getservbyport - Always returning NULL servent Post: 302272700

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Top Forums Programming getservbyport - Always returning NULL servent Post 302272700 by goon12 on Wednesday 31st of December 2008 11:54:48 AM

12-31-2008

Registered User

getservbyport - Always returning NULL servent

Hi,

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

Code:

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

Code:

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:

Code:

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:

Code:

       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

goon12

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LEARN ABOUT NETBSD

setservent

GETSERVENT(3)						   BSD Library Functions Manual 					     GETSERVENT(3)

NAME

     getservent, getservbyport, getservbyname, setservent, endservent -- get service entry

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <netdb.h>

     struct servent *
     getservent();

     struct servent *
     getservbyname(const char *name, const char *proto);

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

     void
     setservent(int stayopen);

     void
     endservent(void);

DESCRIPTION

     The getservent(), getservbyname(), and getservbyport() functions each return a pointer to an object with the following structure containing
     the broken-out fields of a line in the network services data base, /etc/services.

	   struct  servent {
		   char    *s_name;	   /* official name of service */
		   char    **s_aliases;    /* alias list */
		   int	   s_port;	   /* port service resides at */
		   char    *s_proto;	   /* protocol to use */
	   };

     The members of this structure are:

     s_name	The official name of the service.

     s_aliases	A NULL terminated list of alternative names for the service.

     s_port	The port number at which the service resides.  Port numbers must be given and are returned in network byte order.

     s_proto	The name of the protocol to use when contacting the service.

     The getservent() function reads the next line of the file, opening the file if necessary.

     The setservent() function opens and rewinds the file.  If the stayopen flag is non-zero, the net data base will not be closed after each call
     to getservbyname() or getservbyport().

     The endservent() function closes the file.

     The getservbyname() and getservbyport() functions sequentially search from the beginning of the file until a matching protocol name or port
     number is found, or until EOF is encountered.  If a protocol name is also supplied (non-NULL), searches must also match the protocol.

FILES

     /etc/services

DIAGNOSTICS

     Null pointer (0) returned on EOF or error.

SEE ALSO

     getprotoent(3), services(5)

HISTORY

     The getservent(), getservbyport(), getservbyname(), setservent(), and endservent() functions appeared in 4.2BSD.

BUGS

     These functions use static data storage; if the data is needed for future use, it should be copied before any subsequent calls overwrite it.
     Expecting port numbers to fit in a 32 bit quantity is probably naive.

BSD
								   May 25, 1995 							       BSD

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setservent