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I have a school homework to read the partition table from a binary file mbr.bin
root@marlboro ~/fdisk # file mbr.bin
mbr.bin: x86 boot sector, code offset 0x48
http://cs.pub.ro/~programare/Teme_de_casa/tema6/mbr.bin
how can i do that? any ideea can help!
i've tryed
char a[512];
f=fopen("mbr.bin","rb");
fread(a,1,512,f);
but i don't know how to interpret the data from the char array a.
Please pretty please help!
You mean to tell us that they didn't give you any pointers on interpreting the data in the mbr.bin file?
Or did you just not pay attention?
RTFM
they only gave us the same aproach you gave me... so please try to have a more constructive advice.
If you have a sugestion is really apreciated!
Thank you very much!
Quote:
Originally Posted by spank
http://www.google.comQuote:
Originally Posted by spoon_
i've searched one hour on google and nothing really usefull came up... any ideeas ?
Thanks!
i've read some docs. now i don't understand why the first byte from mbr.bin is displayed "eb" in a hex editor and if i read it in c
fread(a,1,512,f);
x=a[0];
printf("%x ",x);
it shows me "ffffffeb". This happens not only with the first byte. But there are some that are displayed the same like in the hex editor
Help!
Use unsigned char rather than char. A plain char may be signed, as appears to be the case on your system.
i`ve found a python code:
part_types = {
0x00: 'Empty',
0x01: 'DOS 12-bit FAT',
.....
0x50: 'DM (disk manager)',
0x51: 'DM6 Aux1 (or Novell)',
...
i was wondering if i can do smthing like that on c.
if i say part_type[0x51] to go to DM6 Aux1 (or Novell)
You could (for example) use an array of strings and access the strings via array indices (just assign the unused indices an empty string). And do yourself a favour and read a nice tutorial. You'll find plenty.
yes i've solved that... now how can i read the bites from a byte.
for ex i have a char and i want to separate all his bits
thank you!
You should read the chapter about "bitwise operators" in your favourite tutorial/book. Especially bitwise AND (&) and bitshift (<< >>) will be important.
do you have other ideeas?Code:void byte2bit(unsigned char a)
{
char s[8];
int ibit;
s[0] = '\0';
for (ibit = 0; ibit < 8; ibit++)
{
if ((a >> ibit) & 1)
{
strcat (s, "1");
}
else
{
strcat (s, "0");
}
}
puts(s);
}
Replace 8 with CHAR_BIT if you want to be accurate. Oh and you have that whole problem of not having enough room in your string for the null character. This means either:
1 - You're running off the end of your array, with strcat.
2 - You somehow escape trashing your buffer, and instead have no null, so puts vomits the contents of your memory to your screen, assuming that doesn't crash, until it hits a byte that's equivalent to a null, at which point it stops.
3 - You somehow luck out, and it appears to work for a while, however, since you've already done something bad, the rest of us just wait for your next question of "why does my program crash at <some entirely seemingly unrelated point in your program>?"
Quzah.
solved that to... sorry for my lack of experience...
now i want to concatenate two bites from a byte to another byte. i have to use a macro
smthing like this
a: 10101011
b: 11101001
i want to obtain 1011101001 (take the first two from the begining of a and put them in the front of b) can i do this using a macro ?
thank you for your help!
You could for example mask out the 6 unwanted bit from a by using an appropriate bitmask and then shift the result two bits to the left and use bitwise OR to "add" b to the result.
You could also shift a 6 bits to the right, then shift 8 bits to the left and use bitwise OR to "add" b to the result. This will maybe cause a warning by the compiler.
This can of course be done with a makro.
i've done smthing like this because i only needed the decimal number.Code:#define C(x) (x>>7&1)*256+(x>>6&1)*128
next problem... i have a 4 byte entry that is an unsigned int, the problem is that i used a char to read the binary file. how can i obtain the 4 byte int ?
http://www.daniweb.com/code/snippet150.htmlCode:#include <stdio.h>
#include <limits.h>
static const char Bit[] = "01";
char *bits_uchar(char *dest, unsigned char value)
{
char *start = dest;
unsigned char bit;
for ( bit = UCHAR_MAX / 2 + 1; bit > 0; bit >>= 1 )
{
*dest++ = Bit[!!(value & bit)];
}
*dest = 0;
return start;
}
char *bits_ushort(char *dest, unsigned short value)
{
char *start = dest;
unsigned short bit;
for ( bit = USHRT_MAX / 2 + 1; bit > 0; bit >>= 1 )
{
*dest++ = Bit[!!(value & bit)];
}
*dest = 0;
return start;
}
int main(void)
{
unsigned char a = 0xAB, b = 0xE9;
unsigned short c = ((a & 0xC0) << 2) | b;
char binary[sizeof c * CHAR_BIT + 1];
printf("a = 0x%02X = %s\n", a, bits_uchar(binary, a));
printf("b = 0x%02X = %s\n", b, bits_uchar(binary, b));
printf("c = 0x%04X = %s\n", c, bits_ushort(binary, c));
return 0;
}
/* my output
a = 0xAB = 10101011
b = 0xE9 = 11101001
c = 0x02E9 = 0000001011101001
*/
My endianness link was a little vague.
Code:#include <stdio.h>
int main(void)
{
unsigned char b[] = {0x12,0x34,0x56,0x78};
unsigned long value = (b[0] << 24) | (b[1] << 16) | (b[2] << 8) | b[3];
printf("value = 0x%08lX\n", value);
return 0;
}
/* my output
value = 0x12345678
*/
Results may vary depending on the endianness of the target.Code:#include <stdio.h>
int main(void)
{
unsigned char b[] = {0x12,0x34,0x56,0x78};
unsigned long value = (b[3] << 24) | (b[2] << 16) | (b[1] << 8) | b[0];
printf("value = 0x%08lX\n", value);
return 0;
}
/* my output
value = 0x78563412
*/
how can i do this viceversa, i have the long and i want the char [4]
i've tryed smthing like this but it doesn't work well:
Code:int main()
{
unsigned char a[5]={00000};
unsigned long x,i;
printf("x=");scanf("%ld",&x);
a[0]=a[0]|(x>>8);
x=x>>8;
a[1]=a[1]|(x>>8);
x=x>>8;
a[2]=a[2]|(x>>8);
x=x>>8;
a[3]=a[3]|(x>>8);
for(i=0;i<4;i++) printf("%x ",a[i]);
printf("\n");
return 0;
}
Perhaps like this.Code:#include <stdio.h>
#include <limits.h>
void foo(unsigned long value)
{
unsigned char byte[sizeof value];
size_t i;
for ( i = 0; i < sizeof byte / sizeof *byte; ++i )
{
byte[i] = value >> (CHAR_BIT * i);
printf("byte[%lu] = 0x%02X\n", (long unsigned)i, byte[i]);
}
}
int main(void)
{
foo(0x12345678UL);
return 0;
}
/* my output
byte[0] = 0x78
byte[1] = 0x56
byte[2] = 0x34
byte[3] = 0x12
*/