appending binary data to file in c

Question

I open a file in C and perform a CRC32 algorithm on the data. From this I get a checksum which I now want to append to the file so that the bit-code of the int is at the end of the bitcode of the file. But when I write the integer to the file all numbers are interpreted as chars and not the bitcode of the int is written. So I tried this:

    int r, tmp, sum3; 
    for(r = 0; r < 25; r+=8){
        int s;
        sum3 = 0;
        for(s = r; s < r+8; s++){
            tmp = 1;
            int v;
            if(binzahl2[s] == '1'){ //binzahl2 contains the bitcode of the checksum as char array
                for(v = 7; v > s-r; v--)
                    tmp*=2;
                sum3 += tmp;
            }
        }
        int y=fprintf(file, "%c", (char) sum3);
    }

But of course every time sum3 is greater than 127 there's a problem with the cast to char so that as first digit of the byte is written 0 and not 1.

Is there any way to fix this so that the 1 is written at the beginning of the byte? Or is there (hopefully) a better way to append the right binary data?

Answer 1

fprintf is for outputting formatted data (that's what the f at the end stands for). You want to use fwrite instead.

Answer 2

fseek(file, 0, SEEK_END);

fwrite("\n", sizeof(char), 1, file);

char binzahl2[33];
unsinged int checkSum; //some value you have calculated
unsinged int b = 1;

for(i = 31; i > -1 ; i--){
    if( checkSum & (b << i) ){binzahl2[31 - i] = '1';}
    else{binzahl2[31 - i] = '0';}
}

binzahl2[32] = 0;

size_t charCount = strlen(binzahl2);

fwrite(binzahl2, sizeof(char), charCount, file);