Read and Write
Read and write operations are available, as shown in Table 8.2.
Table 8.2: Read and write operations.
The read operation returns data from the pipe to the calling task. The task specifies how much data to read. The task may choose to block waiting for the remaining data to arrive if the size specified exceeds what is available in the pipe. Remember that a read operation on a pipe is a destructive operation because data is removed from a pipe during this operation, making it unavailable to other readers. Therefore, unlike a message queue, a pipe cannot be used for broadcasting data to multiple reader tasks.
A task, however, can consume a block of data originating from multiple writers during one read operation.
The write operation appends new data to the existing byte stream in the pipe. The calling task specifies the amount of data to write into the pipe. The task may choose to block waiting for additional buffer space to become free when the amount to write exceeds the available space.
No message boundaries exist in a pipe because the data maintained in it is unstructured. This issue represents the main structural difference between a pipe and a message queue. Because there are no message headers, it is impossible to determine the original producer of the data bytes. As mentioned earlier, another important difference between message queues and pipes is that data written to a pipe cannot be prioritized. Because each byte of data in a pipe has the same priority, a pipe should not be used when urgent data must be exchanged between tasks.