Semaphore Shared Memory example

This is an example of using semaphores to synchronize communication over shared memory between two separate processes. The two processes, server and client, use two semaphores and two shared memory segments to communicate.

The two semaphores are used to signal when each process is done accessing shared memory. The two shared memory segments are a "header" segment and a "data" segment. The header segment is used to describe the size and metadata of the contents of the data segment.

Communication overview

server will ask for a file from the client. The name of the file is chosen at random, and the length (bytes) of this filename is placed in the header segment. This is done so client knows how many bytes to read from the data segment. The filename is then dropped into the data segment. server increments its semaphore, letting client know that it's done. server then waits to decrement the client semaphore; it is blocked until client is finished.

client then reads the length of the filename from the header segment, and then reads that many bytes from the data segment. With the filename known, client loads the file. The size of the file is then placed into the header segment, overwriting the previous value. In this case, since the file is an image, the dimensions and number of channels are also written to the header segment. client then writes the image file's bytes to the data segment and increments its semaphore, letting server know the file is ready to be read.

Finally, server is unblocked and can read the header segment to determine how to read the file in the data segment.

Required knowledge

server and client need to know a few things to cooperate.

• The two semaphore names must be known by both processes
  • POSIX named semaphores are used because they exist as a file outside the processes memory spaces. Unnamed semaphores cannot be used because they exist only within the current process' memory. System V semaphores are not used because they are ancient
• The two shared memory segment names must be known by both processes

API

• The server command is a string whose length is placed in the header segment as a single uint64_t.
• The client response is an image file. The total number of bytes, the dimensions of the image, and the number of channels in the image are placed in the header segment as an array of 4 uint64_t
• Shared memory segments can be resized up, but not down. This shaves off a bit of time when varying size files are sent because the OS is not re-allocating shared memory with each command and response