Binary read from file

**dereach** · 02-27-2008

Hi!
I'm trying read data from binary file.

File looks in hex editor:
12 1A 15 00 AD AE 11 00 16 00

I use getc to recive data.

Code:

...

char num_1, num_2;
int mask, i,;
...

data_file = fopen(where, "rb"); 

...

for(i=1;i< end_file ;i=i+2)
		{
		         num_1 = getc (data_file);
		         num_2 = getc (data_file);
	
                  mask= 0x0000000000000000;	

                 printf ("i=%d mask=%016x mask=%d num_1=%016x num_2=%016x \n", i, mask,  mask, num_1, num_2);
               }
...

But I'm confused with results:
i=0, mask=0000000000000000, mask=0, num_1=0000000000000012, num_2=000000000000001A
i=1, mask=0000000000000000, mask=0, num_1=0000000000000015, num_2=0000000000000000
i=2, mask=0000000000000000, mask=0, num_1=00000000FFFFFFAD, num_2=00000000FFFFFFAE
i=3, mask=0000000000000000, mask=0, num_1=0000000000000011, num_2=0000000000000000
i=4, mask=0000000000000000, mask=0, num_1=0000000000000016, num_2=0000000000000000

Why for i=2 num_1 is FFFFFFAD instead AD ??

Daniel

**vart** · 02-27-2008

getc returns int (you implicetly conver it to char)

char could be unsigned or signed - in your case it is signed, so when it is converted back to int and then printed - you see what you see.

Try to avoid implicit conversions
For example something like

Code:

unsigned char num[2];
size_t read;

...

read = fread(data_file,1,2,num);
if(read == 2)
{
   fprintf("&#37;02X %02X", (unsigned int)num[0], (unsigned int)num[1]);
}

**matsp** · 02-27-2008

It should, however, be sufficient to change the char to unsigned char - it will then be automatically converted to int in the call to printf, but since unsigned char fits without touching the sign bit in an int, it is fine to do that.

--
Mats

**vart** · 02-27-2008

Originally Posted by matsp

It should, however, be sufficient to change the char to unsigned char - it will then be automatically converted to int in the call to printf, but since unsigned char fits without touching the sign bit in an int, it is fine to do that.

--
Mats

As long as getc does not returns EOF

**matsp** · 02-27-2008

Originally Posted by vart

As long as getc does not returns EOF

Of course.

--
Mats

**dereach** · 02-27-2008

Thats right.

Thanks for help.

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