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Zval is one of the most important data structures in PHP, which contains information about the values and types of variables in PHP.

1. The structure of zval1.1 zval (zend_types.h) typedef struct _ zval_struct zval;struct _ zval_struct {zend_value value / * value * / union {struct {ZEND_ENDIAN_LOHI_4 (zend_uchar type, / * active type * / zend_uchar type_flags, zend_uchar const_flags, zend_uchar reserved) / * call info for EX (This) * /} v Uint32_t type_info;} U1; union {uint32_t var_flags; uint32_t next; / * hash collision chain * / uint32_t cache_slot; / * literal cache slot * / uint32_t lineno; / * line number (for ast nodes) * / uint32_t num_args / * arguments number for EX (This) * / uint32_t fe_pos; / * foreach position * / uint32_t fe_iter_idx; / * foreach iterator index * /} U2;}

Zval has a relatively simple structure and consists of three parts:

Zend_value: holds a value or pointer to a specific variable type

The core function of U1 is to distinguish types.

U2 is the auxiliary value

In order to take a more intuitive look at the structure of the zval and the values of the core fields, we have the previous figure.

Description of u1.v.type:

IS_UNDEF: the tag is not defined, indicating that the data can be overwritten or deleted. For example, when unset an array element, PHP 7 will not delete the data directly from the memory allocated to HashTable, but will first mark the location of the Bucket where the element is located as IS_UNDEF, and when the number of IS_ Undef elements in the HashTable reaches a certain threshold, the elements marked by IS_UNDEF will be overwritten or deleted during the rehash operation.

IS_TRUE and IS_FALSE: here the IS_BOOL is optimized into two, recording Boolean tags directly in type.

IS_REFERENCE: a new type that uses different processing methods to handle "&" in PHP7.

IS_INDIRECT: it is also a new type. Because the design of HashTable in PHP 7 is very different from that in PHP5, the IS_INDRECT type is introduced to solve the problem of global symbol table accessing CV variable table.

IS_PTR: this type is defined as a pointer type and is used to parse value.ptr, usually on function types. Such as declaring a function or method.

1.2 zend_value

The definition of Zend _ value in zval is as follows:

Typedef union _ zend_value {zend_long lval; / * long value * / double dval; / * double value * / zend_refcounted * counted; zend_string * str; zend_array * arr; zend_object * obj; zend_resource * res; zend_reference * ref; zend_ast_ref * ast Zval * zv; void * ptr; zend_class_entry * ce; zend_function * func; struct {uint32_t w1; uint32_t w2;} ww;} zend_value;1.3 zval memory footprint

In a zval:

Zend_value is union and occupies only 8 bytes. It just happens to hold a zend_long or a double, or a pointer.

U1 is 4 bytes and stores a v or type_info

U2 is 4 bytes

So a zval takes up 16 bytes. In the corresponding php5, the size of a zval is 48 bytes, which is indeed a huge improvement.

two。 Variable storage 2.1 true, false, null

Can be distinguished directly according to zval.u1.v.type, without the participation of zend_value

2.2 long,double

It is stored directly in lval or dval of zend_value.

2.3 other types (string,array,object,resource, etc.)

Use the pointer corresponding to zend_value to point to its specific structure.

For example, the structure of a string type is

Struct _ zend_string {zend_refcounted_h gc; zend_ulong h; / * hash value * / size_t len; char val [1];}

The memory organization of a string variable is shown in the following figure, with zval.value.str pointing to the zend_string structure.

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