变量赋值 php内核的实现(二）

2022-10-12 14:08:19

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

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; 　　　//对于$b=&$a , variable_ptr存放的是$b对应的的值zval所对应的内存地址(这里并没有zval) ， variable_ptr_ptr实现存放的是$b变量的地址， $b存放的是对应zval的内存地址
    zval *value_ptr = *value_ptr_ptr; 　　　　　　//对于$b=&$a , value_ptr为 $a的值1的地址，value_ptr_ptr存放的是$a变量的地址，$a存放的是1的内存地址

    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);
    }
}

#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;                                                                                                                                        
}

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变量赋值 php内核的实现(二）

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