C - creating a packet using char and int

Question

I'm looking to build a binary string that consists of hex values... I've learned sprintf is not the correct way to accomplish this. I've got a real simple gui to collect configuration settings that need to be sent a device. The values can be signed and will always be numerical. I have a set of values like:

#define PKTHEAD 0xA5
#define VALONE  0xA0
#define VALTWO  0xA1
#define VALTHREE 0xA2
#define PKTTAIL 0x5A

signed int val1 = 10000; //0x2710
signed int val2 = -84;   //0xFFAC
signed int val3 = -175001 //0xFFFD5467

Right now I'm doing this:

int main(int argc, char *argv[])
{
    int numBytes = 10;
    signed char *tmp;
    signed char *pyld;
    pyld = calloc(numBytes, sizeof(char));

    signed int val1 = 10000; //0x2710
    signed int val2 = -84;   //0xFFAC
    signed int val3 = -175001 //0xFFFD5467

    tmp = pyld;

    *(tmp + (numBytes - 1)) = DACHEAD;
    *(tmp + (numBytes - 2)) = DACHEAD;
    *(tmp + (numBytes - 3)) = DACHEAD;
    *(tmp + (numBytes - 4)) = VALONE;
    *(tmp + (numBytes - 5)) = val1;
    printf("payload = %hhx%hhx%hhx%hhx%hx%hx%hhx%hx\n", pyld[9], pyld[8], pyld[7], pyld[6], pyld[5], pyld[4], pyld[3], pyld[2], pyld[1], pyld[0]);
}

The output is: "payload = a5a5a5a0100a1ffac"

My val1 value is incorrect and I'm wondering if there is a better way to compose this packet. Should I be doing bit shifts instead?

Any help is greatly appreciated.

Thanks!

score 0 · Accepted Answer · edited May 23 '17 at 11:56

A better way of sending data over network is using a serialization library. For example, see this question

Also, if you do not want to use external libraries you should first understand integer sizes that you need, either it is 16 bit or 32 bit, the endianness (and convert accordingly) that your device uses and always prefer using memcpy for putting values of correct size into binary buffer.

C - creating a packet using char and int

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