GRASS GIS 8 Programmer's Manual 8.3.2(2024)-exported
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get_row.c
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1/**
2 * \file lib/segment/get_row.c
3 *
4 * \brief Segment row retrieval routines.
5 *
6 * This program is free software under the GNU General Public License
7 * (>=v2). Read the file COPYING that comes with GRASS for details.
8 *
9 * \author GRASS GIS Development Team
10 *
11 * \date 2005-2018
12 */
13
14#include <stdio.h>
15#include <unistd.h>
16#include <string.h>
17#include <errno.h>
18#include <grass/gis.h>
19#include "local_proto.h"
20
21/**
22 * \fn int Segment_get_row (SEGMENT *SEG, void *buf, int row)
23 *
24 * \brief Read row from segment file.
25 *
26 * Transfers data from a segment file, row by row, into memory
27 * (which can then be written to a regular matrix file). <b>Seg</b> is the
28 * segment structure that was configured from a call to
29 * <i>Segment_init()</i>.
30 *
31 * <b>Buf</b> will be filled with <em>ncols*len</em> bytes of data
32 * corresponding to the <b>row</b> in the data matrix.
33 *
34 * \param[in] seg segment
35 * \param[in,out] buf
36 * \param[in] row
37 * \return 1 if successful
38 * \return -1 if unable to seek or read segment file
39 */
40
41int Segment_get_row(const SEGMENT *SEG, void *buf, off_t row)
42{
43 int size;
44 off_t ncols, col;
45 int scols;
46 int n, index;
47
48 if (SEG->cache) {
49 memcpy(buf, SEG->cache + ((size_t)row * SEG->ncols) * SEG->len,
50 SEG->len * SEG->ncols);
51
52 return 1;
53 }
54
55 ncols = SEG->ncols - SEG->spill;
56 scols = SEG->scols;
57 size = scols * SEG->len;
58
59 for (col = 0; col < ncols; col += scols) {
60 SEG->address(SEG, row, col, &n, &index);
61 SEG->seek(SEG, n, index);
62
63 if (read(SEG->fd, buf, size) != size) {
64 G_warning("Segment_get_row: %s", strerror(errno));
65 return -1;
66 }
67
68 /* The buf variable is a void pointer and thus points to anything. */
69 /* Therefore, it's size is unknown and thus, it cannot be used for */
70 /* pointer arithmetic (some compilers treat this as an error - SGI */
71 /* MIPSPro compiler for one). Since the read command is reading in */
72 /* "size" bytes, cast the buf variable to char * before incrementing */
73 buf = ((char *)buf) + size;
74 }
75 if ((size = SEG->spill * SEG->len)) {
76 SEG->address(SEG, row, col, &n, &index);
77 SEG->seek(SEG, n, index);
78
79 if (read(SEG->fd, buf, size) != size) {
80 G_warning("Segment_get_row: %s", strerror(errno));
81 return -1;
82 }
83 }
84
85 return 1;
86}
int Segment_get_row(const SEGMENT *SEG, void *buf, off_t row)
Definition get_row.c:41
void G_warning(const char *msg,...)
Print a warning message to stderr.
Definition gis/error.c:203