Convert uint8 hex array in reverse order to double/uint16/string/etc

[ngay528]

I'm retrieving a size 4 uint8 array from a buffer that arrives like the following:

c4 ed e5 d7    (hex format)

and need to convert it so it reads:

d7e5edc4 (hex)

3622170052   (decimal format)

I know it can be done in a loop using some <<'s or something.  I can't remember all this low level stuff so help would be greatly appreciated.  I can rearrange the initial data array, but not sure how to convert to decimal number.  I assume a simple for loop starting at the end would be the easiest way to solve.

PS on this subject, where is a good place that explains all the uses of << or >> or ^= etc

Thanks

[RobertoBozzolo · Accepted solution]

I happened to use this routine in the past to accomplish a task like yours:

unsigned char   string[5];

int  number;

string[0] = 0xc4;

string[1] = 0xed;

string[2] = 0xe5;

string[3] = 0xd7;

string[4] = 0x0;    // string terminator

number = conv_dword_to_int (string);
 

int conv_dword_to_int (unsigned char * buf)
{
 return ((*(buf + 3)) << 24)  +
   ((*(buf + 2)) << 16)  +
   ((*(buf + 1)) << 😎 +
   (*(buf + 0));
}

[jr_2005]

An alternative approach is to use unions:

typedef union {

    int i;

    char b [4];

} unionInt;

int EndianInt (int *in, int *out) {

    unionInt ui, uo;

    ui.i = *in;             // Start by copying the integer value

    uo.b [3] = ui.b [0];    // Just swap the bytes over

    uo.b [2] = ui.b [1];

    uo.b [1] = ui.b [2];

    uo.b [0] = ui.b [3];

    return *out = uo.i;     // Allows in and out to point to the same data, if required

}

To process an array of bytes, simply cast it at the point of call:

    char dat [] = {0xc4, 0xed, 0xe5, 0xd7};

    int value;

    EndianInt ((int *) dat, &value);

JR

[ebalci]

There is already a function for this (can be used for that purpose) in ANSI C library.

It is memcpy ! 🙂

Copy this into Interactive Execution and run:

#include <ansi_c.h>

static unsigned char string[5];

static unsigned int  number;

string[0] = 0xc4;

string[1] = 0xed;

string[2] = 0xe5;

string[3] = 0xd7;

string[4] = 0x0;    // string terminator

memcpy (&number, string, 4);

printf ("0x%04x\n", number); 

[mrp]

That trick only works on little-endian processors. Admittedly that accounts for the vast majority of (Intel) CPUs out there, but some of us work with big-endian ones as well and we have to be careful when using a mix of the two types in any system.

Mike