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Array and Long manipulations

Juru Dala

Hi everyone,

I have an array eg:

command[9] = {0x3C , 0x4A , 0x33 , 0x25 , 0xB1 , 0xD3 , 0x59 , 0x34 , 0x2E }

the last 4 byte is the value (in hex) that I need to manipulate, let say increment by 3.

and after manipulation I need to send back the value together with other data in one array to serial port.

What I had try is:

//init outside
unsigned char k[4];
long value;


k[0] = command [5]; k[1] = command [6]; k[2] = command [7]; k[3] = command [8];

value= strtol (k, NULL, 4);
value= value+3;
sprintf ( k, "%Ld\n"  , value);

buf[4] = k[0]; buf[5] = k[1]; buf[6] = k[2]; buf[7] = k[3];

sendcom ( buf , 15);//send to serial function

But it not working.

Can anyone give me help, and honestly I'm lack of C basics

Thanks

Juru

Parents
  • 0 in reply to Dan Henry

    Lots of ways to go about this.

    If your data is actually structured, then why not structure it?

    typedef struct
    {
    CmdHeader header;  // 5 bytes
    U32       value;
    } Command;

Command command;

...
    command.value += 3;

    sendcom ((U8*)&command, sizeof(command));

    Just because the sendcom routine sends a series of bytes doesn't mean the entire rest of your program needs to treat the data that way.

    Presumably that "CmdHeader" has some further structure, but your post doesn't describe the data.

    If you do want to keep the command in a byte array, then you can also do some pointer surgery:

    U8 command[9] = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
#define MyCmdValueOffset 5

...
   // find a pointer to value by adding 5
   // to the start of the command buffer,
   // and casting to long,
   *(long*)(command + MyCmdValueOffset) += 3;

    Note that this is exactly what the version with the struct does, except that the compiler does the work for you so that you do not have to manually maintain the value offset.

    sprintf() operates on strings. It looks like you have an actual binary integer in that array.

Reply
  • 0 in reply to Dan Henry

    Lots of ways to go about this.

    If your data is actually structured, then why not structure it?

    typedef struct
    {
    CmdHeader header;  // 5 bytes
    U32       value;
    } Command;

Command command;

...
    command.value += 3;

    sendcom ((U8*)&command, sizeof(command));

    Just because the sendcom routine sends a series of bytes doesn't mean the entire rest of your program needs to treat the data that way.

    Presumably that "CmdHeader" has some further structure, but your post doesn't describe the data.

    If you do want to keep the command in a byte array, then you can also do some pointer surgery:

    U8 command[9] = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
#define MyCmdValueOffset 5

...
   // find a pointer to value by adding 5
   // to the start of the command buffer,
   // and casting to long,
   *(long*)(command + MyCmdValueOffset) += 3;

    Note that this is exactly what the version with the struct does, except that the compiler does the work for you so that you do not have to manually maintain the value offset.

    sprintf() operates on strings. It looks like you have an actual binary integer in that array.

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