Question

I need to do simple client program, when I add port number and client will be serch all services for this port. Now is problem with segmentation fault in if statement.
How to return all services? In my program it will be return just one, I think.

my code:

   int main (int argc, char *argv[])
{
    int sockfd, n,pol, s;
    int numer;
    char recvline[MAXLINE +1];
    char  p;
    struct sockaddr_in servaddr;
    struct servent *sp;

    if (argc != 3)
        err_sys("Aby uruchomić podaj: klient <Adres IP> <port>");

    s = atoi(argv[2]);
    if((sp = getservbyport(s,NULL)) == NULL)
        {
            printf("port (s): %d \n", s);
            printf("port (sp): %d \n", sp->s_port); //segmentation fault
            err_sys("problem with port");   
        }

    if((sockfd = socket(AF_INET, SOCK_STREAM, 0))<0)
        err_sys("Blad utworzenia polaczenia");
    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = sp->s_port;
    if(inet_pton(AF_INET, argv[1], &servaddr.sin_addr)<=0)
        err_sys_kom("Blad konwersji do adresu IP dla %s", argv[1]);

    printf("%s", sp->s_name);
    pol = connect(sockfd, (SA*) &servaddr, sizeof(servaddr));
    if (pol < 0)
       {
          err_sys_kom("Blad polaczenie z serwerem");
          close(sockfd);
          exit(-1);
       }
    else
        str_cli(stdin, sockfd , 1);

exit(0);
}

Answer 1

EDIT (in response to new new problem – see comments below):

You probably need to get a list of protocols and work with those in a loop. The contents of the loop should roughly be:

1. Call getprotoent
2. If the result is NULL, exit the loop.
3. Else, dig out the protocol name from the returned structure.
4. Use that name as the second argument for getservbyport
5. Do what you want to with the result

EDIT (in response to new problem):

if((sp = getservbyport(s,NULL)) == NULL)

So your logic is to read sp ONLY IF sp is NULL. Obviously it will segfault.

It should be:

if((sp = getservbyport(s,NULL)) != NULL)

But then you will point out another new problem:

Why is sp NULL?

This could be because (as per the earlier version of my answer), you did an atoi on something which was not an integer. It could be because of any other reason. We can’t say because we don’t know what input you give.

---

This following part of the answer was in response to an old problem that the OP asked in the same question, and has since then chosen to edit over it:

First of all: Since you’re using getservbyport, you really should read about services, if you haven’t already.

Now on to the error:

getservbyport is of type:

struct servent *getservbyport(int port, const char *proto);

You are passing argv[2] which is of type char * instead of an int for port.

I believe the user inputs this as an argument in your program?

If you know that a char * points to a set of characters which look like an integer, like “1024”, then you can convert it to an integer with atoi.

Do this instead, on the line with the error, when calling getservbyport, while making sure you’ve included stdlib.h:

getservbyport(atoi(argv[2]),NULL)

If argv[2] is NOT representable as an integer, you’ll get undefined behavior, so maybe, you’ll want to check this first.