perl socket 프로그래밍 도중 기이한 현상 정리

RETURN VALUES
If successful, the number of bytes actually read is returned. Upon read-
ing end-of-file, zero is returned. Otherwise, a -1 is returned and the
global variable errno is set to indicate the error.

ERRORS
The read(), readv(), and pread() system calls will succeed unless:

[EBADF] The d argument is not a valid file or socket descrip-
tor open for reading.

[EFAULT] The buf argument points outside the allocated address
space.

[EIO] An I/O error occurred while reading from the file sys-
tem.

[EINTR] A read from a slow device was interrupted before any
data arrived by the delivery of a signal.

[EINVAL] The pointer associated with d was negative.

[EAGAIN] The file was marked for non-blocking I/O, and no data
were ready to be read.

[EISDIR] The file descriptor is associated with a directory
residing on a filesystem that does not allow regular
read operations on directories (e.g. NFS).

[EOPNOTSUPP] The file descriptor is associated with a filesystem
and file type that do not allow regular read opera-
tions on it.