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:

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    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:

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    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:

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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

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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

binary file reading in C

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