Question

I am using following api to initialize sockfd at client side:(sockfd=3)

              if ((sockfd = socket(p->ai_family, p->ai_socktype,p->ai_protocol))  == -1) {
                    perror("client: socket");
                    continue;
            }

& initializing my TPKT_Buff to {3,0,0,0} value by using function :

            if(Fill_TPKT(PStack,TPKT_Buff) != 0)
  {
   printf("Error while filling TPKT Buffer");
   return 1;
  }printf("tpkt/2_Buff%x %x\n",TPKT_Buff[0],TPKT_Buff[1]);printf("sockfd=%d\n",sockfd);

But, after calling function :

 if(Fill_COTP(PStack,&cotp) != 0)
    {
     printf("Error while filling COTP Structure!");
     return 1;
    }

my socfd & TPKT_Buff values changed to zero TPKT_Buff={0,0,0,0} & sockfd=0 :

printf("sockfd=%d\n",sockfd);
 printf("TPKT/2_Buff=%x %x\n",TPKT_Buff[0],TPKT_Buff[1]);

Definitions of functions Fill_COTP & Fill_TPKT are as follows :

  int Fill_TPKT(FILE *fptr,unsigned char *buf)
  {
   fseek(fptr,14,SEEK_SET);
   fscanf(fptr,"%d",buf+0);

   fseek(fptr,15,SEEK_CUR); 
   fscanf(fptr,"%d",buf+1);

   return 0;
  }

 int Fill_COTP(FILE *fptr, COTP *cotp)
 {
  unsigned short temp;

  fseek(fptr,13,SEEK_CUR);
  fscanf(fptr,"%d",&temp);
  cotp->Destination_Ref[1] = temp;
  cotp->Destination_Ref[0] = temp>>8;
  printf("%x %x\n",cotp->Destination_Ref[0],cotp->Destination_Ref[1]);
  fseek(fptr,13,SEEK_CUR);
  fscanf(fptr,"%d",&temp);
  cotp->Source_Ref[1] = temp;
  cotp->Source_Ref[0] = temp>>8;
  printf("%x %x\n",cotp->Source_Ref[0],cotp->Source_Ref[1]);
  fseek(fptr,14,SEEK_CUR);
  fscanf(fptr,"%d",&temp);
  cotp->Source_Tsap[1] = temp;
  cotp->Source_Tsap[0] = temp>>8;
  printf("%x %x\n",cotp->Source_Tsap[0],cotp->Source_Tsap[1]);
  fseek(fptr,14,SEEK_CUR);
  fscanf(fptr,"%d",&temp);
  cotp->Destination_Tsap[1] = temp;
  cotp->Destination_Tsap[0] = temp>>8;
  printf("%x %x\n",cotp->Destination_Tsap[0],cotp->Destination_Tsap[1]);
  fseek(fptr,17,SEEK_CUR);
  fscanf(fptr,"%d",&(cotp->TPDU_size));
  printf("%x\n",cotp->TPDU_size);
  return 0;
 }

Here PStack is a file pointer.
I am not getting why my sockfd & TPKT_Buff values changing to zero even I am not using these values in my function Fill_COTP();
Please give some suggestion.
Definition of COTP is:

       typedef struct
         {
          unsigned char PDU_type;
          unsigned char Destination_Ref[2];
          unsigned char Source_Ref[2];
          unsigned char Source_Tsap[2];
          unsigned char Destination_Tsap[2];
          unsigned char TPDU_size;
         } COTP;

There is no relation between sockfd & TPKT_Buff.

Answer 1

The trouble appears to be in the line:

fscanf(fptr,"%d",&(cotp->TPDU_size)); 

Your TPCU_size is unsigned char TPDU_size; which is only 1 byte (assuming this to be the size of ‘char’) in size, but you are trying put 4 bytes (assuming that to be the size of ‘int’) into it during fscanf, thereby potentially overwriting the memory around it.