, Ns_CsDestroy, Ns_CsEnter, Ns_CsInit, Ns_CsLeave - Manage and use critical section locks

#include "ns.h"

void
Ns_CsDestroy(Ns_Cs *csPtr)

void
Ns_CsEnter(Ns_Cs *csPtr)

void
Ns_CsInit(Ns_Cs *csPtr)

void
Ns_CsLeave(Ns_Cs *csPtr)

Critical section locks are used to prevent more than one thread from executing a specific section of code at one time. They are implemented as "objects", which simply means that memory is allocated to hold the lock state. They can also be called "sychronization objects".

While a thread is executing a critical section of code, all other threads that want to execute that same section of code must wait until the lock surrounding that critical section has been released.

This is crucial to prevent race conditions which could put the server into an unknown state. For example, if a section of code frees a pointer and then decrements a counter that stores how many pointers exist, it is possible that the counter value and the actual number of pointers may be different. If another section of the server relies on this counter and reads it when the pointer has been freed, but the counter has not yet been decremented, it could crash the server or put it into an unknown state.

Critical section locks should be used sparingly as they will adversely impact the performance of the server or module. They essentially cause the section of code they enclose into behaving in a single-threaded manner. If a critical section executes slowly or blocks, other threads that must execute that section of code will begin to block as well until the critical section lock is released.

You will normally want to wrap sections of code that are used to both read and write values, create and destroy pointers and structures or otherwise look at or modify data in the system. Use the same named lock for both read and write operations on the same data.

Threads that are waiting for a critical section lock to be released do not have to poll the lock. The critical section lock functions use thread condition functions to signal when a lock is released.

Ns_CsDestroy(csPtr)

Destroy a critical section object. Note that you would almost never need to call this function as synchronization objects are typically created at startup and exist until the server exits.

The underlying objects in the critical section are destroyed and the critical section memory returned to the heap.

Ns_CsEnter(csPtr)

Lock a critical section object, initializing it first if needed. If the critical section is in use by another thread, the calling thread will block until it is no longer so.

Note that critical sections are recursive and must be exited the same number of times as they were entered.

Ns_CsInit(csPtr)

Initialize a critical section object. Memory will be allocated to hold the object's state.

Ns_CsLeave(csPtr)

Unlock a critical section once. A count of threads waiting to enter the critical section is kept, and a condition is signaled if this is the final unlock of the critical section so that other threads may enter the critical section.

nsd(1), info(n), Ns_MasterLock(3), Ns_MasterUnlock(3), Ns_CondDestroy(3), Ns_CondSignal(3), Ns_CondWait(3), Ns_MutexLock(3), Ns_MutexUnlock(3)