int check_file_header(char *addr);
int check_header_length(unsigned char *addr, long offset);
unsigned long first_idat(unsigned char *addr);
int total_idat(unsigned char *addr);
void build_png_file(struct PNG_FILE_STRUCT *png_file, char *out_file_name);
int change_idat_content(unsigned char *addr, struct PNG_FILE_STRUCT *png_file, char *message, int accuracy, unsigned long offset, char *out_file_name);