Hello again...
On Friday I was asking about how to quickly update a few
cells in a large raster layer. After looking at the source to
libgis again I wrote the following hack to allow opening raster layers
for update (permitting successful access to both 'G_get_map_row'
and 'G_put_map_row_random'). It **seems** to work (but not thoroughly
tested, probably has bugs, and absolutely no guarantees about it).
The layer to be updated must be uncompressed first.
Tom
------
#include "G.h"
#define FCB G__.fileinfo[fd]
int
G_open_cell_old_update(char *name, char *mapset) {
int fd, old_fd, n;
/* try to open the file */
fd = G_open_cell_old(name,mapset);
if (fd <0)
return(fd);
/* give up if it is compressed (run 'r.compress -u' first) */
if (FCB.cellhd.compressed) {
G_close_cell(fd);
return -1;
}
/* now try to reopen the file in update mode */
old_fd = fd;
close(fd);
fd = G_open_update("cell",name);
if (fd < 0) {
G__.fileinfo[old_fd].open_mode = -1;
return(fd);
}
/* copy the file header if the fd is different */
if (fd!=old_fd) {
bcopy(&G__.fileinfo[fd],&G__.fileinfo[old_fd],sizeof(struct fileinfo));
G__.fileinfo[old_fd].open_mode = -1;
}
/* toggle the open_mode flag to RANDOM */
FCB.open_mode = OPEN_NEW_RANDOM;
/* allocate a work buffer (if not done already) */
n = FCB.cellhd.cols * (sizeof(CELL) + 1) + 1;
if (n > G__.work_buf_size)
{
if (G__.work_buf_size <= 0)
G__.work_buf = (unsigned char *) G_malloc (n);
else
G__.work_buf = (unsigned char *) G_realloc(G__.work_buf,n);
G__.work_buf_size = n;
}
return (fd);
}
void
G_close_cell_update(int fd) {
/* set the open_mode flag back to OPEN_OLD before closing the file */
FCB.open_mode = OPEN_OLD;
G_close_cell(fd);
}
---- end of sample code ----