咸鱼

抓HTTP包的时候发现很多网站的响应报文是gzip压缩后的数据，存放在一个或多个chunk里面（参见HTTP响应报文中的chunked）。这些gzip数据是不可阅读的，需要进行解压。一开始在网上找到了一份可以正常运行的代码，贴出来：

http://hi.baidu.com/xzq2000/blog/item/c5429f2fd6a646301f308991.html/cmtid/332e72f08f0b53a2a40f5237

char* ungzip(char* source,int len)
{
int err;
z_stream d_stream;
Byte compr[segment_size]={0}, uncompr[segment_size*4]={0};
memcpy(compr,(Byte*)source,len);
uLong comprLen, uncomprLen;
comprLen = sizeof(compr) / sizeof(compr[0]);
uncomprLen = 4*comprLen;
strcpy((char*)uncompr, "garbage");

d_stream.zalloc = (alloc_func)0;
d_stream.zfree = (free_func)0;
d_stream.opaque = (voidpf)0;

d_stream.next_in = compr;
d_stream.avail_in = 0;
d_stream.next_out = uncompr;

err = inflateInit2(&d_stream,47);
if(err!=Z_OK)
{
   printf("inflateInit2 error:%d",err);
   return NULL;
}
while (d_stream.total_out < uncomprLen && d_stream.total_in < comprLen) {
d_stream.avail_in = d_stream.avail_out = 1;
err = inflate(&d_stream,Z_NO_FLUSH);
    if(err == Z_STREAM_END) break;
    if(err!=Z_OK)
    {
    printf("inflate error:%d",err);
    return NULL;
   }
}
err = inflateEnd(&d_stream);
if(err!=Z_OK)
{
   printf("inflateEnd error:%d",err);
   return NULL;
}
char* b = new char[d_stream.total_out+1];
memset(b,0,d_stream.total_out+1);
memcpy(b,(char*)uncompr,d_stream.total_out);
return b;
}
后来看了zlib usage example（参见zlib使用范例），模仿写了一段代码，可以正常运行，而且感觉比上面的代码要快，因为上面的代码把z_stream的avail_in和avail_out都设为1了，只能一字节一字节地解压，非常慢。

#include <stdio.h>
#include <string.h>
#include <assert.h>
#include <zlib.h>
#define segment_size 1460//largest tcp data segment
int ungzip(char* source,int len,char*des)
{
int ret,have;
int offset=0;
z_stream d_stream;
Byte compr[segment_size]={0}, uncompr[segment_size*4]={0};
memcpy(compr,(Byte*)source,len);
uLong comprLen, uncomprLen;
comprLen =len;//一开始写成了comprlen=sizeof(compr)以及comprlen=strlen(compr)，后来发现都不对。

//sizeof(compr)永远都是segment_size，显然不对，strlen(compr)也是不对的，因为strlen只算到\0之前，

//但是gzip或者zlib数据里\0很多。
uncomprLen = segment_size*4;
strcpy((char*)uncompr, "garbage");

d_stream.zalloc = Z_NULL;
d_stream.zfree = Z_NULL;
d_stream.opaque = Z_NULL;

d_stream.next_in = Z_NULL;//inflateInit和inflateInit2都必须初始化next_in和avail_in
d_stream.avail_in = 0;//deflateInit和deflateInit2则不用

ret = inflateInit2(&d_stream,47);
if(ret!=Z_OK)
{
   printf("inflateInit2 error:%d",ret);
   return ret;
}
d_stream.next_in=compr;
d_stream.avail_in=comprLen;
do
{
 d_stream.next_out=uncompr;
 d_stream.avail_out=uncomprLen;
 ret = inflate(&d_stream,Z_NO_FLUSH);
 assert(ret != Z_STREAM_ERROR);
 switch (ret)
 {
  case Z_NEED_DICT:
              ret = Z_DATA_ERROR;   
        case Z_DATA_ERROR:
        case Z_MEM_ERROR:
              (void)inflateEnd(&d_stream);
               return ret;
    }
 have=uncomprLen-d_stream.avail_out;
 memcpy(des+offset,uncompr,have);//这里一开始我写成了memcpy(des+offset,d_stream.next_out,have);

 //后来发现这是不对的，因为next_out指向的下次的输出，现在指向的是无有意义数据的内存。见下图

offset+=have;

}while(d_stream.avail_out==0);
inflateEnd(&d_stream);
memcpy(des+offset,"\0",1);
return ret;
}