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memtalk

Credit to Julian Goldstein from virt.so for giving me the following prompt as a take-home assignment.

NAME

memtalk - A utility for two processes to talk to each other via a POSIX shared memory object.

SYNOPSIS

memtalk [-f Path to a POSIX shared memory object]

DESCRIPTION

memtalk is a process that can talk to another memtalk processes through a POSIX shared memory object.

memtalk functions in the following way:

  • When memtalk is executed, it checks its -f argument for a valid path to a POSIX shared memory object. If the -f argument is missing, the program returns -1.

  • If the POSIX shared memory object passed to it via the -f argument does not currently exist on the filesystem, memtalk creates it, before opening it.

  • memtalk then uses the resulting file descriptor to map in a page of memory into into its address space that has shared permissions. The size of this allocation is determined via sysconf(3).

  • Once the shared page has been mapped, if memtalk is the one that created the POSIX shared memory object, it writes a 32-bit magic number to the base address to signify that it has initialized the memory.

  • Memory is structured like so:

  +--------------------+
  |  32-bit magic num  |
  +--------------------+
  | ring buffer struct |
  +--------------------+
  |  ring buffer slab  |
  +--------------------+
  | ring buffer struct |
  +--------------------+
  |  ring buffer slab  |
  +--------------------+

  • One ring buffer is used by the first instance of memtalk to send data to the second instance of memtalk and the other is used by the second instance of memtalk to send data to the first instance.

  • memtalk reads stdin and forwards the bytes to its sending ring buffer and writes the receiving ring buffer's bytes to stdout.

EXAMPLE USAGE

Open up terminal 1 and write:

$ cat /usr/share/dict/words | memtalk -f /someobject

Then open up terminal 2 and write:

$ find / | memtalk -f /someobject