[SOLVED] How do I print the value of a byte in C?

stf92 · 07-21-2014, 10:24 AM

Hi: Let's say I read 512 bytes from a sector into variable sector.

Code:

unsigned char sector[512];
FILE * infile;

int main(){
    .................
    infile= fopen("/dev/sda", "r");
    fread(sector, sizeof(sector), 1, infile);
    .................

What I read into sector could be something like

Code:

00000000  fa eb 21 01 b4 01 4c 49  4c 4f 17 02 76 1e c4 53  |úë!.´.LILO..v.ÄS|
00000010  00 00 00 00 57 b9 e5 01  42 07 f3 4c 81 00 80 60  |....W¹å.B.óL...`|

Now, how could I print the contents of sector? The first byte is 0xfa. So, I should print "f" and "a" to see the first byte. How do I translate 0xfa to the string "fa"?

|Assuming the byte is in c, of type char, I could do

Code:

char car;
car = c >> 4;
if(car > 9) car = car + 7;
car = car + 0x30 // "0" is ASCII char 0x30
printf("%s", car);

And I can print the low nibble in an analogous way. And repeat this operation for each of the 512 bytes in sector. But is there not a more simple way to do this?

gengisdave · 07-21-2014, 10:32 AM

use printf("%x", car);

ntubski · 07-21-2014, 10:59 AM

use printf("%02x", car);

NevemTeve · 07-21-2014, 11:09 AM

use

Code:

printf("%02x", (unsigned char)car);

stf92 · 07-21-2014, 11:14 AM

Thank you very much. I could print the contents of the MBR this way! First time I do something like this in Linux. What is weird is that the program does not need to get any kind of permissions to read the hard disk sectors. The only provision was to run it as root! I wonder what would happen if I open the device for write instead of read.

@NevemTeve: I already took the precaution of declaring car of type unsigned char. But it took me a while to realize I should not declare it just char.

NevemTeve · 07-21-2014, 11:25 AM

> The only provision was to run it as root!

No biggie, huh?