为什么arr[56]=“ ”；里面加了空格或者其他字符就能打印，而不加空格就无法打印出来。

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

char arr[56]=" ";
sem_t sem,sem1;
int i=0;

void *Fun(void *arg)
{
	while(i<=56)
	{
		sem_wait(&sem1);
		i++;
		arr[i]='b';
		sem_post(&sem);
	}
	return NULL;
}

void *Fun1(void *arg)
{
	while(i<=56)
	{
		sem_wait(&sem);
		i++;
		arr[i]='a';
		sem_post(&sem1);
	}
	return NULL;
}

int main(int argc, const char *argv[])
{
	int ret=0;

	pthread_t pid,pid1;

//	memset(arr,0,56);

	ret=sem_init(&sem,0,1);
	if(ret!=0)
	{
		printf("sem_init is fail\n");
		return -1;
	}
	ret=sem_init(&sem1,0,0);
	if(ret!=0)
	{
		printf("sem_init is fail\n");
		return -1;
	}

	ret=pthread_create(&pid,NULL,Fun,NULL);
	if(ret!=0)
	{
		printf("Fun create is failed\n");
		return -1;
	}
	ret=pthread_create(&pid1,NULL,Fun1,NULL);
	if(ret!=0)
	{
		printf("Fun1 create is failed\n");
		return -1;
	}

	pthread_join(pid,NULL);
	pthread_join(pid1,NULL);

	printf("%s\n",arr);

	return 0;
}

stu@stu-VirtualBox:~/day5$ ./a.out
 ababababababababababababababababababababababababababababab
stu@stu-VirtualBox:~/day5$ 

为什么arr[56]=“ ”；里面加了空格或者其他字符就能打印，而不加空格就无法打印出来。