If your z/OS® UNIX XL C/C++ application program forks processes that need to communicate among themselves for work to be done, you can take advantage of POSIX.1-defined unnamed pipes. If your application program's processes need to communicate with other processes that it did not fork, you should use the POSIX.1-defined named pipe (FIFO special file) support. See Using named pipes for more information.
When you code the pipe() function to create a pipe, you pass a pointer to a two-element integer array where pipe() puts the file descriptors it creates. One descriptor is for the input end of the pipe, and the other is for the output end of the pipe. You can code your application so that one process writes data to the input end of the pipe and another process reads from the output end on a first-in-first-out basis. You can also build a stream on the pipe by using fdopen(), and use buffered I/O functions. The result is that you can communicate data between a parent process and any of its child processes.
The opened pipe is assigned the two lowest-numbered file descriptors available.
z/OS UNIX provides no security checks for unnamed pipes, because such a pipe is accessible only by the parent process that creates the pipe and any of the parent process's descendent processes. When the parent process ends, an unnamed pipe created by the process can still be used, if needed, by any existing descendant process that has an open file descriptor for the pipe.
Consider the sample program (CCNGHF1) in Figure 1. In this example, where you open a pipe, do a write operation, and later do a read operation from the pipe. For more information on the pipe() function and the file I/O functions, see z/OS XL C/C++ Runtime Library Reference.
/* this example shows how unnamed pipes may be used */
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
int main() {
int ret_val;
int pfd[2];
char buff[32];
char string1[]="String for pipe I/O";
ret_val = pipe(pfd); /* Create pipe */
if (ret_val != 0) { /* Test for success */
printf("Unable to create a pipe; errno=%d\n",errno);
exit(1); /* Print error message and exit */
}
if (fork() == 0) {
/* child program */
close(pfd[0]); /* close the read end */
ret_val = write(pfd[1],string1,strlen(string1)); /*Write to pipe*/
if (ret_val != strlen(string1)) {
printf("Write did not return expected value\n");
exit(2); /* Print error message and exit */
}
}
else {
/* parent program */
close(pfd[1]); /* close the write end of pipe */
ret_val = read(pfd[0],buff,strlen(string1)); /* Read from pipe */
if (ret_val != strlen(string1)) {
printf("Read did not return expected value\n");
exit(3); /* Print error message and exit */
}
printf("parent read %s from the child program\n",buff);
}
exit(0);
}