Jun 1, 2006

What the heck is TSRMLS_CC, anyway?

If you've ever worked on the PHP internals or built an extension, you've seen this construct floating around here and there, but noone ever talks about it. Those who know what this is typically answer questions from those who don't with "Don't worry about what it is, just use it here here here and here. And if the compiler says you're missing a tsrm_ls, put it there too..." This isn't laziness on the part of the person answering the question (okay, maybe it is a little bit), it's just that the engine goes so far out of its way to simplify what this magic values does, that there's no profit in a new extension developer knowing the mechanics of it. The information is like a cow's opinion, it doesn't matter, it's Moo.


Since I love to listen to myself rattle on about pointless topics (and I havn't blogged much this month), I thought I'd cover this topic and see if anyone manages to stay awake through it. You can blame Lukas, he got me rolled onto planet-php.net...

Glossary

TSRM
Thread Safe Resource Manager - This is an oft overlooked, and seldom if ever discussed layer hiding in the /TSRM directory of your friendly neighborhood PHP source code bundle. By default, the TSRM layer is only enabled when compiling a SAPI which requires it (e.g. apache2-worker). All Win32 builds have this layer enabled enabled regardless of SAPI choice.


ZTS
Zend Thread Ssafety - Often used synonymously with the term TSRM. Specifically, ZTS is the term used by ./configure ( --enable-experimental-zts for PHP4, --enable-maintainer-zts for PHP5), and the name of the #define'd preprocessor token used inside the engine to determine if the TSRM layer is being used.


tsrm_ls
TSRM local storage - This is the actual variable name being passed around inside the TSRMLS_* macros when ZTS is enabled. It acts as a pointer to the start of that thread's independent data storage block which I'll cover in just a minute


TSRMLS_??
A quartet of macros designed to make the differences between ZTS and non-ZTS mode as painless as possible. When ZTS is not enabled, all four of these macros evaluate to nothing. When ZTS is enabled however, they expand out to the following definitions:
  • TSRMLS_C tsrm_ls
  • TSRMLS_D void ***tsrm_ls
  • TSRMLS_CC , tsrm_ls
  • TSRMLS_DC , void ***tsrm_ls

Globals

In any normal C program (just like in PHP) you have two methods of getting data access to the same block of data in two different functions. One method is to pass the value on the parameter stuck like so:

#include 

void output_func(char *message)
{
printf("%s\n", message);
}

int main(int argc, char *argv[])
{
output_func(argv[0]);

return 0;
}

Alternately, you could store the value in a variable up in the global scope and let the function access it there:

#include 

char *message;

void output_func(void)
{
printf("%s\n", message);
}

int main(int argv, char *argv[])
{
message = argv[0];
output_func();

return 0;
}

Both approaches have their merits and drawbacks and typically you'll see some combination of the two used in a real application. Indeed, PHP is covered in global variables from resource type identifiers, to function callback pointers, to request specific information such as the symbol tables used to store userspace variables. Attempting to pass these values around in the parameter stack would be more than unruly, it'd be impossible for an application like PHP where it's often necessary to register callbacks with external libraries which don't support context data.


So common information, like the execution stack, the function and class tables, and extension registries all sit up in the global scope where they can be picked up and used at any point in the application. For single-threaded SAPIs like CLI, Apache1, or even Apache2-prefork, this is perfectly fine. Request specific structures are initialized during the RINIT/Activation phase, and reset back to their original values during the RSHUTDOWN/Deactivation phase in preparation for the next request. A given webserver like Apache1 can serve up multiple pages at once because it spawns multiple processes each in their own process space with their own independant copies of global data.


Now let's introduce threaded webservers like Apache2-worker, or IIS. Under these conditions, only one process space is active at a given time with multiple threads spun off. Each of these threads then act in the same manner as a single-threaded process might; Servicing requests one-at-a-time as dispatched by inbound requests. The trouble starts to brew as two or more threads try to service the a request at the same time. Each thread wants to use the global scope to store its request-specific information, and tries to do so by writing to the same storage space. At the least, this would result in userspace variables declared in one script showing up in another. In practice, it leads to quick and disasterous segfaults and completely unpredictable behavior as memory is double freed or written with conflicting information by separate threads.


Non-Global Globals

The solution is to require the engine, the core, and any extension using global storage to determine how much memory will be used by request-specific data. Then, at the spin-up of each new thread, allocate a chunk of memory for each of these players to store their data into thus giving each thread its own local storage. In order to group all the individual chuncks used by a given thread together, one last vector of pointers is allocated to store the individual sub-structure pointers into. It's the pointer to this vector which is passed around as the tsrm_ls variable by the TSRMLS_* family of macros. To see how this works, let's look at a example extension:


typedef struct _zend_myextension_globals {
int foo;
char *bar;
} zend_myextension_globals;

#ifdef ZTS
int myextension_globals_id;
#else
zend_myextension_globals myextension_globals;
#endif

/* Triggered at the beginning of a thread */
static void php_myextension_globals_ctor(zend_myextension_globals *myext_globals TSRMLS_DC)
{
myext_globals->foo = 0;
myext_globals->bar = NULL;
}

/* Triggered at the end of a thread */
static void php_myextension_globals_dtor(zend_myextension_globals *myext_globals TSRMLS_DC)
{
if (myext_globals->bar) {
efree(myext_globals->bar);
}
}

PHP_MINIT_FUNCTION(myextension)
{
#ifdef ZTS
ts_allocate_id(&myextension_globals_id, sizeof(zend_myextension_globals),
php_myextension_globals_ctor, php_myextension_globals_dtor);
#else
php_myextension_globals_ctor(&myextension_globals TSRMLS_CC);
#endif

return SUCCESS;
}

PHP_MSHUTDOWN_FUNCTION(myextension)
{
#ifndef ZTS
php_myextension_globals_dtor(&myextension_globals TSRMLS_CC);
#endif

return SUCCESS;
}

Here you can see the extension declaring its global requirements to the TSRM layer by stating that it needs sizeof(zend_myextension_globals) bytes of storage, and providing callbacks to use when initializing (or destroying) a given thread's local storage. The value populated into myextension_globals_id represents the offset (common to all threads) into the tsrm_ls vector where the pointer to that thread's local storage can be found. In the event that ZTS is not enabled, the data storage is simply placed into the true global scope and the thread initialization and shutdown routines are called manually during the Module's Startup and Shutdown phases. If you're wondering why TSRMLS_CC was included in the non-ZTS blocks, then I clearly havn't made you fall asleep yet. Those aren't needed there since we know they evaluate to nothing, but it helps encourage good habits to include them anywhere the function's prototype calls for them.


Putting it all together

The final piece of this thread-safe puzzle comes from the question: "How do I access data in these structures?" And the answer to that question comes in the form of another familiar looking macro. Each extension or core component defines, in one of its header files, a macro which looks something like the following:


#ifdef ZTS
# define MYEXTENSION_G(v) \
(((zend_myextension_globals*)(*((void ***)tsrm_ls))[(myextension_globals_id)-1])->v)
#else
# define MYEXTENSION_G(v) (myextension_globals.v)
#endif

Thus, when ZTS is not enabled, this macro simply plucks the right value out of the imediate value in the global scope, otherwise it uses the ID to locate the thread's local storage copy of the structure and derefence the value from there.


Wanna know more, like how to deal with foreign callbacks where tsrm_ls isn't available? Buy my book!

2 comments:

  1. Sara, I found your post and bought your book. Thanks!

    I'm wondering if in an embedded scenario it is feasible to have the globals persist after a thread has ended, and re-access them from a new thread?

    It looks like ts_resource_ex looks up the globals based on the thread_id (which is problematic, as the thread ID would obviously change). I'm envisioning an "Alter Globals Thread ID" method that could be called at termination of a thread to change the "thread ID" for the globals to a unique "pseudo-thread ID" (i.e. unique, but not associated with a thread.)

    And then from the new thread, this method could be called to change the "pseudo-thread ID" to the "real" thread ID of the new thread.

    Are there other considerations that would make such an idea infeasible?

    (I'm considering this in the context of a stateful implementation of PHP for an application server, where the PHP environment could remain in-memory at the server for the duration of a user session, and subsequent browser interactions could interact with this resident PHP environment without having to explicitly marshal "state" into a blank environment on each call.)

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  2. Excellent article, thanks.

    Considering how widespread PHP is, it is baffling how little information is available on PHP thread safety.

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