getservbyport - Always returning NULL servent

Applications Programming
1

Hi,

I am having an issue using getservbyport. Here is a little program to demonstrate the problem (removed the includes):

int 
main(void) {

        struct servent *service;
        int memsize = sizeof(struct servent);

        service = (struct servent *)malloc(memsize);
        bzero(service, memsize);
        service = getservbyport(25, NULL);
        if (service == NULL)
        {
                perror("getservbyport()");
                exit(1);
        }
        printf("service name = %s\n", service->s_name);

        return 0;
}

Here is how I am compiling and running this program

me@host:~$ gcc -o foo foo.c 
me@host:~$ ./foo
getservbyport(): Success

I was expecting to see "service name = smtp", since my /etc/services file contains the following two lines:

smtp             25/tcp    mail         #Simple Mail Transfer
smtp             25/udp    mail         #Simple Mail Transfer

I tried this on two hosts with the same result. One has gcc 4.1.2 and the other has gcc 2.96. I also get the same result, on both hosts, if I specify the protocol to use when calling getservbyport (instead of NULL which matches any protocol).

Can anyone see what I am doing wrong, or why a NULL result from the call to getservbyport would say "Success" from the perror call? The man page for getservbyport says the following:

       The getservent(), getservbyname() and getservbyport() functions  return
       the  servent structure, or a NULL pointer if an error occurs or the end
       of the file is reached.

It could be reaching EOF, but again, the two lines are present for smtp in my /etc/services file.

Thanks,
goon12

2

You're getting a bit tied up with pointer handling, but line 5 is where the real change is.

[highlight=c]
int
main(void) {

    struct servent *service;
    service = getservbyport\(htons\(25\), NULL\);
    if \(service == NULL\)
    \{
            perror\("getservbyport\(\)"\);
            exit\(1\);
    \}
    printf\("service name = %s\\n", service->s_name\);

    return 0;

}
[/highlight]

3

Thanks reborg, I missed that in the man page.

       The getservbyport() function returns a servent structure for the line that  matches
       the  port  port given in network byte order using protocol proto. If proto is NULL,
       any protocol will be matched.

Thanks again,
Joe