why define size_t ssize_t ?

http://blog.csdn.net/yuwenliang/archive/2010/01/18/5207491.aspx

ssize_t说明

Data Type:
ssize_t

This data type is used to represent the sizes of blocks that can be read or written in a single operation. It is similar to size_t
, but must be a signed type.

Function:
ssize_t read (int filedes
, void *buffer
, size_t size
)

The read
function reads up to size
bytes from the file with descriptor filedes
, storing the results in the buffer
. (This is not necessarily a character string, and no terminating null character is added.)

The return value is the number of bytes actually read. This might be less than size
;
for example, if there aren't that many bytes left in the file or if
there aren't that many bytes immediately available. The exact behavior
depends on what kind of file it is. Note that reading less than size
bytes is not an error.

A value of zero indicates end-of-file (except if the value of the size
argument is also zero). This is not considered an error. If you keep calling read
while at end-of-file, it will keep returning zero and doing nothing else.

If read
returns at least
one character, there is no way you can tell whether end-of-file was
reached. But if you did reach the end, the next read will return zero.

In case of an error, read
returns -1
.