Why does octal dump output the ascii code for an integer variable instead of the binary code?

This is the code in C to input a character and integer variable to a file.

int main(void)
{
    FILE *fp;
    char file[20];
    int i = 40;

    printf("Enter filename: ");
    scanf("%s", file);

    fp = fopen(file, "wb");

    fprintf(fp, "%d", i);
    fprintf(fp, "%c%c", '4', '0');

    fclose(fp);

    return 0;
}

So lets say the file name is text and the od command is invoked..

od -xc text

This is the output of the command

0000000    3034    3034
            4 0     4 0

Why does it print the ASCII values for both of the value 40 and not the hex code 0028 for the integer variable?

Answer

fprintf formats all output as text – fprintf( fp, "%d", 42 ) writes the character values '4' and '2' to the output stream, same as fprintf( fp, "%c%c", '4', '2' ) and fprintf( fp, "%s", "42" ).

If you want to write the binary representation of the integer to your output stream, then you need to use fwrite( &i, sizeof i, 1, fp );.