Thread: Socket Programming getting segfault

Hello,
I am new here and hopefully I can get some help. I am trying to send a packet across a client/server communication. I am getting a seg fault (while running the program(It compiles fine)) when I try to read the neighbor file, You should be able to see this below the comment /***** Read neighbor file***/ :
Please forgive any mistakes and thanks for any help.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 


typedef struct {
  int dist;
  //int  Port;
  char source[21];
  char dest[21];
  //int  Min, Max;
} PACKET;




/*struct packet {
    int dist;
    char source[21];
    char dest[21];
};*/


void error(const char *msg)
{
    perror(msg);
    exit(0);
}


int main(int argc, char *argv[])
{
    int sockfd, portno, n;
    struct sockaddr_in serv_addr;
    struct hostent *server;


    char buffer[256];
    if (argc < 3) {
        fprintf(stderr,"usage: IPaddress hostname\n");
        exit(1);
    }
    
    FILE *pFile;
    char virtual[21];
    int port;
    char addr[21];
    char line[21];
    
    PACKET *pkt;
    //struct packet *pkt;
    FILE *neighbor;
    char s[21];
    char d[21]; 
     
    if (argc < 2) {
        fprintf(stderr,"ERROR: Enter an IP Addr\n");
        exit(1);
    }
     
    pFile = fopen("node.config", "r");
     
    if (pFile != NULL)
    {
         
         while (!feof(pFile))
         {
             fscanf(pFile, "%s %d %s", virtual, &port, addr);
             if(strcmp(virtual, argv[1]) == 0)
                 break;
             //printf("%s %d %s\n", virtual, port, addr);
           }


         printf("%d\n", port);
         fclose(pFile);
    }
    else
    {
        printf("Could not open file.\n");
    }
    
    /******** Read the neighbor file *********/
    
    neighbor = fopen("neighbor.config", "r");
    
    if (neighbor != NULL)
    {
         
         while (!feof(neighbor))
         {
             printf("here1\n");
             //fscanf(neighbor, "%s %s %d", pkt->source, pkt->dest, &pkt->dist);
             //fscanf(neighbor, "%s %s %d", s, d, &pkt->dist);
             //if(strcmp(virtual, argv[1]) == 0)
                 //break;
             printf("here\n");
             printf("%s %s %d\n", pkt->source, pkt->dest, pkt->dist);
           }


         printf("%d\n", port);
         fclose(neighbor);
    }
    else
    {
        printf("Could not open file.\n");
    }
    
    //portno = atoi(argv[2]);
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) 
        error("ERROR opening socket");
    server = gethostbyname(argv[2]);
    if (server == NULL) {
        fprintf(stderr,"ERROR, no such host\n");
        exit(0);
    }
    bzero((char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    bcopy((char *)server->h_addr, 
         (char *)&serv_addr.sin_addr.s_addr,
         server->h_length);
    serv_addr.sin_port = htons(port);
    if (connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0) 
        error("ERROR connecting");
    printf("Please enter the message: ");
    bzero(buffer,256);
    fgets(buffer,255,stdin);
    
    //n = write(sockfd,buffer,strlen(buffer));
    //n = send(sockfd, buffer, strlen(buffer), 0);
    n = send(sockfd, pkt, sizeof(pkt), 0);
    
    if (n < 0) 
         error("ERROR writing to socket");
    bzero(buffer,256);
    //n = read(sockfd,buffer,255);
    n = recv(sockfd, buffer, 255, 0);
    if (n < 0) 
         error("ERROR reading from socket");
    printf("%s\n",buffer);
    close(sockfd);
    return 0;
}

Code:

PACKET *pkt;

isn't enough. It will point to a random (and likely invalid) address. You need to allocate memory for the structure.

Code:

while (!feof(pFile))

FAQ > Why it's bad to use feof() to control a loop - Cprogramming.com

Bye, Andreas

Thanks a bunch Andreas. I was able to solve the problem by allocating space for PACKET
pkt = (PACKET *)malloc(sizeof(PACKET));