dcmds can be composed into a pipeline using the | operator. The purpose of a pipeline is to pass a list of values, typically virtual addresses, from one dcmd or walker to another. Pipeline stages might be used to map a pointer from one type of data structure to a pointer to a corresponding data structure, to sort a list of addresses, or to select the addresses of structures with certain properties.
MDB executes each dcmd in the pipeline in order from left to right. The left-most dcmd is executed using the current value of dot, or using the value specified by an explicit expression at the start of the command. When a | operator is encountered, MDB creates a pipe (a shared buffer) between the output of the dcmd to its left and the MDB parser, and an empty list of values.
As the dcmd executes, its standard output is placed in the pipe and then consumed and evaluated by the parser, as if MDB were reading this data from standard input. Each line must consist of an arithmetic expression terminated by a NEWLINE or semicolon (;). The value of the expression is appended to the list of values associated with the pipe. If a syntax error is detected, the pipeline is aborted.
When the dcmd to the left of a | operator completes, the list of values associated with the pipe is then used to invoke the dcmd to the right of the | operator. For each value in the list, dot is set to this value and the right-hand dcmd is executed. Only the rightmost dcmd in the pipeline has its output printed to standard output. If any dcmd in the pipeline produces output to standard error, these messages are printed directly to standard error and are not processed as part of the pipeline.