Using Named Pipes

Another way to pass data between COBOL and Java programs is through named pipes. Named pipes are a method for exchanging information between two unrelated processes.

Technically, named pipes are files with known pathnames. Because a named pipe is associated with a pathname, unrelated processes can open the file to begin communications with one another. Because a Java program can open a named pipe just as it would a normal file, no special Java or JNI code is required. By opening the file for reading, a process has access to the reading end of the pipe, and by opening the file for writing, a process has access to the writing end of the pipe. In effect, named pipes allow independent processes to "rendezvous" their I/O streams.

Named pipes can be created in two ways–via the command line or from within a program.

In UNIX, to create a named pipe with the file named "npipe" you can use the following command on the command line:

% mkfifo npipe

Alternatively, you could create the named pipe from within your program using:

int mkfifo(const char *path, mode_t mode)

where "path" is the path of the file and "mode_t" is the mode (permissions) with which the file should be created.

A named pipe can be opened using the open( ) system call or the fopen( ) standard C library function. See Working with C and C++ Programs for information on interfacing ACUCOBOL-GT programs to C routines.

As with normal files, if the call succeeds, you get either a file descriptor or a "FILE" structure pointer, depending on how you opened the file. You can then use this information for reading or writing, depending on the parameters you passed to open( ) or fopen( ).

Reading from and writing to a named pipe are very similar to reading from and writing to a normal file. You can use the standard C library function calls read( ) and write( ).

Named pipes can also be used on Windows systems. You create Windows pipes with the CreateNamedPipe( ) API. You can then use the OpenFile( ) API to access the other end of the newly created named pipe.

Although named pipes can be very effective for communicating between COBOL and Java applications, bear in mind the following issues:

• Named pipes work only for processes on the same host machine.
• Named pipes can be created only in the local file system of the host.
• Named pipe data is a byte stream, and no record identification exists.
• Named pipes provide only a half-duplex flow of data. They are also known as "fifos" for their method of "first in, first out" communication. To establish full-duplex communication, you must create and manage two pipes, which can be complicated and result in file deadlocks if you are not careful.