将互斥锁属性的类型设置为PTHREAD_MUTEX_RECURSIVE

嵌套锁，允许同一个线程对同一个锁成功获得多次，并通过多次unlock解锁。如果是不同线程请求，则在加锁线程解锁时重新竞争。

#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
pthread_mutex_t mutex;
pthread_mutexattr_t attr;
void* func(void* arg){
    if(*(char*)arg == '\0') return NULL;
    pthread_mutex_lock(&mutex);
    char* str = (char*)arg;
    while(*str != '\0'){
        fputc(*str, stdout);
        str++;
    }
    fputc('\n', stdout);
    func((char*)arg+1);
    pthread_mutex_unlock(&mutex);
    return NULL;
}
int main(){
    int ret;
    if(( ret = pthread_mutexattr_init(&attr)) != 0){
        fprintf(stderr, "create mutex attribute error. msg:%s", strerror(ret));
        exit(1);
    }
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
    pthread_mutex_init(&mutex, &attr);
    pthread_t p1, p2;
    char str1[8], str2[8];
    sprintf(str1, "abcdefg");
    sprintf(str2, "1234567");
    if((ret = pthread_create(&p1, NULL, func, str1)) != 0){
        fprintf(stderr, "create thread error. msg:%s", strerror(ret));
        exit(1);
    }
    if((ret = pthread_create(&p2, NULL, func, str2)) != 0){
        fprintf(stderr, "create thread error. msg:%s", strerror(ret));
        exit(1);
    }
    pthread_join(p1, NULL);
    pthread_join(p2, NULL);
}