起因

最近在看PHP7的源码，想要看一下一个zval一个执行流程，此时需要用到gdb调试，然后按照相关的步骤进行
每一个zval的字节数是16，zval的结构类型

struct _zval_struct {
  zend_value  value; //8个字节
  union u1; //4个字节
  union u2; //4个字节
}

具体的,这个是php-7.4.19中的

struct _zval_struct {
    zend_value        value;            /* value */ //8个字节
    union { //4个字节
        struct {
            ZEND_ENDIAN_LOHI_3(
                zend_uchar    type,         /* active type */ //通过type值来区分类型
                zend_uchar    type_flags,
                union {
                    uint16_t  extra;        /* not further specified */
                } u)
        } v;
        uint32_t type_info;
    } u1;
    union {//4个字节
        uint32_t     next;                 /* hash collision chain */ //解决哈希冲突的
        uint32_t     cache_slot;           /* cache slot (for RECV_INIT) */  //运行时缓存
        uint32_t     opline_num;           /* opline number (for FAST_CALL) */ //
        uint32_t     lineno;               /* line number (for ast nodes) */  //php的哪一行
        uint32_t     num_args;             /* arguments number for EX(This) */  //参数的个数
        uint32_t     fe_pos;               /* foreach position */   
        uint32_t     fe_iter_idx;          /* foreach iterator index */ //游标的索引位置
        uint32_t     access_flags;         /* class constant access flags */ //prviate protected public
        uint32_t     property_guard;       /* single property guard */  //防止类中魔术方法的循环引用
        uint32_t     constant_flags;       /* constant flags */
        uint32_t     extra;                /* not further specified */
    } u2;
};

gdb php
b ZEND_ECHO_SPEC_CV_HANDLER #设置断点
r zval.php
n
n
p z
p *z
.
.
.

zval.php

<?php
$a = 2;
echo $a;
$b = 1.1;
echo $b;
$c = null;
echo $c;
$d = true;
echo $d;
$e = false;
echo $e;
$f = 'string';
echo $f;
$g = [1, 2, 3];
echo $g;
$h = new stdClass();
echo $h;// FATAL ERROR