port_init.c

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/*!
   \file diglib/port_init.c
 
   \brief Vector library - portability (lower level functions)
 
   Lower level functions for reading/writing/manipulating vectors.
 
   This code is a quick hack to allow the writing of portable
   binary data files.
   The approach is to take known values and compare them against
   the current machine's internal representation.   A cross reference
   table is then built, and then all file reads and writes must go
   through these routines to correct the numbers if need be.
 
   As long as the byte switching is symmetrical, the conversion routines
   will work both directions.
 
   The integer test patterns are quite simple, and their choice was
   arbitrary, but the float and double valued were more critical.
 
   I did not have a specification for IEEE to go by, so it is possible
   that I have missed something.  My criteria were:
 
   First, true IEEE numbers had to be chosen to avoid getting an FPE.
   Second, every byte in the test pattern had to be unique.   And
   finally, the number had to not be sensitive to rounding by the
   specific hardware implementation.
 
   By experimentation it was found that the number  1.3333  met
   all these criteria for both floats and doubles
 
   See the discourse at the end of this file for more information
 
   The 3.0 dig, and dig_plus files are inherently non-portable.  This
   can be seen in moving files between a SUN 386i and other SUN machines.
   The recommended way to transport files was always to convert to ASCII
   (b.a.vect) and copy the ASCII files:  dig_ascii and dig_att to the
   destination machine.
 
   The problem lies in the way that different architectures internally
   represent data.   If a number is internally store as  0x01020304 on
   a 680x0 family machine, the same number will be stored as
   0x04030201 on an 80386 class machine.
 
   The CERL port of GRASS to the Compaq 386 already has code to deal
   with this incompatibility.  This code converts all files that are written
   out to conform to the 680x0 standard.  These binary files can then be
   shared between machines without conversion.
   This code is designed to work with the majority of computers in use
   today that fit the following requirements:
   byte     ==  8 bits
   int      ==  4 bytes
   long     ==  4 bytes
   double   ==  IEEE standard 64 bit
   float    ==  IEEE standard 32 bit
 
   bytes can be swapped around in any reasonable way, but bits within each
   byte must be maintained in normal high to low ordering:  76543210
   is this a problem?
 
   If this ability is desired on a SUN 386i, for example, you simply
   define the compiler flag  CERL_PORTABLE in the src/CMD/makehead  file
   and recompile all of the mapdev programs.
   needs update, makehead/mapdev no longer exist
 
   Binary DLG files are NOT supported by this code, and will continue to
   be non-portable between different architectures.
   applies to the files coor/topo/cidx, needs testing
 
   SPDX-FileCopyrightText: 2001-2009 GRASS Development Team
   SPDX-License-Identifier: GPL-2.0-or-later
 
   \author Original author CERL, probably Dave Gerdes
   \author Update to GRASS 5.7 Radim Blazek
 */
 
#include <stdio.h>
#include <sys/types.h>
#include <grass/vector.h>
#include <grass/glocale.h>
 
#define TEST_PATTERN 1.3333
#ifdef HAVE_LONG_LONG_INT
#define LONG_LONG_TEST 0x0102030405060708LL
#endif
#define LONG_TEST  0x01020304
#define INT_TEST   0x01020304
#define SHORT_TEST 0x0102
 
static double u_d = TEST_PATTERN;
static float u_f = TEST_PATTERN;
off_t u_o; /* depends on sizeof(off_t) */
static long u_l = LONG_TEST;
static int u_i = INT_TEST;
static short u_s = SHORT_TEST;
 
/* dbl_cmpr holds the bytes of an IEEE representation of  TEST_PATTERN */
static const unsigned char dbl_cmpr[] = {0x3f, 0xf5, 0x55, 0x32,
                                         0x61, 0x7c, 0x1b, 0xda};
/* flt_cmpr holds the bytes of an IEEE representation of  TEST_PATTERN */
static const unsigned char flt_cmpr[] = {0x3f, 0xaa, 0xa9, 0x93};
static const unsigned char off_t_cmpr[] = {0x01, 0x02, 0x03, 0x04,
                                           0x05, 0x06, 0x07, 0x08};
static const unsigned char lng_cmpr[] = {0x01, 0x02, 0x03, 0x04};
static const unsigned char int_cmpr[] = {0x01, 0x02, 0x03, 0x04};
static const unsigned char shrt_cmpr[] = {0x01, 0x02};
 
/* Find native sizes */
int nat_dbl = sizeof(double);
int nat_flt = sizeof(float);
int nat_off_t = sizeof(off_t);
int nat_lng = sizeof(long);
int nat_int = sizeof(int);
int nat_shrt = sizeof(short);
 
int dbl_order;
int flt_order;
int off_t_order;
int lng_order;
int int_order;
int shrt_order;
 
unsigned char dbl_cnvrt[sizeof(double)];
unsigned char flt_cnvrt[sizeof(float)];
unsigned char off_t_cnvrt[sizeof(off_t)];
unsigned char lng_cnvrt[sizeof(long)];
unsigned char int_cnvrt[sizeof(int)];
unsigned char shrt_cnvrt[sizeof(short)];
 
/*
 * match search_value against each char in basis.
 * return offset or -1 if not found
 */
static int find_offset(const unsigned char *basis, unsigned char search_value,
                       int size)
{
    int i;
 
    for (i = 0; i < size; i++)
        if (basis[i] == search_value)
            return (i);
 
    return (-1);
}
 
static int find_offsets(const void *pattern, unsigned char *cnvrt,
                        const unsigned char *cmpr, int port_size, int nat_size,
                        const char *typename)
{
    int big, ltl;
    int i;
 
    for (i = 0; i < port_size; i++) {
        int off = find_offset(pattern, cmpr[i], nat_size);
 
        if (off < 0)
            G_fatal_error(_("Unable to find '%x' in %s"), cmpr[i], typename);
 
        cnvrt[i] = off;
    }
 
    big = ltl = 1;
 
    for (i = 0; i < port_size; i++) {
        if (cnvrt[i] != (nat_size - port_size + i))
            big = 0; /* isn't big endian */
        if (cnvrt[i] != (port_size - 1 - i))
            ltl = 0; /* isn't little endian */
    }
 
    if (big)
        return ENDIAN_BIG;
 
    if (ltl)
        return ENDIAN_LITTLE;
 
    return ENDIAN_OTHER;
}
 
/*!
   \brief Initialize Port_info structures
 */
void port_init(void)
{
    static int done;
 
    if (done)
        return;
 
    done = 1;
 
    /* Following code checks only if all assumptions are fulfilled */
    /* Check sizes */
    if (nat_dbl != PORT_DOUBLE)
        G_fatal_error("sizeof(double) != %d", PORT_DOUBLE);
    if (nat_flt != PORT_FLOAT)
        G_fatal_error("sizeof(float) != %d", PORT_DOUBLE);
    /* off_t size is variable, depending on the vector size and LFS support */
    if (nat_lng < PORT_LONG)
        G_fatal_error("sizeof(long) < %d", PORT_LONG);
    if (nat_int < PORT_INT)
        G_fatal_error("sizeof(int) < %d", PORT_INT);
    if (nat_shrt < PORT_SHORT)
        G_fatal_error("sizeof(short) < %d", PORT_SHORT);
 
    /* Find for each byte in big endian test pattern (*_cmpr)
     * offset of corresponding byte in machine native order.
     * Look if native byte order is little or big or some other (pdp)
     * endian.
     */
 
    if (nat_off_t == 8)
#ifdef HAVE_LONG_LONG_INT
        u_o = (off_t)LONG_LONG_TEST;
#else
        G_fatal_error("Internal error: can't construct an off_t literal");
#endif
    else
        u_o = (off_t)LONG_TEST;
 
    dbl_order =
        find_offsets(&u_d, dbl_cnvrt, dbl_cmpr, PORT_DOUBLE, nat_dbl, "double");
    flt_order =
        find_offsets(&u_f, flt_cnvrt, flt_cmpr, PORT_FLOAT, nat_flt, "float");
    off_t_order = find_offsets(&u_o, off_t_cnvrt, off_t_cmpr, nat_off_t,
                               nat_off_t, "off_t");
    lng_order =
        find_offsets(&u_l, lng_cnvrt, lng_cmpr, PORT_LONG, nat_lng, "long");
    int_order =
        find_offsets(&u_i, int_cnvrt, int_cmpr, PORT_INT, nat_int, "int");
    shrt_order = find_offsets(&u_s, shrt_cnvrt, shrt_cmpr, PORT_SHORT, nat_shrt,
                              "short");
}