Using shared memory
As with pipes, the MQ data is lost once consumed. Duplex message data copying increases user space-kernel space calls, therefore an overhead is to be expected. The shmem mechanism is fast. As you learned in the previous chapter and the previous section, the synchronization of the data access is an issue that must be resolved by the system programmer, especially when it comes to race conditions.
An important remark is that the term shared memory is vague in itself. Is it a global variable that two threads could access simultaneously? Or is it a shared region of RAM, which multiple CPU cores use as a common ground to transfer data between each other? Is it a file in the filesystem that many processes modify? Great questions – thanks for asking! In general, all of those are kinds of shared resources, but when we speak about the term memory, we should really think about a region in the main memory that is visible to many processes and where multiple tasks...
