Linux write()：ssize_t write (int fd, const void * buf, size_t count)
有可能返回成功值，但实际并未成功写入的情况
因此，使用write()返回值作为文件写成功与否不完全正确，还需判断一下errno
open()
open("in.txt", O_RDONLY); // 阻塞模式 open("in.txt", O_RDONLY|O_NONBLOCK); // 非阻塞模式
O_NONBLOCK
When opening a FIFO with O_RDONLY or O_WRONLY set:
* If O_NONBLOCK is set, an open() for reading-only shall return without delay. An open() for writing-only shall return an error if no
process currently has the file open for reading.
* If O_NONBLOCK is clear, an open() for reading-only shall block the calling thread until a thread opens the file for writing. An open()
for writing-only shall block the calling thread until a thread opens the file for reading.