深入解析PHP的引用计数机制_php技巧
PHP的变量声明并赋值后,变量名存在符号表中,而值和类信息存在zval中,zval中包含四个变量,is_ref,refcount,value,type,zval源码如下
struct _zval_struct {
/* Variable information */
zvalue_value value; /* value */
zend_uint refcount__gc;
zend_uchar type; /* active type */
zend_uchar is_ref__gc;
};
refcount表示value地址与其相同的zval共有多少个,refcount=0时,zval被销毁
is_ref表示一个zval是否被引用,有“0”和“1”两种状态
此处分析一下什么时候zval会被复制或者开辟新的内存空间呢
1.当is_ref=0,且refcount>1时,如果改变某个指向该zval的变量的值,会生成新zval,原zval的refcount--,例如:$a=1;$b=$a;$b=2;,zval将被复制,也就是说原先ab指向同一个zval,后来b会使用新开辟的zval
2.当is_ref=0,且refcount>1时,如果将zval赋值给某个引用变量,那么用来赋值和变量和被赋值的变量会使用同一个原zval,而其他指向原zval的变量将会指向一个新复制的zval,且refcount会被重新计算,例如:$a=1;$b=$a;$c=$a;$d=&$a;,此时ad使用原zval,bc使用新复制出来的zval
3.当is_ref=1,且refcount>1时,如果将zval复制给某个非引用变量,该非引用变量会使用一个新复制的zval,元zval的refcount不变,例如:$a=1;$b=&$a;$c=$a,那么ab使用原zval,而c使用新复制的zval
type表示该zval的值类型,宏定义如下
#define IS_NULL 0
#define IS_LONG 1
#define IS_DOUBLE 2
#define IS_BOOL 3
#define IS_ARRAY 4
#define IS_OBJECT 5
#define IS_STRING 6
#define IS_RESOURCE 7
#define IS_CONSTANT 8
#define IS_CONSTANT_ARRAY 9
value表示该zval的值,他也是个共同体,代码如下
typedef union _zvalue_value {
long lval; /* long value */
double dval; /* double value */
struct {
char *val;
int len;
} str;
HashTable *ht; /* hash table value */
zend_object_value obj;
} zvalue_value;
现在你知道php是如何类型变换的了,因为他的值存的其实是个可以代表任何类型的结构体,而具体的取值则根据type来决定是用共同体里的哪个变量来存值的
见下面的例子1
.-----------
$a = 1;
$b = $a;
$c = $a;
.-----------
$d = &$a;
.-----------
$a = 2;
.-----------
$b = null;
查看refcount,is_ref,zval的变化
执行完第一部分后来看看输出
1-----------------------------
a:(refcount=3, is_ref=0),int 1
b:(refcount=3, is_ref=0),int 1
c:(refcount=3, is_ref=0),int 1
可以看出来a,b,c使用同一个zval
再看执行完第二部分的
2----------------------------
a:(refcount=2, is_ref=1),int 1
b:(refcount=2, is_ref=0),int 1
c:(refcount=2, is_ref=0),int 1
d:(refcount=2, is_ref=1),int 1
注意此时a,d在一起了,他们使用同一个zval,而bc使用一个新生成的zval,同时重新计算两个zval的refcount和is_ref
3----------------------------
a:(refcount=2, is_ref=1),int 2
b:(refcount=2, is_ref=0),int 1
c:(refcount=2, is_ref=0),int 1
d:(refcount=2, is_ref=1),int 2
可以知道ad这两个is_ref=1的好基友的值是同时改变的
4----------------------------
a:(refcount=2, is_ref=1),int 2
b:(refcount=1, is_ref=0),null
c:(refcount=1, is_ref=0),int 1
d:(refcount=2, is_ref=1),int 2
bc由于他们的zval的is_ref=0,所以他们不是好基友,他们的值不会同时改变,于是bc的zval再次分裂,b = null c = 1

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