Introduction to globals in a PHP extension

In a language such as C, a "global" variable is a variable that can be accessed from any function without any extra declaration. These traditional globals have a few drawbacks:

• Barring any special options passed to the compiler, a global varaible can be accessed and changed by any piece of code anywhere in the program, whether or not that code should be doing so.
• A typical global variable is not thread safe.
• The names of global variables are as global as the variables themselves.

A PHP extension's globals are more properly called the "extension state", since most modules must remember what they're doing between function calls. The "counter" extension is a perfect example of this need: The basic interface calls for a counter with a persistant value. A programmer new to Zend and PHP might do something like this in counter.c to store that value:

/* ... */
static long basic_counter_value;

/* ... */

PHP_FUNCTION(counter_get)
{
    RETURN_LONG(basic_counter_value);
}

On the surface this appears a viable solution, and indeed in a simple test it would function correctly. However, there are a number of situations in which more than one copy of PHP is running in the same thread, which means more than one instance of the counter module. Suddenly these multiple threads are sharing the same counter value, which is clearly undesireable. Another problem shows itself when considering that another extension might someday happen to have a global with the same name, and due to the rules of C scoping, this has the potential to cause a compile failure, or worse, a runtime error. Something more elaborate is needed, and so exists Zend's support for threadsafe per-module globals.

Declaring module globals

Whether a module uses only a single global or dozens, they must be defined in a structure, and that structure must be declared. There are some macros that assist with doing so in a way that avoids name conflicts between modules: ZEND_BEGIN_MODULE_GLOBALS(), ZEND_END_MODULE_GLOBALS(), and ZEND_DECLARE_MODULE_GLOBALS(). All three take as a parameter the short name of the module, which in the case of the counter module is simply "counter". Here is the global structure declaration from php_counter.h:

ZEND_BEGIN_MODULE_GLOBALS(counter)
    long        basic_counter_value;
ZEND_END_MODULE_GLOBALS(counter)

And this is the declaration from counter.c:

ZEND_DECLARE_MODULE_GLOBALS(counter)

Accessing module globals

As discussed above, per-module globals are declared inside a C structure whose name is obscured by Zend macros. As a result, the ideal way to access members of this structure is by the use of further macros. Accordingly, most if not all extensions which have globals have a declaration like this somewhere in their header file:

#ifdef ZTS
#define COUNTER_G(v) TSRMG(counter_globals_id, zend_counter_globals *, v)
#else
#define COUNTER_G(v) (counter_globals.v)
#endif

Note: This could have been generalized into a macro of its own by the Zend API, but as of PHP 5.3 (and PHP 6 at the time of this writing), that hasn't happened. The global accessor construct is written into the header by ext_skel and thus is generally left alone by extension writers, unless they wish to change the name of the accessor macro.

Note: COUNTER_G was the name given to the macro by ext_skel, but it's not necessary for it to have that name and could just as easily be called FOO instead.

Any code in the counter extension that accesses a global must thus wrap it in the macro COUNTER_G.