gfarm_error_t gfs_pio_read (GFS_File gf, void * buffer, int size, int * np);
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.