README

Checkpoint synchronization primitive.
Checkpoints can be used as a primitive for any type of synchronization or as a base for implementations 
of other custom synchronization primitives(rwlocks, semaphores, upgradable_lockables etc).

Checkpoint is a point in program where thread awaits until some condition will hold.
Checkpoints are implemented through method pass(checkp_get(), checkp_set(), name) of global object of type 'checkpoint'. 
Threads wait till checkp_get() condition will hold and then execute checkp_set() function. 
Depending on checkp_get() and checkp_set() arguments to pass(), checkpoints can implement 
any kind of synchronization, and might be used instead of mutexes, rwlocks or condition variables. 
Method pass() has following arguments:
- checkp_get() function is a condition that checks values of flags. If checkp_get() returns zero, 
thread will sleep in kernel until condition will hold and checkp_get() return non-zero.
- checkp_set() function is to update values of flags. Function checkp_set() returns integer value
that can be used by calling code. 
- optional string 'name' to identify checkpoint for debugging.
Execution of program with checkpoints depends on values of flags for checkp_get(). These flags must be
updated in checkp_set(). After each checkp_set() call, all awaiting checkp_get() are checked 
to resume execution of awaiting threads. Functions checkp_get() and checkp_set() are executed 
under global checkpoints lock.

Checkpoint interface:
typedef std::function<int ()> checkp_get;
typedef std::function<int ()> checkp_set;
class checkpoint {
public:
	int pass(checkp_get get, checkp_set set, std::string name = "default");
	int try_pass(checkp_get get, checkp_set set);
	std::string print();
};

For example:
   int shared = 0;
   int locked = 0;
...
        chkp.pass([]()->int {return !locked;}, []()->int {locked = 1; return 0;});
	shared++;
        chkp.pass([]()->int {return locked;}, []()->int {locked = 0; return 0;});
Here checkpoint synchronizes access to shared data

Other checkpoint methods:
try_pass() returns -1 if checkpoint can't be passes instantly, or passes checkpoint and returns set value.
print() returns names of all currently awaiting checkpoints for debugging purposes.

Examples are in the directory test:
test1.cpp is an example of using checkpoints to synchronize access to shared data.
test2.cpp is an example of using checkpoints to synchronize several writer and reader threads.

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