Recipe 10.17 Writing Data Streams from C

Problem

You need to exchange binary data between C and Java.

Solution

Use the network byte-ordering macros.

Discussion

The program that created the file random.dat read by the program in the previous recipe was not written in Java, but in C. Since the earliest days of the TCP/IP protocol in the 1980s, and particularly on the 4.2 BSD version of Unix, there was an awareness that not all brands of computers store the bytes within a word in the same order, and there was a means for dealing with it. For this early heterogeneous network to function at all, it was necessary that a 32-bit word be interpreted correctly as a computer's network address, regardless of whether it originated on a PDP-11, a VAX, a Sun workstation, or any other kind of machine then prevalent (no "IBM PC" machines were powerful enough to run TCP/IP at that time). So network byte order was established, a standard for which bytes go in which order on the network. And the network byte order macros were written: ntohl for network-to-host order for a long (32 bits), htons for host-to-network order for a short (16 bits), and so on. In most Unix implementations, these C macros live in one of the Internet header files, although in some newer systems, they have been segregated out into a file like <machine/endian.h>, as on our OpenBSD system.

The designers of Java, working at Sun, were well aware of these issues and chose to use network byte order in the Java Virtual Machine. Thus, a Java program can read an IP address from a socket using a DataInputStream or write an integer to disk that will be read from C using read( ) and the network byte order macros.

This C program writes the file random.dat read in Recipe 10.16. It uses the network byte order macros to make sure that the long integer (32 bits on most C compilers on the IBM PC) is in the correct order to be read as an int in Java:

/* Create the random-access file for the RandomAccessFile example  */ #include <stdio.h> #include <fcntl.h> #include <stdlib.h> #include <unistd.h> #include <sys/types.h> #include <machine/endian.h> const off_t OFFSET = 1234; const char* FILENAME = "random.dat"; const int MODE = 0644; const char* MESSAGE = "Ye have sought, and ye have found!\r\n"; int main(int argc, char **argv) {     int fd;     int java_offset;     if ((fd = creat(FILENAME, MODE)) < 0) {         perror(FILENAME);         return 1;     }     /* Java's DataStreams etc. are defined to be in network byte order,      * so convert OFFSET to network byte order.      */     java_offset = htonl(OFFSET);     if (write(fd, &java_offset, sizeof java_offset) < 0) {         perror("write");         return 1;     }     if (lseek(fd, OFFSET, SEEK_SET) < 0) {         perror("seek");         return 1;     }     if (write(fd, MESSAGE, strlen(MESSAGE)) != strlen(MESSAGE)) {         perror("write2");         return 1;     }     if (close(fd) < 0) {         perror("close!?");         return 1;     }     return 0; }

The same technique can be used in the other direction, of course, and when exchanging data over a network socket, and anyplace else you need to exchange binary data between Java and C.