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tcpip数据包编码解析（chunk and gzip）

分类： Wincap 2009-07-25 11:23 1089人阅读 评论(0) 收藏 举报

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tcpip数据包编码解析（chunk and gzip）

做tcpip协议栈数据包重组时遇到一些编码问题，主要是chunk和gzip编码。

 

先看chunk：

RFC2616中对Chunked的定义：
Chunked-Body = *chunk
last-chunk
trailer
CRLF

chunk = chunk-size [ chunk-extension ] CRLF
chunk-data CRLF
chunk-size = 1*HEX
last-chunk = 1*("0") [ chunk-extension ] CRLF

chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
chunk-ext-name = token
chunk-ext-val = token | quoted-string
chunk-data = chunk-size(OCTET)
trailer = *(entity-header CRLF)

以下是解码过程的伪代码：
length := 0//用来记录解码后的数据体长度
read chunk-size, chunk-extension (if any) and CRLF//第一次读取块大小
while (chunk-size > 0) {//一直循环，直到读取的块大小为0
read chunk-data and CRLF//读取块数据体，以回车结束
append chunk-data to entity-body//添加块数据体到解码后实体数据
length := length + chunk-size//更新解码后的实体长度
read chunk-size and CRLF//读取新的块大小
}
read entity-header//以下代码读取全部的头标记
while (entity-header not empty) {
append entity-header to existing header fields
read entity-header
}
Content-Length := length//头标记中添加内容长度
Remove "chunked" from Transfer-Encoding//头标记中移除Transfer-Encoding

伪码的逻辑有点混乱，研究了下，自己写了C语言的解码代码：

//////////////////////////////////////

char * unchunk (char *filename)
{
        char cmdbuf[1024];
        /*if (strstr (filename, ".trunk") == 0)
        {
                strcat (filename, ".trunk");
                memset (cmdbuf, 0x0, sizeof (tmpfile));
                sprintf (cmdbuf, "mv %s %s", chunkfile, filename);
                system (cmdbuf);
        }*/

        FILE *fp = fopen (filename, "ab+");
        char newfile[128];
        memset (newfile, 0x0, sizeof (tmpfile));
        strcpy (newfile, filename);
        char *ptr = strstr(newfile, ".trunk");
        *ptr = 0;
        printf ("%s/n", newfile);

        FILE *fp_unchunk = fopen (newfile, "wb+");

        char chunk_head[8];
        memset (chunk_head, 0x0, sizeof (chunk_head));
        fgets (chunk_head, sizeof (chunk_head), fp);
        char *p = strstr (chunk_head, "/r/n");

        if (p)
        {
                int chunk_size = strtol (chunk_head, NULL, 16);
                char *chunk_data;
                while (chunk_size > 0)
                {
                        chunk_data = (char *)malloc (chunk_size);
                        memset (chunk_data, 0x0, chunk_size);
                        fread (chunk_data, chunk_size, 1, fp);
                        fwrite (chunk_data, chunk_size, 1, fp_unchunk);

                        fseek (fp, 2, SEEK_CUR);

                        //reread chunk head
                        memset (chunk_head, 0x0, sizeof (chunk_head));
                        fgets (chunk_head, sizeof (chunk_head), fp);
                        char *p = strstr (chunk_head, "/r/n");
                        if (p)
                        {
                                chunk_size = strtol (chunk_head, NULL, 16);
                                free (chunk_data);
                        }
                        else
                                break;
                }

                //remove old file
                memset (cmdbuf, 0x0, sizeof (cmdbuf));
                sprintf (cmdbuf, "rm %s", filename);
                system (cmdbuf);

                fclose (fp_unchunk);
                fclose (fp);

                return newfile;
        }
        else
        {
                fclose (fp_unchunk);
                fclose (fp);

                return filename;
        }
}

//////////////////////////////////////

 

接下来看gzip的解码

gzip的解码相对更加简单一些，两种方法实现：

一种是直接调用系统gzip命令解压，没有技术含量；

另一种是使用zlib库，通用性更高，但是要使用zlib库，开发过程稍复杂，下面给出gzip文件的解压C代码：

/////////////////////////////////

//调用系统gzip命令的代码（无难度）

void ungzip (char *filename)
{
        char cmdbuf[1024];

        if (strstr (filename, ".gz") == 0)
        {
                memset (cmdbuf, 0x0, sizeof (cmdbuf));
                sprintf (cmdbuf, "mv %s %s.gz", filename, filename);
                system (cmdbuf);
        }

        memset (cmdbuf, 0x0, sizeof (cmdbuf));
        sprintf (cmdbuf, "gzip -d %s", filename);
        system (cmdbuf);
}

 

// 使用zlib库的代码

#include "zlib/zlib.h"

void uncompresstorrent(char *src, char *dst)
{
        gzFile *gzfp=gzopen(src,"rb");
        FILE *fp=fopen(dst,"wb");
        char in[CHUNK];
        int retlen = -1;

        while(0 != (retlen=gzread(gzfp,in,CHUNK)))
        {
                fwrite(in,1,retlen,fp);
        }
        gzclose(gzfp);
        fclose(fp);
}

(编译是需要带 -Lzlib -lz 参数)

/////////////////////////////////