Perl 内部结构详解 -- PerlGuts Illustrated (2 SV)

科技前沿 • 2025-02-22 08:25 • 阅读 81

大家好，我是讯享网，很高兴认识大家。

_SV_HEAD and struct sv

第一个字段Any可以指向任意结构，除了RV，所有的其他类型都是由Any指向的附加数据来实现。
第二个字段REFCNT表明了有多少pointers引用了这个object。初始置为1，当有pointers指向它或者被销毁的时候，这个值需要相应的加1或者减1，当值为0的时候，内存释放。
第三个字段包含了FLAGS & TYPE，是一个32 bit unsigned int。如下图：

C:\>perl -MDevel::Peek -e "Dump $a" SV = NULL(0x0) at 0x182a9fc   REFCNT = 1   FLAGS = () C:\>perl -MDevel::Peek -e "Dump \$a" SV = RV(0x) at 0x29914c   REFCNT = 1   FLAGS = (TEMP,ROK)   RV = 0x182a9fc   SV = NULL(0x0) at 0x182a9fc     REFCNT = 2     FLAGS = ()

讯享网

SvIV and SvNV

整数和小树，结构如图所示：

讯享网C:\>perl -MDevel::Peek -e "$a = 123; Dump $a" SV = IV(0x182aa20) at 0x182aa24 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 123 C:\>perl -MDevel::Peek -e "$a = 123.9; Dump $a" SV = NV(0xc) at 0x182aa2c REFCNT = 1 FLAGS = (NOK,pNOK) NV = 123.9

SvPV

字符串，结构如图所示：

除了SV以外，额外的结构xpv被分配，它包含三部分：

PVX 指向实际的字符串。
CUR 标记字符串的长度，PVX+CUR 处的字符应该为'\0'，标记字符串的结束。
LEN 标记内存分配给char * 的长度，以4为增量。

C:\>perl -MDevel::Peek -e "$a = 't'; Dump $a" SV = PV(0x296fec) at 0x182aa24 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0xc "t"\0 CUR = 1 LEN = 4 C:\>perl -MDevel::Peek -e "$a = 'test'; Dump $a" SV = PV(0x296fec) at 0x182aa2c REFCNT = 1 FLAGS = (POK,pPOK) PV = 0xc "test"\0 CUR = 4 LEN = 8 (随着长度增加，以4为增量) C:\>perl -MDevel::Peek -e "$a = 'test'; $a = undef; Dump $a" SV = PV(0x296ffc) at 0x182aa34 REFCNT = 1 FLAGS = () (包含字符串，但是无效) PV = 0x "test"\0 CUR = 4 LEN = 8

SvOOK

为了提高移除字符串开头字符的速度，使用了OOK标记，IVX存储着偏移量，结构如图所示：

讯享网C:\>perl -MDevel::Peek -e "$a = 'xtesting'; $a=~s/.//; Dump $a" SV = PVIV(0xc) at 0x182aa4c REFCNT = 1 FLAGS = (POK,OOK,pPOK) IV = 1 (OFFSET) PV = 0xd ( "x" . ) "testing"\0 CUR = 7 LEN = 11

SvPVIV and SvPVNV

类似字符串，但是PVIV储存了额外的整数或者小数信息，可以根据flag直接进行数学运算，结构如图所示：

C:\>perl -MDevel::Peek -e "$a = '123.0'; 0+$a; Dump $a" SV = PVNV(0x29838c) at 0x182aa34 REFCNT = 1 FLAGS = (IOK,NOK,POK,pIOK,pNOK,pPOK) IV = 123 NV = 123 PV = 0x "123.0"\0 CUR = 5 LEN = 8 C:\>perl -MDevel::Peek -e "$a = '123testing'; 0+$a; Dump $a" SV = PVNV(0x) at 0x182aa34 REFCNT = 1 FLAGS = (POK,pIOK,pNOK,pPOK) IV = 123 NV = 123 PV = 0x "123testing"\0 CUR = 10 LEN = 12

SvRV

类似指针，指向任意其他结构，结构如图所示：

讯享网C:\>perl -MDevel::Peek -e "$a = '123testing'; $b = \$a ; Dump $b" SV = RV(0x182ab68) at 0x182ab5c REFCNT = 1 FLAGS = (ROK) RV = 0x182aa4c SV = PV(0x296ffc) at 0x182aa4c REFCNT = 2 FLAGS = (POK,pPOK) PV = 0xc "123testing"\0 CUR = 10 LEN = 12