binary file reading in C

[flgeyer]

Hello all,

I'm trying to read in a binary file in C. I have the specification of the file format and everything. I can parse the file with a software called "010 Editor". This way I can check if the values I read in in my C program are correct.

The problem I have is parsing 32 bit integer values in C.

This is the pretty simple code:

    int temp = 0;
    fread(&temp,sizeof(int),1,fp);
    printf("%i\t",temp);
 

It works fine for most of the values in the file. But for some values in the file it is just not printing out the correct integer value.

E.g. :
binary file hex values: 8A 1A 32 01
integer value I get in C: 138
It should be : 20060810

-> 138 is the unsigned byte value of 8A. Am I just reading in 8A?

For the following example it works:
binary file hex values: E1 7A 7F 00
integer value I get in C: 8354529
It should be : 8354529

I also tried to read in the data bytewise from the file and combinig it to an integer by shifting bytewise.

Code:

    int temp = 0;
    temp  = getc(fp);
    temp |= getc(fp) << 8;
    temp |= getc(fp) << 16;
    temp |= getc(fp) << 24;
    printf("%i\t",temp);
 

The results are the same...

I'm using the MS Visual Studio compiler on Windows.
I'm stuck with this problem for quite a while now and I would appreciate any help I can get.

Thank You!

[Banfa]

Have you opened the file in binary mode (add a b to the end of the mode string) because

0x1A = 26 = ASCII Ctrl-Z

which is often(well used to be) used to indicate the end of a text file so the read could be stopping because it thinks it has reached the end of file if you have opened the file in text mode.

[flgeyer]

Thank you very much!

I think that solved my problem. I never thought about this..
I just added that "b" and everything seems to work now.

I also have to read in 24 bit and 8 bit integers.
I thought this piece of code should do the trick:

int read_int24(FILE *fp)
{
    int temp = 0;
    temp  = (int)getc(fp);
    temp |= (int)getc(fp) << 8;
    temp |= (int)getc(fp) << 16;
 
    return temp;
}
 
int read_int8(FILE *fp)
{
    int temp = (int)getc(fp);
    return temp;
}
 

It turns out that I'm just reading in 255 or 0 for 8 bit integers. I'm also not sure about the 24 bit integer values.
Do you have an idea what I'm doing wrong here? I think it's somthing with those byte-shifting operators I don't understand... :(
Maybe there is also an easier way to do this?

Thanks again, you are a great help!

[Banfa]

That code looks correct to me. The only possibility I can think of is that you have got to the end of the file. At the end of the file getc returns EOF and that often has a value -1 (all bits set), but that doesn't explain the 0's.

I would be stepping through the code with a symbolic debugger at this point.

[Zebi Ben]

Hi
I have a problem in reading binary files plz help
i have used fread function to read in binary files

Header *head;//structure holding 
if ((fp = fopen("abc.bin", "rb+")) == NULL) return -1;
    bytes_read=fread((char *)head, sizeof(Header ),1, fp);
    //bytes_read = fread(buf, sizeof(buf), 1, fp);
    if ( bytes_read ) {
                printf( "Number of characters has been read = %i\n", bytes_read );
        printf( "buffer = %s\n", head);}
 

But this code reads only 1st byte
I want to read the entire file .. How can i do tht.. plz help...Also how can i get the values from binary file in the structure variable