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Linux RPC中XDR 外部数据编码实例

网上找了很多XDR编码的内容,但是大多都是介绍相关的,很少有编程实例。因为分布式的课程学习了XDR外部数据编码,并应用在了RPC远程过程调用的实现中。本篇博客先暂时描述XDR相关,下一篇将介绍Socket通信。

这一篇博客介绍了XDR的内部实现 http://blog.csdn.net/chdhust/article/details/9004496 ,需要了解实现的可以参考一下。

XDR的主要作用就是在不同进程间传递消息参数时,避免因为计算机平台的不一致而导致数据传送接收异常。它可以对消息参数按照一定的顺序编码,放在一个数据包里(通常是在内存中申请一个一定大小的字符串缓冲区),然后把这个数据包发送给其他平台,然后在按照之前编码的顺序依次解码,并可以获得原来的消息参数。

这一篇博客简单的描述了一个XDR编码实例 http://blog.csdn.net/seulww/article/details/23093913 

环境要求:

  Linux 操作系统

  rpc/xdr.h

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<malloc.h>
 4 #include<rpc/xdr.h>
 5 #include<cstring>
 6 using namespace std;
 7 
 8 char* func()
 9 {
10     char *res = (char *)malloc(sizeof(char)*1024);
11     sprintf(res,"nihao,你好 \n");
12     return res;
13 }
14 
15 int main(int argc,char* argv[])
16 {
17     printf("begin...\n");
18 
19     short age = 34;
20     char sex = M;
21     double weights = 66.66;
22     char name[20];
23     strcpy(name,"LJ");
24     char *p = name;
25 
26 
27     XDR xdr;
28     char buff[120];
29 
30     xdrmem_create(&xdr,buff,120,XDR_ENCODE);
31     xdr_string(&xdr,&p,sizeof p);
32     xdr_short(&xdr,&age);
33     xdr_char(&xdr,&sex);
34     xdr_double(&xdr,&weights);
35 
36     xdrmem_create(&xdr,buff,120,XDR_DECODE);
37     char name2[20];
38     char *p2 = name2;
39     cout<<strlen(p2)<<endl;
40     xdr_string(&xdr,&p2,sizeof p2);
41     short age2;
42     xdr_short(&xdr,&age2);
43     char sex2;
44     xdr_char(&xdr,&sex2);
45     double weights2;
46     xdr_double(&xdr,&weights2);
47 
48     printf("%s %d %c %lf\n",p2,age2,sex2,weights2);
49 
50     memset(buff,0,sizeof buff);
51     xdrmem_create(&xdr,buff,sizeof buff,XDR_ENCODE);
52     char *p4 = (char *)malloc(sizeof(char)*1024);
53     p4 = func();
54     printf("p4 = %s\n",p4);
55     printf("sizeof p4 = %d\n",(int)(sizeof p4));
56     xdr_string(&xdr,&p4,1024);
57 
58     xdrmem_create(&xdr,buff,sizeof buff,XDR_DECODE);
59     char str2[1024];
60     char *p5 = str2;
61     xdr_string(&xdr,&p5,1024);
62 
63     printf("str2 = %s\n",str2);
64 
65     return 0;
66 }
67 
68 /*  FILE* pFile = fopen("in.txt","ab+");
69     XDR xdr;
70     xdrstdio_create(&xdr,pFile,XDR_ENCODE);
71     int n = 299;
72     xdr_int(&xdr,&n);
73     xdr_destroy(&xdr);
74     fclose(pFile);
75 */

 

详解:

  XDR xdr;  //  定义一个xdr对象

  xdrmem_create(&xdr,buff,120,XDR_ENCODE);  //  创建编码模式,buff是自定义的缓冲区,存放参数,120其实是buff的大小,XDR_ENCODE是编码,相应的XDR_DECODE解码

  xdr_int(&xdr,&n);  //  根据参数类型进行编码  xdr_type(&xdr,&type);

      需要注意的是,字符串的编码要用 xdr_string,并且引用&的参数必须是变量,而不能是字符串的数组名,这是C语言的规则,数组名虽然是数组的首地址指针,但是不是变量,不能引用&。

  xdrmem_create(&xdr,buff,120,XDR_DECODE);  //  解码过程和编码过程放过来,具体参考代码

注意代码中绿色被注释部分:

  是xdr文件编码的方式,而不是使用内存中的缓冲区来存储信息,使用方式类似的。

 

列举一些常用的编码函数(来自Linux中xdr.h):

/* XDR using memory buffers */
extern void xdrmem_create (XDR *__xdrs, const caddr_t __addr,
               u_int __size, enum xdr_op __xop) __THROW;

/* XDR using stdio library */
extern void xdrstdio_create (XDR *__xdrs, FILE *__file, enum xdr_op __xop)
     __THROW;

/*
 * These are the "generic" xdr routines.
 * None of these can have const applied because it‘s not possible to
 * know whether the call is a read or a write to the passed parameter
 * also, the XDR structure is always updated by some of these calls.
 */
extern bool_t xdr_void (void) __THROW;
extern bool_t xdr_short (XDR *__xdrs, short *__sp) __THROW;
extern bool_t xdr_u_short (XDR *__xdrs, u_short *__usp) __THROW;
extern bool_t xdr_int (XDR *__xdrs, int *__ip) __THROW;
extern bool_t xdr_u_int (XDR *__xdrs, u_int *__up) __THROW;
extern bool_t xdr_long (XDR *__xdrs, long *__lp) __THROW;
extern bool_t xdr_u_long (XDR *__xdrs, u_long *__ulp) __THROW;
extern bool_t xdr_hyper (XDR *__xdrs, quad_t *__llp) __THROW;
extern bool_t xdr_u_hyper (XDR *__xdrs, u_quad_t *__ullp) __THROW;
extern bool_t xdr_longlong_t (XDR *__xdrs, quad_t *__llp) __THROW;
extern bool_t xdr_u_longlong_t (XDR *__xdrs, u_quad_t *__ullp) __THROW;
extern bool_t xdr_int8_t (XDR *__xdrs, int8_t *__ip) __THROW;
extern bool_t xdr_uint8_t (XDR *__xdrs, uint8_t *__up) __THROW;
extern bool_t xdr_int16_t (XDR *__xdrs, int16_t *__ip) __THROW;
extern bool_t xdr_uint16_t (XDR *__xdrs, uint16_t *__up) __THROW;
extern bool_t xdr_int32_t (XDR *__xdrs, int32_t *__ip) __THROW;
extern bool_t xdr_uint32_t (XDR *__xdrs, uint32_t *__up) __THROW;
extern bool_t xdr_int64_t (XDR *__xdrs, int64_t *__ip) __THROW;
extern bool_t xdr_uint64_t (XDR *__xdrs, uint64_t *__up) __THROW;
extern bool_t xdr_quad_t (XDR *__xdrs, quad_t *__ip) __THROW;
extern bool_t xdr_u_quad_t (XDR *__xdrs, u_quad_t *__up) __THROW;
extern bool_t xdr_bool (XDR *__xdrs, bool_t *__bp) __THROW;
extern bool_t xdr_enum (XDR *__xdrs, enum_t *__ep) __THROW;
extern bool_t xdr_array (XDR * _xdrs, caddr_t *__addrp, u_int *__sizep,
             u_int __maxsize, u_int __elsize, xdrproc_t __elproc)
     __THROW;
extern bool_t xdr_bytes (XDR *__xdrs, char **__cpp, u_int *__sizep,
             u_int __maxsize) __THROW;
extern bool_t xdr_opaque (XDR *__xdrs, caddr_t __cp, u_int __cnt) __THROW;
extern bool_t xdr_string (XDR *__xdrs, char **__cpp, u_int __maxsize) __THROW;
extern bool_t xdr_union (XDR *__xdrs, enum_t *__dscmp, char *__unp,
             const struct xdr_discrim *__choices,
             xdrproc_t __dfault) __THROW;
extern bool_t xdr_char (XDR *__xdrs, char *__cp) __THROW;
extern bool_t xdr_u_char (XDR *__xdrs, u_char *__cp) __THROW;
extern bool_t xdr_vector (XDR *__xdrs, char *__basep, u_int __nelem,
              u_int __elemsize, xdrproc_t __xdr_elem) __THROW;
extern bool_t xdr_float (XDR *__xdrs, float *__fp) __THROW;
extern bool_t xdr_double (XDR *__xdrs, double *__dp) __THROW;
extern bool_t xdr_reference (XDR *__xdrs, caddr_t *__xpp, u_int __size,
                 xdrproc_t __proc) __THROW;
extern bool_t xdr_pointer (XDR *__xdrs, char **__objpp,
               u_int __obj_size, xdrproc_t __xdr_obj) __THROW;
extern bool_t xdr_wrapstring (XDR *__xdrs, char **__cpp) __THROW;
extern u_long xdr_sizeof (xdrproc_t, void *) __THROW;

 

Linux RPC中XDR 外部数据编码实例