PHP函数二进制安全有关问题
PHP函数二进制安全问题
本文主要从三个角度来阐述php的二进制安全：1. 什么叫php的二进制安全；2. 什么结构确保了php的二进制安全；3. 这种结构还有哪些其它方面的应用？
做到知其然，也知其所以然。
一句话解释：
php的内部函数在操作二进制数据时能保证达到预期的结果，例如str_replace、stristr、strcmp等函数，我们就说这些函数是二进制安全的。
举个列子：
我们来对比一下C和php下的strcmp函数。
C代码如下
- main(){
- char ab[] = "aa\0b";
- char ac[] = "aa\0c";
- printf("%d\n", strcmp(ab, ac));
- printf("%d\n", strlen(ab));
- }
结果：
0
2
解读：
也就是说C语言认为ab和ac这两个字符串是相等的，而且ab的长度为2.
php代码如下
- $ab = "aa\0b";
- $ac = "aa\0c";
- var_dump(strcmp($ab, $ac));
- var_dump(strlen($ab));
- ?>
结果：
int(-1)
int(4)
解读：
也就是php语言认为ab和ac这两个字符串是相等的，而且ab的长度为4。
聪明的你，应该已经发现问题在哪了吧，不错，对于c语言‘\0’是字符串的结束符，所以在C语言中对于字符串“aa\0b”，它读到'\0'就会默认字符读取已经结束，而抛掉后面的字符串'b',导致我们看到strlen(“aa\0b”)的值为2
那问题又来了，php都是C来开发的，为什么php做到了二进制安全呢？
先来看看php的变量存储zval结构
php会根据type的值来决定访问value的哪个成员，为字符串时，我们会访问红框标识的str结构，这便是底层字符串的存储结构，它有两个值，一个是指向字符串的指针val，另一个是记录字符串长度的len值，就是因为有len这个值，导致了php是二进制安全的：因为它不需要像C一样通过是否遇到'\0'结尾符来判断整个字符串是否读取完毕，而是通过len这个值指定的长度进行读取。
可见一个小小的数据结构改进，为我们带来了更多想象的空间，可谓是结构的一小步，功能的一大步。
拓展：
这么好用的结构，显然会被到处使用，我们常用的redis，在底层存储数据时就用到了这种结构，Redis 没有直接使用 C 语言传统的字符串表示（以空字符结尾的字符数组），而是自己构建了一种名为简单动态字符串（simple dynamic string，SDS）的抽象类型，并将 SDS 用作 Redis 的默认字符串表示
看下SDS的结构定义
- struct sdshdr {
- // 记录 buf 数组中已使用字节的数量
- // 等于 SDS 所保存字符串的长度
- int len;
- // 记录 buf 数组中未使用字节的数量
- int free;
- // 字节数组，用于保存字符串
- char buf[];
- };
以下内容足以阐明这一点：（摘自http://redisbook.com/preview/sds/different_between_sds_and_c_string.html#id6）
C 字符串中的字符必须符合某种编码（比如 ASCII），并且除了字符串的末尾之外，字符串里面不能包含空字符，否则最先被程序读入的空字符将被误认为是字符串结尾 ——这些限制使得 C 字符串只能保存文本数据，而不能保存像图片、音频、视频、压缩文件这样的二进制数据。
举个例子，如果有一种使用空字符来分割多个单词的特殊数据格式，如图 2-17 所示，那么这种格式就不能使用 C 字符串来保存，因为 C 字符串所用的函数只会识别出其中的 "Redis" ，而忽略之后的 "Cluster" 。
虽然数据库一般用于保存文本数据，但使用数据库来保存二进制数据的场景也不少见，因此，为了确保 Redis 可以适用于各种不同的使用场景，SDS 的 API 都是二进制安全的（binary-safe）：所有 SDS API 都会以处理二进制的方式来处理 SDS 存放在 buf 数组里的数据，程序不会对其中的数据做任何限制、过滤、或者假设 ——数据在写入时是什么样的，它被读取时就是什么样。
这也是我们将 SDS 的 buf 属性称为字节数组的原因 ——Redis 不是用这个数组来保存字符，而是用它来保存一系列二进制数据。
比如说，使用 SDS 来保存之前提到的特殊数据格式就没有任何问题，因为 SDS 使用 len 属性的值而不是空字符来判断字符串是否结束，如图 2-18 所示。
buf"]; buf [label = " { 'R' | 'e' | 'd' | 'i' | 's' | '\\0' | 'C' | 'l' | 'u' | 's' | 't' | 'e' | 'r' | '\\0' | '\\0' } "]; // sdshdr:buf -> buf;}" />
通过使用二进制安全的 SDS ，而不是 C 字符串，使得 Redis 不仅可以保存文本数据，还可以保存任意格式的二进制数据。
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