Array Example 3

The well-known parameters to main, argc and argv can be combined into a structure. An array of these structures can make up a history of commands. The declarations and XDR routines might look like Example A-9.

Example A-9 Array Example #3

struct cmd {
 	u_int c_argc;
 	char **c_argv;
};
#define ALEN 1000           /* args cannot be > 1000 chars */
 #define NARGC 100          /* commands cannot have > 100 args */

struct history {
 	u_int h_len;
 	struct cmd *h_cmds;
};
#define NCMDS 75            /* history is no more than 75 commands */

bool_t
xdr_wrapstring(xdrs, sp)
 	XDR *xdrs;
 	char **sp;
{
 	return(xdr_string(xdrs, sp, ALEN));
}

bool_t
xdr_cmd(xdrs, cp)
 	XDR *xdrs;
 	struct cmd *cp;
{
 	return(xdr_array(xdrs, &cp->c_argv, &cp->c_argc, NARGC,
 	        sizeof (char *), xdr_wrapstring));
}
bool_t
xdr_history(xdrs, hp)
 	XDR *xdrs;
 	struct history *hp;
{
 	return(xdr_array(xdrs, &hp->h_cmds, &hp->h_len, NCMDS,
 	        sizeof (struct cmd), xdr_cmd));
}

The most confusing part of this example is that the routine xdr_wrapstring() is needed to package the xdr_string() routine, because the implementation of xdr_array() passes only two parameters to the array element description routine; xdr_wrapstring() supplies the third parameter to xdr_string().

By now the recursive nature of the XDR library should be obvious. Let's continue with more constructed data types.