变量赋值(引用) php内核的实现(二）

<?php
$a=1;
$b=&$a;
$c=2;
$a=&$c;
echo $a."\n";
echo $b;

2

1

结论:

首先保存 左值的内存地址， 因这个内存地址会被再次被赋值

1)右值是引用

　　进入2.2 2.3 2.4步骤

　　例子：

<?php
　　$a=1;
　　$c=2;
　　$b=&$a;  //执行到这里时，属于第2种情况
　　$c=&$a;  //执行到这里时，属于第1种情况,

2)右值不是引用，右值的refcount_gc减1

　　2.1)如果refcount_gc减1，大于0 ，说明有别的变量也共同使用了zval,需要单独分配内存给右值

　　2.2）将右值（内存地址）赋值给左值

　　2.3）refcount_gc 加1，并设置 is_ref=1　

　　2.4)销毁左值

　　　　2.3.1）将上面保存的左值的zval的refcount_gc减1

　　　　　　2.3.1.1）上面值为0，则zval_dtor

　　　　　　2.3.1.2)上面值大于0，则进入GC buffer, 但zval类型必须为 object或 array

　　



可以发现$a,$b,$c全是CV变量

当php解释器执行到$b=&$a时，会执行到下面的handler

static int ZEND_FASTCALL  ZEND_ASSIGN_REF_SPEC_CV_CV_HANDLER(ZEND_OPCODE_HANDLER_ARGS)
{
USE_OPLINE
zend_free_op free_op2;
zval **variable_ptr_ptr;
zval **value_ptr_ptr;

SAVE_OPLINE();
value_ptr_ptr = _get_zval_ptr_ptr_cv_BP_VAR_W(EX_CVs(), opline->op2.var TSRMLS_CC);

if (IS_CV == IS_VAR &&
value_ptr_ptr &&
!Z_ISREF_PP(value_ptr_ptr) &&
opline->extended_value == ZEND_RETURNS_FUNCTION &&
!EX_T(opline->op2.var).var.fcall_returned_reference) {
if (free_op2.var == NULL) {
PZVAL_LOCK(*value_ptr_ptr); /* undo the effect of get_zval_ptr_ptr() */
}
zend_error(E_STRICT, "Only variables should be assigned by reference");
if (UNEXPECTED(EG(exception) != NULL)) {

HANDLE_EXCEPTION();
}
return ZEND_ASSIGN_SPEC_CV_CV_HANDLER(ZEND_OPCODE_HANDLER_ARGS_PASSTHRU);
} else if (IS_CV == IS_VAR && opline->extended_value == ZEND_RETURNS_NEW) {
PZVAL_LOCK(*value_ptr_ptr);
}
if (IS_CV == IS_VAR && UNEXPECTED(EX_T(opline->op1.var).var.ptr_ptr == &EX_T(opline->op1.var).var.ptr)) {
zend_error_noreturn(E_ERROR, "Cannot assign by reference to overloaded object");
}

variable_ptr_ptr = _get_zval_ptr_ptr_cv_BP_VAR_W(EX_CVs(), opline->op1.var TSRMLS_CC);
if ((IS_CV == IS_VAR && UNEXPECTED(value_ptr_ptr == NULL)) ||
(IS_CV == IS_VAR && UNEXPECTED(variable_ptr_ptr == NULL))) {
zend_error_noreturn(E_ERROR, "Cannot create references to/from string offsets nor overloaded objects");
}
zend_assign_to_variable_reference(variable_ptr_ptr, value_ptr_ptr TSRMLS_CC); //在这里执行分配的操作

if (IS_CV == IS_VAR && opline->extended_value == ZEND_RETURNS_NEW) {
Z_DELREF_PP(variable_ptr_ptr);
}

if (RETURN_VALUE_USED(opline)) {
PZVAL_LOCK(*variable_ptr_ptr);
AI_SET_PTR(&EX_T(opline->result.var), *variable_ptr_ptr);
}

CHECK_EXCEPTION();
ZEND_VM_NEXT_OPCODE();
}

static void zend_assign_to_variable_reference(zval **variable_ptr_ptr, zval **value_ptr_ptr TSRMLS_DC)
{
zval *variable_ptr = *variable_ptr_ptr; 　　　
zval *value_ptr = *value_ptr_ptr; 　　　　　　

if (variable_ptr == &EG(error_zval) || value_ptr == &EG(error_zval)) {
variable_ptr_ptr = &EG(uninitialized_zval_ptr);
} else if (variable_ptr != value_ptr) {
if (!PZVAL_IS_REF(value_ptr)) { //此时右值不是一个引用
/* break it away */
Z_DELREF_P(value_ptr);  //refcount_gc减1 的作用 是看 是否还有其他变量也使用了valu_ptr_ptr对应的zval,如果有，则重新分配zval
if (Z_REFCOUNT_P(value_ptr)>0) {
ALLOC_ZVAL(*value_ptr_ptr);
ZVAL_COPY_VALUE(*value_ptr_ptr, value_ptr);
value_ptr = *value_ptr_ptr;
zendi_zval_copy_ctor(*value_ptr);
}
Z_SET_REFCOUNT_P(value_ptr, 1);  //因为上面减1了，所以这里要加1，
Z_SET_ISREF_P(value_ptr);//设置 is_ref为1
}

*variable_ptr_ptr = value_ptr; //将variable_ptr_ptr这个地址指针内容 为 1 的地址
Z_ADDREF_P(value_ptr); //还要将 refcount_gc加1

zval_ptr_dtor(&variable_ptr); //根据情况释放内存

} else if (!Z_ISREF_P(variable_ptr)) {
if (variable_ptr_ptr == value_ptr_ptr) {
SEPARATE_ZVAL(variable_ptr_ptr);
} else if (variable_ptr==&EG(uninitialized_zval)
|| Z_REFCOUNT_P(variable_ptr)>2) {
/* we need to separate */
Z_SET_REFCOUNT_P(variable_ptr, Z_REFCOUNT_P(variable_ptr) - 2);
ALLOC_ZVAL(*variable_ptr_ptr);
ZVAL_COPY_VALUE(*variable_ptr_ptr, variable_ptr);
zval_copy_ctor(*variable_ptr_ptr);
*value_ptr_ptr = *variable_ptr_ptr;
Z_SET_REFCOUNT_PP(variable_ptr_ptr, 2);
}
Z_SET_ISREF_PP(variable_ptr_ptr);
}
}

//zend_variables.c

ZEND_API void _zval_copy_ctor_func(zval *zvalue ZEND_FILE_LINE_DC)
{
switch (Z_TYPE_P(zvalue) & IS_CONSTANT_TYPE_MASK) {
case IS_RESOURCE: {
TSRMLS_FETCH();

zend_list_addref(zvalue->value.lval);
}
break;
case IS_BOOL:
case IS_LONG:
case IS_NULL:
break;
case IS_CONSTANT:
case IS_STRING:
CHECK_ZVAL_STRING_REL(zvalue);
if (!IS_INTERNED(zvalue->value.str.val)) {
zvalue->value.str.val = (char *) estrndup_rel(zvalue->value.str.val, zvalue->value.str.len);
}
break;
case IS_ARRAY:
case IS_CONSTANT_ARRAY: {
zval *tmp;
HashTable *original_ht = zvalue->value.ht;
HashTable *tmp_ht = NULL;
TSRMLS_FETCH();

if (zvalue->value.ht == &EG(symbol_table)) {
return; /* do nothing */
}
ALLOC_HASHTABLE_REL(tmp_ht);
zend_hash_init(tmp_ht, zend_hash_num_elements(original_ht), NULL, ZVAL_PTR_DTOR, 0);
zend_hash_copy(tmp_ht, original_ht, (copy_ctor_func_t) zval_add_ref, (void *) &tmp, sizeof(zval *));
zvalue->value.ht = tmp_ht;
}
break;
case IS_OBJECT:
{
TSRMLS_FETCH();
Z_OBJ_HT_P(zvalue)->add_ref(zvalue TSRMLS_CC);
}
break;
}
}

//zend_API.h

#define CHECK_ZVAL_STRING_REL(z) \
if (Z_STRVAL_P(z)[ Z_STRLEN_P(z) ] != '\0') { zend_error(E_WARNING, "String is not zero-terminated (%s) (source: %s:%d)", Z_STRVAL_P(z) ZEND_FILE_LINE_RELAY_CC); }

//zend_alloc.h

#define estrndup_rel(s, length)                    _estrndup((s), (length) ZEND_FILE_LINE_RELAY_CC ZEND_FILE_LINE_CC)

//zend_alloc.c

ZEND_API char *_estrndup(const char *s, uint length ZEND_FILE_LINE_DC ZEND_FILE_LINE_ORIG_DC)
{
char *p;
#ifdef ZEND_SIGNALS
TSRMLS_FETCH();
#endif

HANDLE_BLOCK_INTERRUPTIONS();

p = (char *) _emalloc(length+1 ZEND_FILE_LINE_RELAY_CC ZEND_FILE_LINE_ORIG_RELAY_CC);
if (UNEXPECTED(p == NULL)) {
HANDLE_UNBLOCK_INTERRUPTIONS();
return p;
}
memcpy(p, s, length);
p[length] = 0;
HANDLE_UNBLOCK_INTERRUPTIONS();
return p;
}

//zend_execute_API.c
ZEND_API void _zval_ptr_dtor(zval **zval_ptr ZEND_FILE_LINE_DC) /* {{{ */
{
#if DEBUG_ZEND>=2
printf("Reducing refcount for %x (%x): %d->%d\n", *zval_ptr, zval_ptr, Z_REFCOUNT_PP(zval_ptr), Z_REFCOUNT_PP(zval_ptr) - 1);
#endif
Z_DELREF_PP(zval_ptr);
if (Z_REFCOUNT_PP(zval_ptr) == 0) {
TSRMLS_FETCH();

if (*zval_ptr != &EG(uninitialized_zval)) {
GC_REMOVE_ZVAL_FROM_BUFFER(*zval_ptr);
zval_dtor(*zval_ptr);
efree_rel(*zval_ptr);
}
} else {
TSRMLS_FETCH();

if (Z_REFCOUNT_PP(zval_ptr) == 1) {
Z_UNSET_ISREF_PP(zval_ptr);
}

GC_ZVAL_CHECK_POSSIBLE_ROOT(*zval_ptr);
}
}

//zend_variables.c

ZEND_API void _zval_dtor_func(zval *zvalue ZEND_FILE_LINE_DC)
{
switch (Z_TYPE_P(zvalue) & IS_CONSTANT_TYPE_MASK) {
case IS_STRING:
case IS_CONSTANT:
CHECK_ZVAL_STRING_REL(zvalue);
STR_FREE_REL(zvalue->value.str.val);
break;
case IS_ARRAY:
case IS_CONSTANT_ARRAY: {
TSRMLS_FETCH();

if (zvalue->value.ht && (zvalue->value.ht != &EG(symbol_table))) {
/* break possible cycles */
Z_TYPE_P(zvalue) = IS_NULL;
zend_hash_destroy(zvalue->value.ht);
FREE_HASHTABLE(zvalue->value.ht);
}
}
break;
case IS_OBJECT:
{
TSRMLS_FETCH();

Z_OBJ_HT_P(zvalue)->del_ref(zvalue TSRMLS_CC);
}
break;
case IS_RESOURCE:
{
TSRMLS_FETCH();

/* destroy resource */
zend_list_delete(zvalue->value.lval);
}
break;
case IS_LONG:
case IS_DOUBLE:
case IS_BOOL:
case IS_NULL:
default:
return;
break;
}
}

//zend.h

#define STR_FREE_REL(ptr) if (ptr && !IS_INTERNED(ptr)) { efree_rel(ptr); }

#define efree_rel(ptr)                            _efree((ptr) ZEND_FILE_LINE_RELAY_CC ZEND_FILE_LINE_CC)

//zend_alloc.c

ZEND_API void _efree(void *ptr ZEND_FILE_LINE_DC ZEND_FILE_LINE_ORIG_DC)
{
TSRMLS_FETCH();

if (UNEXPECTED(!AG(mm_heap)->use_zend_alloc)) {
AG(mm_heap)->_free(ptr);
return;
}
_zend_mm_free_int(AG(mm_heap), ptr ZEND_FILE_LINE_RELAY_CC ZEND_FILE_LINE_ORIG_RELAY_CC);
}

#include <stdio.h>
#include <stdlib.h>
int main()
{
char *variable="abc";
char **variable_ptr_ptr=variable;

char *variable_ptr=*variable_ptr_ptr;

char *value="def";
char **value_ptr_ptr=value;
char *value_ptr=*value_ptr_ptr;

*variable_ptr_ptr=value_ptr;
return 0;
}