产品第一次AppStore审核通过，过年时遇到一个“神奇”的bug，在显示某些特殊字符时App会崩溃。后来经过各种尝试找到原因。

第一层原因

一个函数返回的NSString为nil，使用者没有判断nil，导致崩溃。

第二层原因

部分业务数据库中的信息不是utf-8，而是PC客户端存入的cp936（数据库是SQL Server）
部分信息iOS拿到的是cp936编码的文字。
文字中包含欧元符号 € 。（https://en.wikipedia.org/wiki/Euro）
iOS把cp936当做gb18030解码（主要是受gbk转utf-8影响，网上一把这类文章）
cp936中如果有欧元符号，则会返回nil。

第三层原因

Windows的CP936并符合标准的GBK定义。

Windows中CP936代码页使用0x80来表示欧元符号，而在GB18030编码中没有使用0x80编码位

Code page 936 is not identical to GBK because a code page encodes characters while the GBK only defines code points. In addition, the Euro sign (€), encoded as 0x80 in CP936, is not defined in GBK.

Windows CP936 code page 0x80 to represent the euro symbol in the GB18030 encoding 0x80 encoding, with other locations to represent the euro symbol. This can be understood to be a little problem on the GB18030 backward compatibility; also be understood as 0x80 CP936 expansion of GBK, GB18030 is only compatible and GBK.

search 0x80 in https://en.wikipedia.org/wiki/Code_page_936

http://www.databasesql.info/article/8106207727/

Microsoft later added the euro sign to Codepage 936 and assigned the code 0x80 to it. This is not a valid code point in GBK 1.0.

https://en.wikipedia.org/wiki/GBK

https://msdn.microsoft.com/zh-cn/goglobal/cc305153

https://msdn.microsoft.com/zh-cn/goglobal/bb688113.aspx

解决办法

并没有找到一个靠谱的编码转换工具（iconv貌似可以，但据同事说比较麻烦、且坑较多，使用的话未知风险太多）

手动替换0x80。

关键代码如下：

@implementation NSString(EncodingUtil)

+(NSString *)stringFromGBK:(const char *)srcString{
    // fix Windows CP936 0x80 extension for standard GBK
    
    const int x80 = 0x80;
    const char * toBeConvert = srcString;
    
    size_t x80count = 0;
    const size_t length = strlen(srcString);
    for(size_t idx = 0; idx < length; ++idx){
        int code = (int)(unsigned char)srcString[idx];
        if(code == x80){
            ++x80count;
        }
    }
    
    char *newString = NULL;
    if(x80count > 0){
        size_t newLength = length + x80count * sizeof(char);
        newString = (char*)malloc(newLength + 1);
        memset(newString,0,newLength + 1);
        
        BOOL ignoreNext = NO;
        size_t newPos = 0;
        for(size_t idx = 0; idx < length; ++idx){
            int code = (int)(unsigned char)srcString[idx];
            if(ignoreNext){
                ignoreNext = NO;
                newString[newPos++] = srcString[idx];
                continue;
            }
            
            if(code > x80)
                ignoreNext = YES;
            
            if(code == x80){
                newString[newPos++] = 0xa2;
                newString[newPos++] = 0xe3;
            }else{
                newString[newPos++] = srcString[idx];
            }
        }
        toBeConvert = newString;
    }
    
    NSStringEncoding enc = CFStringConvertEncodingToNSStringEncoding(kCFStringEncodingGB_18030_2000);
    NSString *retString = [[NSString alloc]initWithCString:toBeConvert encoding:enc];
    
    if(newString){
        free(newString);
    }
    
    return retString;
}
@end

测试代码：

char szBadName[101];
        memset(szBadName, 0, sizeof(szBadName));
        int pos = 0;
        szBadName[pos++] = '\xa8';
        szBadName[pos++] = 't';
        szBadName[pos++] = '\xa1';
        szBadName[pos++] = '\xfa';
        szBadName[pos++] = '\x8c';
        szBadName[pos++] = '\xa9';
        szBadName[pos++] = '\x92';
        szBadName[pos++] = '\xf6';
        szBadName[pos++] = '\xa2';
        szBadName[pos++] = '\xd9';
        szBadName[pos++] = '\x8b';
        szBadName[pos++] = '^';
        szBadName[pos++] = '\xc6';
        szBadName[pos++] = 'S';
        szBadName[pos++] = '\xbe';
        szBadName[pos++] = '\x80';
        szBadName[pos++] = '\xa8';
        szBadName[pos++] = '\xcb';
        
        szBadName[pos++] = '\x80';
        szBadName[pos++] = '\x80';
        szBadName[pos++] = '\xbe';
        szBadName[pos++] = '\x80';
        
        
        szBadName[pos++] = '7';
        szBadName[pos++] = '\xa8';
        szBadName[pos++] = '\x80';
        szBadName[pos++] = '\xa8';
        szBadName[pos++] = '\x80';
        szBadName[pos++] = '\xa8';
        szBadName[pos++] = '\x80';
        szBadName[pos++] = '6';
        szBadName[pos++] = '1';
        
        NSLog(@"bad = %@",[NSString stringFromGBK:szBadName]);

总结

这个问题不是普遍问题，应该99.99%的App不会遇到。可是我遇到了……

That’s all.