Thread: RPC

Hello, I was playing around with RPC and wrote a small program to test function of RPC. What i was trying to do was is to send some paramter from the client to the remote procedure. But when i do that i get the expected output. But the server terminated with the dump. Here is the code which i am using. I am sure that it is something to do with the pointer but i cant point it out where.

Server code

Code:

#include <stdio.h>
#include <string.h>
#include <rpc/rpc.h>
#include "print.h"
char dummyMessage[50];
bool_t xdr_dir(XDR *xdrs, char *objp)
{ return ( xdr_string(xdrs, &objp, 50) ); }

PrintChar(char *test)
{
     strcpy(dummyMessage,"This is a test");
     
     return (int)dummyMessage;
}
int main()
{
    registerrpc(PRINTNO, PRINTVER, PRINTNUM, PrintChar, xdr_dir, xdr_dir);
    
    svc_run();
    return 0;
}

client code

Code:

    enum clnt_stat clnt_stat;
    char print[50] = "This is a test from client";
    clnt_stat = callrpc ( "12PHLS", PRINTNO, PRINTVER, PRINTNUM, xdr_dir, print, xdr_dir, print);
    
    printf("%s", print);

And this is the output which i get from the server

Code:

*** glibc detected *** ./server: free(): invalid pointer: 0xbfd998e4 ***
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6[0xb7e2b7cd]
/lib/tls/i686/cmov/libc.so.6(cfree+0x90)[0xb7e2ee30]
/lib/tls/i686/cmov/libc.so.6(xdr_string+0xba)[0xb7ebcefa]
./server[0x8048444]
/lib/tls/i686/cmov/libc.so.6[0xb7ebbca2]
/lib/tls/i686/cmov/libc.so.6[0xb7ebb4d5]
/lib/tls/i686/cmov/libc.so.6(svc_getreq_common+0x292)[0xb7ebaaa2]
/lib/tls/i686/cmov/libc.so.6(svc_getreq_poll+0x85)[0xb7eba605]
/lib/tls/i686/cmov/libc.so.6(svc_run+0xa7)[0xb7ebb077]
./server[0x80484d7]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xdc)[0xb7dd9ebc]
./server[0x80483a1]
======= Memory map: ========
08048000-08049000 r-xp 00000000 08:02 2649556    /home/ssharish/FinalYear/DistributedSystms/Tutorial5/RPCW/server
08049000-0804a000 rwxp 00000000 08:02 2649556    /home/ssharish/FinalYear/DistributedSystms/Tutorial5/RPCW/server
0804a000-0806b000 rwxp 0804a000 00:00 0          [heap]
b7c00000-b7c21000 rwxp b7c00000 00:00 0 
b7c21000-b7d00000 ---p b7c21000 00:00 0 
b7dc3000-b7dc4000 rwxp b7dc3000 00:00 0 
b7dc4000-b7eff000 r-xp 00000000 08:01 5342       /lib/tls/i686/cmov/libc-2.5.so
b7eff000-b7f00000 r-xp 0013b000 08:01 5342       /lib/tls/i686/cmov/libc-2.5.so
b7f00000-b7f02000 rwxp 0013c000 08:01 5342       /lib/tls/i686/cmov/libc-2.5.so
b7f02000-b7f05000 rwxp b7f02000 00:00 0 
b7f09000-b7f14000 r-xp 00000000 08:01 2109       /lib/libgcc_s.so.1
b7f14000-b7f15000 rwxp 0000a000 08:01 2109       /lib/libgcc_s.so.1
b7f15000-b7f18000 rwxp b7f15000 00:00 0 
b7f18000-b7f31000 r-xp 00000000 08:01 2068       /lib/ld-2.5.so
b7f31000-b7f33000 rwxp 00019000 08:01 2068       /lib/ld-2.5.so
bfd89000-bfd9f000 rw-p bfd89000 00:00 0          [stack]
ffffe000-fffff000 r-xp 00000000 00:00 0          [vdso]
Enters the functionAborted (core dumped)

Can any one please explain why is this. Or how do i send paramter to the remote procedure.

Thank you

ssharish

Any help

ssharish

I would first try to get a working example going with a simple "int foo(void)" RPC call. I suspect there is something not right with the processing of the "char*" return type and parameter type - according to your "xdr_dir" filter - even though PrintChar() returns an int.

gg

Originally Posted by Codeplug

I would first try to get a working example going with a simple "int foo(void)" RPC call. I suspect there is something not right with the processing of the "char*" return type and parameter type - according to your "xdr_dir" filter - even though PrintChar() returns an int.

gg

Thanks a lot CodePlug, i did that before i dint this example. Cos i wasn't sure about RPC working. And Indeed it worked if I dont send any parameter to the function. This is how it looks like

My server code

Code:

#include <stdio.h>
#include <string.h>
#include <rpc/rpc.h>
#include "print.h"

char dummyMessage[50];

bool_t xdr_dir(XDR *xdrs, char *objp)
{ return ( xdr_string(xdrs, &objp, 50) ); }

PrintChar()
{
     strcpy(dummyMessage,"This is a test\n");
     
     return (int)dummyMessage;
}
int main()
{
    registerrpc(PRINTNO, PRINTVER, PRINTNUM, PrintChar, xdr_void, xdr_dir);
    
    svc_run();
    return 0;
}

client code

Code:

#include <stdio.h>
#include <rpc/rpc.h>
#include "print.h"

bool_t xdr_dir(XDR *xdrs, char *objp)
{ return ( xdr_string(xdrs, &objp, 50) ); }

int main()
{
    enum clnt_stat clnt_stat;
    char print[50];
    
    clnt_stat = callrpc ( "12PHLS", PRINTNO, PRINTVER, PRINTNUM, xdr_void, (char *)0, xdr_dir, print);
    
    printf("&#37;s", print);
    
    if (clnt_stat != 0) 
       clnt_perrno (clnt_stat);
    
    return 0;
}

And Indeed it works as expected. The problems comes when I start sending parameters.

ssharish

In all the online examples, I didn't see any custom xdr filters that passed the address of a parameter off the stack. In this case, xdr_dir() is passing along the address of the stack parameter "objp".

One other clue that suggests that might not be correct is the built-in "xdr_wrapstring()" filter. This simply calls xdr_string() for you - as your xdr_dir() does - but it uses a parameter of "char**".

Try using xdr_wrapstring() alone - or see if using "char **objp" makes any difference.

My other thought was that perhaps callrpc() should be called with "&print" instead of just "print" - but if the no parameter example is working as expected then perhaps not.

gg