Convert string to hexadecimal

Problem :
Convert a string to a hexadecimal number, for example, "12" is converted to 0x12, "12345678" is converted to 0x12345678.
Note : Before conversion, each character occupies one byte, after conversion two digits occupy one byte, such as "12345678" occupies 8 bytes, after conversion occupies four bytes: 0x12, 0x34, 0x56, x078.
Implementation :

#include <stdio.h>
#include <ctype.h>
int char2bcd(char str, unsigned char *value)
{
	if(!str || !value){
		return -1;
	}
	
	if(isdigit(str)){
		*value = str - '0';
	}else if(islower(str)){
		*value = str - 'a' + 10;
	}else if(isupper(str)){
		*value = str- 'A' + 10;
	}else{
		return -1;
	}
	return 0;
}
int getValueFromStr(char *string, unsigned int length, unsigned char *value)
{
	int i, ret;
	unsigned char p,q;
	if(!string || length < 0 || length>8){
		return -1;
	}

	//*value = 0;
	
	if(length%2){
		ret = char2bcd(string[0], &p);
		if(ret<0){
			return -1;
		}
		value[0] = p;
		
		for( i =1 ;i< ((length+1)>>1); i++){

			ret = char2bcd(string[2*i - 1], &p);
			if(ret < 0){
				return -1;
			}
			ret = char2bcd(string[2*i], &q);
			if(ret < 0){
				return -1;
			}
			
			//*value = (*value << 8) + (p << 4) + q;
			value[i] = (p << 4) + q;
		}
	}else{
		for( i =0 ;i< (length>>1); i++){
			ret = char2bcd(string[2*i], &p);
			if(ret < 0){
				return -1;
			}
			ret = char2bcd(string[2*i + 1], &q);
			if(ret < 0){
				return -1;
			}
			
			//*value = (*value << 8) + (p << 4) + q;
			value[i] = (p << 4) + q;
		}	
	}

	return 0;
}