#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);
DESCRIPTION
read() attempts to read up to count bytes from file descriptor fd into the
buffer starting at buf.
If count is zero, read() returns zero and has no other results. If count is
greater than SSIZE_MAX, the result is unspecified.
RETURN VALUE
On success, the number of bytes read is returned (zero indicates end of file；0表示到了文件的末尾),
and the file position is advanced by this number. It is not an error if this
number is smaller than the number of bytes requested; this may happen for exam-
ple because fewer bytes are actually available right now (maybe because we were
close to end-of-file, or because we are reading from a pipe, or from a termi-
nal), or because read() was interrupted by a signal. On error, -1 is returned,
and errno is set appropriately. In this case it is left unspecified whether
the file position (if any) changes.