为什么os.read在python的第二次调用中返回空

Question

I try this

fd = os.open("myfd.txt",os.O_RDWR)


In [28]: os.read(fd,24)
Out[28]: 'my test is good\n'

In [29]: os.read(fd,24)
Out[29]: ''

why did it return empty during second call

also when print fd it returns 3 as filedescriptor , what is meant by number 3

Answer 1

Because at that point, the file pointer is positioned at the end of the file (due to the first read pulling out all the data). It looks like you need an os.lseek to reset the file pointer:

print os.read(fd,24)
os.lseek(fd,0,0)
print os.read(fd,24)

Note that normal file objects are typically much easier to work with if you can help it:

with open('filename') as fin:
    print fin.read(24)
    fin.seek(0)
    print fin.read(24)

Answer 2

When you made that first read call, the file pointer moved 24 bytes (or characters) ahead, so you probably hit the end of the file.

And 3 is just a descriptor, it doesn't have any meaning to anything else than the operating system. The reason it's 3 is because descriptors 0, 1, and 2 are already taken by default (0 = stdin, 1 = stdout, 2 = stderr)