44.4. Using Pipes to Connect Filters

When a pipe is created, the file descriptors used for the two ends of the pipe are the next lowest-numbered descriptors available. Since, in normal circumstances, descriptors 0, 1, and 2 are already in use for a process, some higher-numbered descriptors will be allocated for the pipe. So how do we bring about the situation shown in Figure 44-1, where two filters (i.e., programs that read from stdin and write to stdout) are connected using a pipe, such that the standard output of one program is directed into the pipe and the standard input of the other is taken from the pipe? And in particular, how can we do this without modifying the code of the filters themselves?

The answer is to use the techniques described in Section 5.5 for duplicating file descriptors. Traditionally, the following series of calls was used to accomplish the desired result: