Converting a HEX value to DECimal value in C - c

Question

I have tried out many ideas from SO. One of them worked (output was DEC 49 for HEX 31) when I tested it in here onlinegdb.
But, when I implemented it in my application, it didn't produce same results. (output was 31 for 31 again).
The idea is to use a string value (full of HEX pairs);
An example; 313030311b5b324a1b5b324a534f495f303032371b
I need to convert each integer pair (HEX) into equivalence decimal value.
e.g.
HEX => DEC
31 => 49
30 => 48
I will then send the DEC value using UART value by value.
The code I test the behavior is below and here;
But, it doesn't have to be that code, I am open to all suggestions as long as it does the job.
#include <stdio.h>
int isHexaDigit(char p) {
return (( '0' <= p && p <= '9' ) || ( 'A' <= p && p <= 'F'));
}
int main(int argc, char** argv)
{
char * str = "31";
char t[]="31";
char* p = t;
char val[3]; // 2 hexa digit
val[2] = 0; //and the final \0 for a string
int number;
while (isHexaDigit(*p) && isHexaDigit(*(p+1))) {
val[0] = *p;
val[1] = *(p+1);
sscanf(val,"%X", &number); // <---- Read hexa string into number
printf("\nNum=%i",number); // <---- Display number to decimal.
p++;
//p++;
if (!*p) break;
p++;
}
return 0;
}
EDIT
I minimized the code.
Odd-length string is ignored for the time being.
The code sends out the data byte by byte. In the terminal application,
I get the values as HEX, e.g. HEX 31 instead of DEC 49. They are actually same. But, a device I use requires DEC 49 version of the value (which is ASCII = 1)
Any pointer highly appreciated.

Answer 1

You can use strtol function to convert your hex string to binary and then convert it to a decimal string in a single line:
snprintf(str_dec, 4, "%ld", strtol(str_hex, NULL, 16));
Your code becomes:
#include <stdio.h>
#include <stdlib.h>
int isHexaDigit(char p) {
return (( '0' <= p && p <= '9' ) || ( 'A' <= p && p <= 'F'));
}
int main(int argc, char** argv)
{
char * str = "31";
char t[]="31";
char* p = t;
char str_hex[3] = {0,};
char str_dec[4] = {0,};
while (isHexaDigit(*p) && isHexaDigit(*(p+1))) {
str_hex[0] = *p;
str_hex[1] = *(p+1);
/* Convert hex string to decimal string */
snprintf(str_dec, 4, "%ld", strtol(str_hex, NULL, 16));
printf("str_dec = %s\n", str_dec);
/* Send the decimal string over UART1 */
if (str_dec[0]) UART1_Write(str_dec[0]);
if (str_dec[1]) UART1_Write(str_dec[1]);
if (str_dec[2]) UART1_Write(str_dec[2]);
/* Reset str_dec variable */
str_dec[0] = 0;
str_dec[1] = 0;
str_dec[2] = 0;
p++;
if (!*p) break;
p++;
}
return 0;
}