Converting hexadecimal to ASCII in c - c

Question

I'm working on an Android rom for a mobile phone and I want to make the kernel load the wifi MAC address from the device's NV partition. My code looks like this:
#include <linux/kernel.h>
#include <linux/random.h>
#include <linux/syscalls.h>
#define ETHER_ADDR_LEN 6
#define FILE_WIFI_MACADDR "/dev/block/mmcblk0p7"
static int bcm_wifi_get_mac_addr(unsigned char *buf)
{
int ret = 0;
mm_segment_t oldfs;
int i;
int fp;
int macbyte;
int readlen = 0;
uint rand_mac;
static unsigned char mymac[ETHER_ADDR_LEN] = {0,};
const unsigned char nullmac[ETHER_ADDR_LEN] = {0,};
const unsigned char bcastmac[] = {0xFF,0xFF,0xFF,0xFF,0xFF,0xFF};
if (buf == NULL)
return -EAGAIN;
memset(buf, 0x00, ETHER_ADDR_LEN);
oldfs = get_fs();
set_fs(get_ds());
fp = sys_open(FILE_WIFI_MACADDR, O_RDONLY, 0);
if (fp < 0) {
pr_err("%s: Failed to read error %d for %s\n",
__FUNCTION__, fp, FILE_WIFI_MACADDR);
goto random_mac;
}
for (i = 0; i<12; i++) {
macbyte=0;
sys_lseek( fp,i+7680,SEEK_SET);
readlen = sys_read(fp,&macbyte,1);
if (i)
sprintf(macaddr,"%s%c",macaddr,macbyte);
else
sprintf(macaddr,"%c",macbyte);
}
if (readlen > 0) {
unsigned char* macbin;
macbin = (unsigned char*)macaddr;
pr_info("%s: READ MAC ADDRESS %02X:%02X:%02X:%02X:%02X:%02X\n",
__FUNCTION__,
macbin[0], macbin[1], macbin[2],
macbin[3], macbin[4], macbin[5]);
if (memcmp(macbin, nullmac, ETHER_ADDR_LEN) == 0 ||
memcmp(macbin, bcastmac, ETHER_ADDR_LEN) == 0) {
sys_close(fp);
goto random_mac;
}
memcpy(buf, macbin, ETHER_ADDR_LEN);
} else {
sys_close(fp);
goto random_mac;
}
sys_close(fp);
return ret;
random_mac:
set_fs(oldfs);
pr_debug("%s: %p\n", __func__, buf);
if (memcmp( mymac, nullmac, ETHER_ADDR_LEN) != 0) {
/* Mac displayed from UI is never updated..
So, mac obtained on initial time is used */
memcpy(buf, mymac, ETHER_ADDR_LEN);
return 0;
}
srandom32((uint)jiffies);
rand_mac = random32();
buf[0] = 0x00;
buf[1] = 0x90;
buf[2] = 0x4c;
buf[3] = (unsigned char)rand_mac;
buf[4] = (unsigned char)(rand_mac >> 8);
buf[5] = (unsigned char)(rand_mac >> 16);
memcpy(mymac, buf, 6);
pr_info("[%s] Exiting. MAC %02X:%02X:%02X:%02X:%02X:%02X\n",
__FUNCTION__,
buf[0], buf[1], buf[2], buf[3], buf[4], buf[5] );
return 0;
}
The idea is to load the nv parition, located at /dev/block/mmcblk0p7, then read the mac address, which is located at offset 7680 on the nv. The problem is that the MAC address is written in hex, so I'm trying to print it to an ASCII string using sprintf().
for (i = 0; i<12; i++) {
macbyte=0;
sys_lseek( fp,i+7680,SEEK_SET);
readlen = sys_read(fp,&macbyte,1);
if (i)
sprintf(macaddr,"%s%c",macaddr,macbyte);
else
sprintf(macaddr,"%c",macbyte);
}
In the nv the MAC looks something like this: 34 30 42 30 46 41 36 35 39 33 34 39, which in ASCII is 40B0FA659349. But instead the resulting MAC is 34:30:42:30:46:41, which tells me that the hex values are not getting converted at all.
What would be the proper way to export the hex values into an ASCII string? I'm new to programming and i was hoping someone could give me some tips.
Thanks in advance.

Answer 1

In your loop you are reading single bytes and converting them to hex strings, while what you actually need to do is read the hex string and convert it byte values. Unless you actually want a hex string, in which case no conversion is necessary.
You have 12 hex characters representing 6 bytes so:
#define MAC_LEN 6
uint8_t macbytes[MAC_LEN] ;
for( i = 0; i < MAC_LEN; i++ )
{
char hex_str[3] ;
unsigned byte_val ;
sys_lseek( fp, (i*2) + 7680, SEEK_SET ) ;
readlen = sys_read( fp, hex_str, 2 ) ;
sscanf( hex_str, "%2X", &byte_val ) ;
macbytes[i] = (uint8_t)byte_val ) ;
}
The data in NV is already ASCII coded hexadecimal; for example 0x34 is the ASCII code for the hex digit '4', and 0x30 that for '0', together the ASCII character pair "40" represent the single 8 bit integer value 0x40. So the conversion you need is ASCII to byte array, not "hex to ASCII" (which makes no semantic sense).

Answer 2

I think this is OP's stubbing block: forming a string version of the MAC address.
I'll make this wiki for anyone to modify, borrow or steal.
sys_lseek( fp,i+7680,SEEK_SET);
char macaddr[100];
char *p = macaddr;
const char *sep = "";
for (i = 0; i < 12; i++) {
unsigned char macbyte;
int readlen = sys_read(fp, &macbyte, 1);
if (readlen != 1) Handle_Error();
p += sprintf(p, "%s%02X", sep, macbyte);
sep = ":";
}
puts(macaddr);