Type Casting problem C

EmtYskY · 12-29-2006, 03:48 PM

Hi

I am to about to make a program reads partition table. I put MBR sector copy on a unsigned char *buffer of 512 bytes of size. I wanted to validate the last fields of MBR which is the Checksum has 2 bytes. First I create a pointer *signature= malloc(2 * sizeof(unsigned char)) and then point it to the 510 position on MBR buffer but It hasn't worked correctly I had only the 1st byte of the checksum.

Any Suggestion please ?

Thanks

dombrowsky · 12-29-2006, 04:22 PM

perhaps my C syntax is a bit rusty, but I believe this is wrong:

*ptr = malloc(size);

In fact, the following code will compile with a warning, but instantly coredumps on my machine:

Code:

int main(void){
  char *ptr = (char*)0;
  printf("%p\n",(void*)ptr);
  *ptr = malloc(16);
  printf("%p\n",(void*)ptr);
  ptr = malloc(16);
  printf("%p\n",(void*)ptr);
}

Comment out the bad line ->

Code:

int main(void){
  char *ptr = (char*)0;
  printf("%p\n",(void*)ptr);
  /* *ptr = malloc(16); */
  printf("%p\n",(void*)ptr);
  ptr = malloc(16);
  printf("%p\n",(void*)ptr);
}

results:

Code:

0x0
0x0
0x660578

Hko · 12-29-2006, 05:11 PM

Quote:

Originally Posted by EmtYskY

First I create a pointer *signature= malloc(2 * sizeof(unsigned char)) and then point it to the 510 position on MBR buffer [..]

If set the pointer signature to some place in some other buffer, then the address of the new malloc()ed memory is lost. This is a memory leak. You can just point signature to buffer+510, or read directly from buffer[510] and buffer[511] to get the checksum.

Quote:

Originally Posted by EmtYskY

[..] but It hasn't worked correctly I had only the 1st byte of the checksum.

You probably need to cast buffer[510] to the type of the checksum. As this is 2 bytes, you need to make sure not to read past the end of the buffer.

Probably a safer way is to transfer the bytes to an certain type using bitwise shifting. For example, if you need to read the checksum in a unsigned int, you could do something like:

Code:

unsigned int read_chksum;
read_checksum = (buffer[511] << 8) | buffer[510];
/* switch [511] and [510] depending on endianess of the on-disk checksum*/

Hope this helps.

tuxdev · 12-29-2006, 06:01 PM

In order to gaurantee a 16 bit type, you could also use uint16_t, although that is C99 only.