GRASS GIS 8 Programmer's Manual 8.2.1(2023)-exported
get_row.c
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1
2/**
3 * \file lib/segment/get_row.c
4 *
5 * \brief Segment row retrieval routines.
6 *
7 * This program is free software under the GNU General Public License
8 * (>=v2). Read the file COPYING that comes with GRASS for details.
9 *
10 * \author GRASS GIS Development Team
11 *
12 * \date 2005-2018
13 */
14
15#include <stdio.h>
16#include <unistd.h>
17#include <string.h>
18#include <errno.h>
19#include <grass/gis.h>
20#include "local_proto.h"
21
22
23/**
24 * \fn int Segment_get_row (SEGMENT *SEG, void *buf, int row)
25 *
26 * \brief Read row from segment file.
27 *
28 * Transfers data from a segment file, row by row, into memory
29 * (which can then be written to a regular matrix file). <b>Seg</b> is the
30 * segment structure that was configured from a call to
31 * <i>Segment_init()</i>.
32 *
33 * <b>Buf</b> will be filled with <em>ncols*len</em> bytes of data
34 * corresponding to the <b>row</b> in the data matrix.
35 *
36 * \param[in] seg segment
37 * \param[in,out] buf
38 * \param[in] row
39 * \return 1 if successful
40 * \return -1 if unable to seek or read segment file
41 */
42
43int Segment_get_row(const SEGMENT * SEG, void *buf, off_t row)
44{
45 int size;
46 off_t ncols, col;
47 int scols;
48 int n, index;
49
50 if (SEG->cache) {
51 memcpy(buf, SEG->cache + ((size_t)row * SEG->ncols) * SEG->len, SEG->len * SEG->ncols);
52
53 return 1;
54 }
55
56 ncols = SEG->ncols - SEG->spill;
57 scols = SEG->scols;
58 size = scols * SEG->len;
59
60 for (col = 0; col < ncols; col += scols) {
61 SEG->address(SEG, row, col, &n, &index);
62 SEG->seek(SEG, n, index);
63
64 if (read(SEG->fd, buf, size) != size) {
65 G_warning("Segment_get_row: %s", strerror(errno));
66 return -1;
67 }
68
69 /* The buf variable is a void pointer and thus points to anything. */
70 /* Therefore, it's size is unknown and thus, it cannot be used for */
71 /* pointer arithmetic (some compilers treat this as an error - SGI */
72 /* MIPSPro compiler for one). Since the read command is reading in */
73 /* "size" bytes, cast the buf variable to char * before incrementing */
74 buf = ((char *)buf) + size;
75 }
76 if ((size = SEG->spill * SEG->len)) {
77 SEG->address(SEG, row, col, &n, &index);
78 SEG->seek(SEG, n, index);
79
80 if (read(SEG->fd, buf, size) != size) {
81 G_warning("Segment_get_row: %s", strerror(errno));
82 return -1;
83 }
84 }
85
86 return 1;
87}
int Segment_get_row(const SEGMENT *SEG, void *buf, off_t row)
Definition: get_row.c:43
void G_warning(const char *msg,...)
Print a warning message to stderr.
Definition: gis/error.c:204