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    <p>Ok, first of all you're not getting a total random intermangled words of HELLO and WORLDs because stdout is buffered.</p> <p>So your parent is printing HELLO until that buffer gets full, and then that whole buffer ends up on your screen. Same for the child.</p> <p>If you want to synchronize this, you could use 2 pipes to communicate between your parent and child. e.g. </p> <ul> <li>parent prints "HELLO", </li> <li>child starts reading from the pipe1. </li> <li>parent writes a character to the pipe1 to signal that the child should print "WORLD"</li> <li>parent starts reading from the pipe2</li> <li>child reads from pipe1, prints "WORLD"</li> <li>child writes a character to pipe2 to signal that the parent should print "HELLO"</li> <li>repeat the above</li> </ul> <p>-</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;unistd.h&gt; void err(const char *error) { perror(error); exit(1); } void run_handler(int readfd,int writefd,const char *message) { for(;;) { char ch; int i; if((i = read(readfd,&amp;ch,1)) == 0) break; else if(i &lt; 0) err("read"); //important: fflush the text so it ends up in the output //and not in an internal FILE* buffer. if(fputs(message,stdout) == EOF || fflush(stdout) == EOF) break; if(write(writefd,&amp;ch,1) != 1) err("write"); } } int main() { int pipe1[2]; int pipe2[2]; pid_t pid; if(pipe(pipe1) != 0) err("pipe1"); if(pipe(pipe2) != 0) err("pipe2"); if((pid = fork()) == 0 ) { //write one char to the parent to get it started char ch = '.'; if(write(pipe1[1],&amp;ch,1) != 1) err("write"); //close the pipe ends we don't need close(pipe1[0]); close(pipe2[1]); run_handler(pipe2[0],pipe1[1],"WORLD\n"); }else if(pid &gt; 0) { //close the pipe ends we don't need close(pipe1[1]); close(pipe2[0]); run_handler(pipe1[0],pipe2[1],"HELLO "); } else { err("fork"); } return 0; } </code></pre>
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