[GRASS-user] problem with Rast_get_x_value functions

GRASS GRASS user
1

Hi
Sorry for GRASS USERS list but I'm totally banned on GRASS-DEV due to my proxyserver error, hope I will change it in the next few days, so for now I ask here.

I have problem with functions
Rast_get_[c,f,d]_value(void*, RASTER_MAP_TYPE)

according to its description it require void* argument as first so:

suppose in_map_type=CELL_TYPE

Rast_allocate_buf(in_map_type);
Rast_get_row(in_map_fd, in_buf, r, in_map_type); // for now is OK, buffer has proper value
But next:
for(c=0;c<ncols;++c)
internal_map_f[r][c] = Rast_get_f_value(&in_buf[c], in_map_type);
// not ok, compiler show warning on dereferencing pointer and program returns garbage

internal_map_f[r][c] = Rast_get_f_value(&((CELL*)in_buf)[c], in_map_type); // OK but I must cast it manually

it is illogical for me, if in_buf (void*) is casted internally to (CELL*) why I must to do it again in a function calling?
sorry again for NTG.
Jarek

2

Jarek Jasiewicz wrote:

Sorry for GRASS USERS list but I'm totally banned on GRASS-DEV due to my
proxyserver error, hope I will change it in the next few days, so for
now I ask here.

I have problem with functions
Rast_get_[c,f,d]_value(void*, RASTER_MAP_TYPE)

according to its description it require void* argument as first so:

suppose in_map_type=CELL_TYPE

Rast_allocate_buf(in_map_type);
Rast_get_row(in_map_fd, in_buf, r, in_map_type); // for now is OK,
buffer has proper value
But next:
for(c=0;c<ncols;++c)
internal_map_f[r][c] = Rast_get_f_value(&in_buf[c], in_map_type);
// not ok, compiler show warning on dereferencing pointer and program
returns garbage

internal_map_f[r][c] = Rast_get_f_value(&((CELL*)in_buf)[c],
in_map_type); // OK but I must cast it manually

it is illogical for me, if in_buf (void*) is casted internally to
(CELL*) why I must to do it again in a function calling?

You cannot dereference a "void*", nor perform arithmetic on one.

If you want to access the map as an array of FCELL values, use
Rast_allocate_f_buf() and Rast_get_f_row(). There is no point in using
the map's native type if you're just going to convert the data anyhow.

If you want to write code which works "natively" with any type, you
need to use G_incr_void_ptr() for array access, e.g.:

  size_t size = Rast_cell_size(in_map_type);

  ...

  /* equivalent to "ptr = &in_buf[c]" */
  ptr = G_incr_void_ptr(in_buf, c * size);

Most of the time, it's preferable to work with a specific type
internally and convert on input and output.

--
Glynn Clements <glynn@gclements.plus.com>

3

Thanks Glynn

You cannot dereference a "void*", nor perform arithmetic on one.

If you want to access the map as an array of FCELL values, use
Rast_allocate_f_buf() and Rast_get_f_row(). There is no point in using
the map's native type if you're just going to convert the data anyhow.
  

The problem was how to write warper function to read and convert data from any type to any type
it is alrady done, I didn't know I must use (c*size) to instead of c passing pointer to function

thanks a lot
Jarek