Pipes provide the user with a more reliable, synchronized means of interprocess communication. Unnamed pipes can be used only with related processes. The popen system call provides the user with an easy way to generate an unnamed pipe to execute a shell command. Named pipes (FIFOs), which exist as actual directory entries, can be shared by unrelated processes. The amount of data a pipe can contain is limited by the system. When a pipe is no longer associated with any processes, its contents are flushed by the system. The read and write system calls, which can be used with pipes, provide the user with an easy means of coordinating the flow of data in a pipe. Care must be taken when using pipes to prevent deadlock situations. Deadlock can occur when one process opens one end of a pipe for writing and another process opens the other end of the same pipe for writing. Each process in turn is waiting for the other to complete its action.  Pipes can be used only by processes that are running on the same platform. Unfortunately, pipes provide no easy way for a reading process to determine who the writing process was. All processes involved with using pipes must have forehand knowledge of their existence.
 As the unnamed pipe generated by popen is done without the user's direct use of the open system call, should the O_NDELAY or O_NBLOCK flags need to be set, the fcntl system call must be used.