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Fixes float encoding/decoding for both big and little endian (fixes #665
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, #694)

* Removes useless memcpy calls and no longer used swap32 and swap16 macros.
* Updated float encoding functions to consider endianess of 16-bit registers (fixes #665)
* Deprecated modbus_get_float() and modbus_set_float() are implemented using cdab variants.
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@ghorwin
ghorwin authored Jul 17, 2024
1 parent 98f5764 commit 13bd584
Showing 1 changed file with 54 additions and 112 deletions.
166 changes: 54 additions & 112 deletions src/modbus-data.c
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Original file line number Diff line number Diff line change
Expand Up @@ -26,50 +26,6 @@

#include "modbus.h"

#if defined(HAVE_BYTESWAP_H)
# include <byteswap.h>
#endif

#if defined(__APPLE__)
# include <libkern/OSByteOrder.h>
# define bswap_16 OSSwapInt16
# define bswap_32 OSSwapInt32
# define bswap_64 OSSwapInt64
#endif

#if defined(__GNUC__)
# define GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__ * 10)
# if GCC_VERSION >= 430
// Since GCC >= 4.30, GCC provides __builtin_bswapXX() alternatives so we switch to them
# undef bswap_32
# define bswap_32 __builtin_bswap32
# endif
# if GCC_VERSION >= 480
# undef bswap_16
# define bswap_16 __builtin_bswap16
# endif
#endif

#if defined(_MSC_VER) && (_MSC_VER >= 1400)
# define bswap_32 _byteswap_ulong
# define bswap_16 _byteswap_ushort
#endif

#if !defined(bswap_16)
# warning "Fallback on C functions for bswap_16"
static inline uint16_t bswap_16(uint16_t x)
{
return (x >> 8) | (x << 8);
}
#endif

#if !defined(bswap_32)
# warning "Fallback on C functions for bswap_32"
static inline uint32_t bswap_32(uint32_t x)
{
return (bswap_16(x & 0xffff) << 16) | (bswap_16(x >> 16));
}
#endif
// clang-format on

/* Sets many bits from a single byte value (all 8 bits of the byte value are
Expand Down Expand Up @@ -124,17 +80,29 @@ uint8_t modbus_get_byte_from_bits(const uint8_t *src, int idx, unsigned int nb_b
/* Get a float from 4 bytes (Modbus) without any conversion (ABCD) */
float modbus_get_float_abcd(const uint16_t *src)
{
/* Suppose the modbus byte stream contained the 32-bit float 0x10203040 - abcd.
On big endian systems, the memory starting at src contains two 16-bit registers 0x1020 and 0x3040
On little endian system, the 16-bit registers memory holds 0x2010 and 0x4030
To convert the data to float32 on big-endian we only need to cast the pointer and we are done.
On little endian systems, we need to swap the bytes in each word again and then assemble
an integer via shift operations and finally cast to float32.
A portable way is to retrieve low and high bytes of both words using shift operations, then assemble
the 32-bit integer.
*/

float f;
uint32_t i;
uint8_t a, b, c, d;

a = (src[0] >> 8) & 0xFF;
b = (src[0] >> 0) & 0xFF;
c = (src[1] >> 8) & 0xFF;
d = (src[1] >> 0) & 0xFF;
a = (src[0] >> 8) & 0xFF; // high byte if first word
b = (src[0] >> 0) & 0xFF; // low byte if first word
c = (src[1] >> 8) & 0xFF; // high byte if second word
d = (src[1] >> 0) & 0xFF; // low byte if second word

i = (a << 24) | (b << 16) | (c << 8) | (d << 0);
memcpy(&f, &i, 4);
// assemble in memory location of float
// from right to left: get address of float, interpret as address to uint32, dereference and write uint32
*(uint32_t *)&f = (a << 24) | (b << 16) | (c << 8) | (d << 0);

return f;
}
Expand All @@ -143,16 +111,16 @@ float modbus_get_float_abcd(const uint16_t *src)
float modbus_get_float_dcba(const uint16_t *src)
{
float f;
uint32_t i;
uint8_t a, b, c, d;

a = (src[0] >> 8) & 0xFF;
b = (src[0] >> 0) & 0xFF;
c = (src[1] >> 8) & 0xFF;
d = (src[1] >> 0) & 0xFF;
d = (src[0] >> 8) & 0xFF;
c = (src[0] >> 0) & 0xFF;
b = (src[1] >> 8) & 0xFF;
a = (src[1] >> 0) & 0xFF;

i = (d << 24) | (c << 16) | (b << 8) | (a << 0);
memcpy(&f, &i, 4);
// assemble in memory location of float
// from right to left: get address of float, interpret as address to uint32, dereference and write uint32
*(uint32_t *)&f = (a << 24) | (b << 16) | (c << 8) | (d << 0);

return f;
}
Expand All @@ -161,16 +129,16 @@ float modbus_get_float_dcba(const uint16_t *src)
float modbus_get_float_badc(const uint16_t *src)
{
float f;
uint32_t i;
uint8_t a, b, c, d;

a = (src[0] >> 8) & 0xFF;
b = (src[0] >> 0) & 0xFF;
c = (src[1] >> 8) & 0xFF;
d = (src[1] >> 0) & 0xFF;
b = (src[0] >> 8) & 0xFF;
a = (src[0] >> 0) & 0xFF;
d = (src[1] >> 8) & 0xFF;
c = (src[1] >> 0) & 0xFF;

i = (b << 24) | (a << 16) | (d << 8) | (c << 0);
memcpy(&f, &i, 4);
// assemble in memory location of float
// from right to left: get address of float, interpret as address to uint32, dereference and write uint32
*(uint32_t *)&f = (a << 24) | (b << 16) | (c << 8) | (d << 0);

return f;
}
Expand All @@ -179,114 +147,88 @@ float modbus_get_float_badc(const uint16_t *src)
float modbus_get_float_cdab(const uint16_t *src)
{
float f;
uint32_t i;
uint8_t a, b, c, d;

a = (src[0] >> 8) & 0xFF;
b = (src[0] >> 0) & 0xFF;
c = (src[1] >> 8) & 0xFF;
d = (src[1] >> 0) & 0xFF;
c = (src[0] >> 8) & 0xFF;
d = (src[0] >> 0) & 0xFF;
a = (src[1] >> 8) & 0xFF;
b = (src[1] >> 0) & 0xFF;

i = (c << 24) | (d << 16) | (a << 8) | (b << 0);
memcpy(&f, &i, 4);
// assemble in memory location of float
// from right to left: get address of float, interpret as address to uint32, dereference and write uint32
*(uint32_t *)&f = (a << 24) | (b << 16) | (c << 8) | (d << 0);

return f;
}

/* DEPRECATED - Get a float from 4 bytes in sort of Modbus format */
float modbus_get_float(const uint16_t *src)
{
float f;
uint32_t i;

i = (((uint32_t) src[1]) << 16) + src[0];
memcpy(&f, &i, sizeof(float));

return f;
return modbus_get_float_cdab(src);
}

/* Set a float to 4 bytes for Modbus w/o any conversion (ABCD) */
void modbus_set_float_abcd(float f, uint16_t *dest)
{
uint32_t i;
uint8_t *out = (uint8_t *) dest;
uint32_t i = *(uint32_t*)(&f);
uint8_t a, b, c, d;

memcpy(&i, &f, sizeof(uint32_t));
a = (i >> 24) & 0xFF;
b = (i >> 16) & 0xFF;
c = (i >> 8) & 0xFF;
d = (i >> 0) & 0xFF;

out[0] = a;
out[1] = b;
out[2] = c;
out[3] = d;
dest[0] = (a << 8) | b;
dest[1] = (c << 8) | d;
}

/* Set a float to 4 bytes for Modbus with byte and word swap conversion (DCBA) */
void modbus_set_float_dcba(float f, uint16_t *dest)
{
uint32_t i;
uint8_t *out = (uint8_t *) dest;
uint32_t i = *(uint32_t*)(&f);
uint8_t a, b, c, d;

memcpy(&i, &f, sizeof(uint32_t));
a = (i >> 24) & 0xFF;
b = (i >> 16) & 0xFF;
c = (i >> 8) & 0xFF;
d = (i >> 0) & 0xFF;

out[0] = d;
out[1] = c;
out[2] = b;
out[3] = a;
dest[0] = (d << 8) | c;
dest[1] = (b << 8) | a;
}

/* Set a float to 4 bytes for Modbus with byte swap conversion (BADC) */
void modbus_set_float_badc(float f, uint16_t *dest)
{
uint32_t i;
uint8_t *out = (uint8_t *) dest;
uint32_t i = *(uint32_t*)(&f);
uint8_t a, b, c, d;

memcpy(&i, &f, sizeof(uint32_t));
a = (i >> 24) & 0xFF;
b = (i >> 16) & 0xFF;
c = (i >> 8) & 0xFF;
d = (i >> 0) & 0xFF;

out[0] = b;
out[1] = a;
out[2] = d;
out[3] = c;
dest[0] = (b << 8) | a;
dest[1] = (d << 8) | c;
}

/* Set a float to 4 bytes for Modbus with word swap conversion (CDAB) */
void modbus_set_float_cdab(float f, uint16_t *dest)
{
uint32_t i;
uint8_t *out = (uint8_t *) dest;
uint32_t i = *(uint32_t*)(&f);
uint8_t a, b, c, d;

memcpy(&i, &f, sizeof(uint32_t));
a = (i >> 24) & 0xFF;
b = (i >> 16) & 0xFF;
c = (i >> 8) & 0xFF;
d = (i >> 0) & 0xFF;

out[0] = c;
out[1] = d;
out[2] = a;
out[3] = b;
dest[0] = (c << 8) | d;
dest[1] = (a << 8) | b;
}

/* DEPRECATED - Set a float to 4 bytes in a sort of Modbus format! */
void modbus_set_float(float f, uint16_t *dest)
{
uint32_t i;

memcpy(&i, &f, sizeof(uint32_t));
dest[0] = (uint16_t) i;
dest[1] = (uint16_t) (i >> 16);
return modbus_set_float_cdab(f, dest);
}

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