How to Calculate CheckSum in objective C - objective-c

Question

I am working on an app that deals with external hardware communication, and I'm having problem getting checksum of a package. (I'm still learning objective C while making this app, so I'm still fairly new in this.)
In another code written in C, the checksum was calculated like this:
byte CalculateCheckSum (byte txcount){
byte local_cs=0;
while(txcount>0){
local_cs+=*x_ptr;
x_ptr += 1;
txcount--;
};
return (~local_cs+1);
}
I tried to use the some code for objective C:
u_int8_t synByteSOH[]= {SYN,SYN,SOH,SETSERIALINFO};
- (Byte)CalcCheckSum:(Byte)i
{ u_int8_t synByteSOH[]= {SYN,SYN,SOH,SETSERIALINFO};
Byte local_cs = 0;
int j = 0;
while (i>0) {
local_cs += synByteSOH[j];
i--;
j++;
};
return (~local_cs+1);
}
No warnings or errors, but it's said clang: error: linker command failed with exit code 1
Does anyone know why is that? And how should I fix it?

Answer 1

All valid C code it valid Objective C code. Don't try rewriting it into a Objective C method - there's zero value to that.
The trick to adapting the original function is that it relies on several things: byte being typedeffed, and a global/static variable called x_ptr.
About x_ptr, where it comes from and how is it initialized in the original, we know nothing from the pasted snippet. So assuming that the byte array synByteSOH is the data block you need a checksum of, just introduce a file static variable x_ptr of type byte* and initialize it to your data block:
typedef unsigned char byte;
static byte *x_ptr;
byte CalculateCheckSum (byte txcount)
//...Follows as pasted
//...
//And now we call it elsewhere:
byte synByteSOH[]= {SYN,SYN,SOH,SETSERIALINFO};
x_ptr = synByteSOH
byte Checksum = CalculateCheckSum(4); //4 is the data block size
In general, even in an ObjC project it's perfectly OK to have C functions. No one ever said every piece of code should be in a class method.