gfs_pio_read() attempts to read up to size bytes from the file referenced by gf into the buffer starting at buffer, and returns the number of bytes read to the address pointed by np. On end of file, the number of bytes read is zero.
The function terminated successfully.
gf does not specify a regular file.
The file descripter specified by gf is not a valid or is not open for reading.
There is no available file system node.
Insufficient memory was available.
An error except the above occurred. The reason is shown by gfarm_error_string(3).