unsigned characters and sprintf() C - c

Question

I have this code:
int main(){
char buffer[1024];
char port = 1;
int length = 255;
char * record = "$TAG ,0 ,89 ,0, 1\n";
if(length < 0 || length > 255){
printf("Error - length out of range for an unsigned char.\n");
exit(1);
}
snprintf(buffer, 1024, "%c%c%s", port, (unsigned char) length, record);
int port_rc = buffer[0];
int length_rc = buffer[1];
printf("port_rc: %d\n",port_rc);
printf("length_rc: %d\n",length_rc);
return 0;
}
Output when I run it:
port_rc: 1
length_rc: -1
I think I am missing something here in terms of snprintf() as i'm not seeing the 255 value when reading the array it created back. My guess is that snprintf() is promoting the variable 'length' to an int or something. Does anyone know how I can achieve this?
Thanks.

Answer 1

I don't think you can use sprintf() to store 255 into the buffer. The buffer argument to sprintf() is a char array. Whether char is signed or unsigned by default is implementation-defined; see Is char signed or unsigned by default?. If 255 is greater than CHAR_MAX, trying to store 255 results in undefined behavior; if the implementation defaults to signed then CHAR_MAX will probably be 127.
I suggest not using sprintf() to store numbers into the buffer. Declare it as:
unsigned char buffer[127];
Then you can do:
buffer[0] = port;
buffer[1] = length;
snprintf((char *)(buffer + 2), sizeof buffer - 2, "%s", record);

Answer 2

"Be careful, though," when judging such a "solution."
In my humble, the root problem – in your original post - is that the variables port_rc and length_rc should have been declared as unsigned integers. You do not want a value such as $FF to be erroneously "sign-extended" to become $FFFFFFFF == -1 ...
Your "solution" is quite different from the original because, as you see, it now stores into both buffer[0] and buffer[1] before then retrieving and examining those values!

Answer 3

WORKING SOLUTION:
int main(){
unsigned char buffer[1024];
char port = 1;
int length = 255;
char * record = "$TAG ,0 ,89 ,0, 1\n";
if(length < 0 || length > 255){
printf("Error - length out of range for an unsigned char.\n");
exit(1);
}
buffer[0] = port;
buffer[1] = length;
snprintf((char *)(buffer), sizeof buffer - 2, "%c%c%s", port,length,record);
int port_rc = buffer[0];
int length_rc = buffer[1];
char char_first = buffer[2];
printf("port_rc: %d\n",port_rc);
printf("length_rc: %d\n",length_rc);
printf("char_first: %c\n",char_first);
return 0;
}
RETURNS:
port_rc: 1
length_rc: 255
char_first: $