This is an example using fork() and pipe() to demonstrate communication from a parent process to a child process.

There some details worth noticing:

• When the program writes error messages, it writes them to stderr. This is common to do with error messages because output to stderr is not buffered. (If the output is buffered, then in some cases we might not get the error message.)
• The loop using read() will end when it finds the EOF marker. The only reason the EOF marker is there is that the parent closed the write end of the pipe after writing into it. If the write end was not closed, there would be no EOF marker and the read() loop would not end.

#include <sys/wait.h>             /* Needed to use wait() */
#include <stdio.h>                  
#include <stdlib.h>
#include <unistd.h>               /* UNIX and POSIX constants and functions (fork, pipe) */
#include <string.h>               /* Needed to use strlen() */

int main(int argc, char *argv[])
{
 int pipefd[2];
 pid_t cpid;
 char buf;
 if (argc != 2) 
  {
   fprintf(stderr, "Usage: %s \n", argv[0]);
   exit(EXIT_FAILURE);
  }
 if (pipe(pipefd) == -1)          /* An error has occurred. */
  {
   fprintf(stderr, "%s", "The call to pipe() has failed.\n");           
   exit(EXIT_FAILURE);
  }
 cpid = fork();                   /* fork() returns the child process's PID  */
 if (cpid == -1)                  /* An error has occurred. */
  {
   fprintf(stderr, "%s", "The call to fork() has failed.\n");
   exit(EXIT_FAILURE);
  }
 if (cpid == 0) 
  {                               /* Child reads from pipe */
   printf("I am the child.\n");
   close(pipefd[1]);              /* Close unused write end */
   printf("The child is about to read from the pipe.\n");
   while (read(pipefd[0], &buf, 1) > 0)
     write(STDOUT_FILENO, &buf, 1);
   write(STDOUT_FILENO, "\n", 1);
   close(pipefd[0]);
   printf("The child has just echoed from the pipe to standard output.\n");
   _exit(EXIT_SUCCESS);
  } 
 else 
  {                               /* Parent writes argv[1] to pipe */
   printf("I am the parent.\n");
   close(pipefd[0]);              /* Close unused read end */
   write(pipefd[1], argv[1], strlen(argv[1]));
   close(pipefd[1]);              /* Closing creates the EOF marker. */
   printf("The parent has just written data into the pipe.\n");
   printf("The parent will now wait for the child to terminate.\n");
   wait(NULL);                    /* Parent waits for the child to terminate */
   exit(EXIT_SUCCESS);
  }
 return 0;
}