Solution
--------

        As  we need  to dump  the combined  call graph  for multiple 
        files,  we  need  to  consider link  time  optimization  (LTO). 
        Thus  we  specify  the  options  "-flto".  Now  if  we  specify 
        the option  "-flto-partiton=none", it  loads the  function body 
        and  call graph.  However when  we  do not  specify the  option 
        "-flto-partiton=none" the default partiton is used. This option 
        loads only the call graph and not the function body.            

          for (node = cgraph_nodes; node; node = node->next)
          {
                /* Nodes without a body, and clone nodes are not interesting. */
                if (!gimple_has_body_p (node->decl) || node->clone_of)
                        continue;

                push_cfun (DECL_STRUCT_FUNCTION (node->decl));

                fprintf (dump_file, "\n Function : ");
                fprintf (dump_file,"%s\t",cgraph_node_name(node));

                /* Check if the node i.e. a function has callers */
                if(node->callers)
                {
                        /* Print the first caller */
                        fprintf (dump_file, "\t Caller :");
                        fprintf (dump_file,"%s",cgraph_node_name(node->callers->caller));

                        /* Extract the next caller */
                        edge = node->callers->next_caller;

                        /* Iterate over all the remaining callers */
                        while(edge)
                        {
                                fprintf (dump_file, "\t Caller :");
                                fprintf (dump_file,"%s",cgraph_node_name(edge->caller));
                                edge = edge->next_caller;
                        }
                }

                /* restoring the context by popping cfun. */
                pop_cfun ();
          }

        We have 2 input files `test1.c' and `test2.c'. 

        Below are the observations to be noted.

        1. If we execute without specifying "-flto" option it generates
            dump file for  test1.c and test2.c but not  for the combined
            output (i.e. all files merged.*.* are missing).

                gcc -fplugin=./plugin.so -c test1.c -fdump-ipa-all
                gcc -fplugin=./plugin.so -c test2.c -fdump-ipa-all
                gcc -fplugin=./plugin.so -o merged test1.o test2.o -fdump-ipa-all

               In order to create these files, we need to enable the lto mode.

        2. If we execute using the  "-flto" option it does not generate
            any data in  the combined output file. In  this case default
            partiton is considered  which loads only the  call graph and
            not the function body.

                gcc -fplugin=./plugin.so -c -flto test1.c -fdump-ipa-all
                gcc -fplugin=./plugin.so -c -flto test2.c -fdump-ipa-all
                gcc -fplugin=./plugin.so -o merged -flto test1.o test2.o -fdump-ipa-all

            As we have the check to test if the function has body, it
            skips the code to dump the functions without callers.

                        if (!gimple_has_body_p (node->decl) || node->clone_of)
                        continue;

            If we  remove this condition,  we get correct  output for
            individual files as well as for combined output.

        3. If  we execute using "-flto  -flto-partition=none" option,
           it  generates  complete  output.  When  we  use  the  option
           "-flto-partition=none". both the call graph and the function
           bodies gets loaded. This gives us the complete output.

                gcc -fplugin=./plugin.so -c -flto -flto-partition=none test1.c -fdump-ipa-all
                gcc -fplugin=./plugin.so -c -flto -flto-partition=none test2.c -fdump-ipa-all
                gcc -fplugin=./plugin.so -o merged -flto -flto-partition=none test1.o test2.o -fdump-ipa-all