the main thread
    }
    else {
        pause(); // Suspend the current thread until all other threads complete
    }
}

void schedule() {
    int start_thread = current_thread;
    do {
        current_thread = (current_thread + 1) % MAX_THREADS;
        if (threads[current_thread] != NULL && threads[current_thread]->joined && threads[current_thread]->active && !threads[current_thread]->completed) {
            // Switch to this thread
            longjmp(threads[current_thread]->env, 1);
        }
    } while (current_thread != start_thread);
}

void mythread_yield() {
    threads[current_thread]->active = 0;  // Mark current thread as inactive
    schedule();  // Find the next thread to execute
    threads[current_thread]->active = 1;  // Mark next thread as active
}

int mythread_create(mythread_t *thread, void *(*start_routine)(void *), void *arg) {
    thread->arg = arg;
    thread->stack = mmap(NULL, STACK_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    if (thread->stack == MAP_FAILED) {
        perror("mmap");
        return -1;
    }
    thread->start_routine = start_routine;

    if (clone((int (*)(void *)) routine_wrapper, (char*)thread->stack + STACK_SIZE, CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND | CLONE_THREAD, thread) == -1) {
        perror("clone");
        return -1;
    }

    thread->joined = 1;
    thread->completed = 0;
    thread->active = 1; // Mark the thread as active
    threads[NUM_ALIVE_THREADS - 1] = thread; // Add the thread to the array
    return 0;
}

void mythread_detach(mythread_t *t) {
    t->joined = 0;
}

int mythread_join(mythread_t *thread, void **ret_val) {
    if (thread->joined == 0) {
        return -1; // Thread is not joinable
    }

    while (thread->completed == 0) {
        sched_yield(); // Suspend the current thread until the thread completes
    }

    if (ret_val != NULL) {
        *ret_val = thread->res;
    }

    munmap(thread->stack, STACK_SIZE); // Free the stack memory

    return 0;
}

void mythread_exit() {
    threads[current_thread]->active = 0;  // Mark current thread as inactive
    if (__sync_fetch_and_add(&NUM_ALIVE_THREADS, -1) != 1) {
        // There are other threads, switch to another thread
        mythread_yield();
    }
    else {
        exit(0);
    }
}
```

In this code, I have ensured that `current_thread` starts at 0, and I've corrected the placement of the thread in the `threads` array. Additionally, I've adjusted the initialization of `NUM_ALIVE_THREADS`. Please test this code and let me know if you encounter any further issues.