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Linux 2.6 - man page for getservbyport_r (linux section 3)

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GETSERVENT_R(3) 		    Linux Programmer's Manual			  GETSERVENT_R(3)

NAME
       getservent_r, getservbyname_r, getservbyport_r - get service entry (reentrant)

SYNOPSIS
       #include <netdb.h>

       int getservent_r(struct servent *result_buf, char *buf,
		       size_t buflen, struct servent **result);

       int getservbyname_r(const char *name, const char *proto,
		       struct servent *result_buf, char *buf,
		       size_t buflen, struct servent **result);

       int getservbyport_r(int port, const char *proto,
		       struct servent *result_buf, char *buf,
		       size_t buflen, struct servent **result);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       getservent_r(), getservbyname_r(), getservbyport_r():
	   _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION
       The  getservent_r(),  getservbyname_r(), and getservbyport_r() functions are the reentrant
       equivalents of, respectively, getservent(3), getservbyname(3), and getservbyport(3).  They
       differ in the way that the servent structure is returned, and in the function calling sig-
       nature and return value.  This manual page describes just the differences  from	the  non-
       reentrant functions.

       Instead of returning a pointer to a statically allocated servent structure as the function
       result, these functions copy the structure into the location pointed to by result_buf.

       The buf array is used to store the string fields pointed to by the returned servent struc-
       ture.  (The nonreentrant functions allocate these strings in static storage.)  The size of
       this array is specified in buflen.  If buf is too small, the call  fails  with  the  error
       ERANGE,	and  the  caller  must	try again with a larger buffer.  (A buffer of length 1024
       bytes should be sufficient for most applications.)

       If the function call successfully obtains a service record, then *result is  set  pointing
       to result_buf; otherwise, *result is set to NULL.

RETURN VALUE
       On  success,  these  functions  return 0.  On error, they return one of the positive error
       numbers listed in errors.

       On error, record not found (getservbyname_r(), getservbyport_r()), or end of  input  (get-
       servent_r()) result is set to NULL.

ERRORS
       ENOENT (getservent_r()) No more records in database.

       ERANGE buf is too small.  Try again with a larger buffer (and increased buflen).

CONFORMING TO
       These functions are GNU extensions.  Functions with similar names exist on some other sys-
       tems, though typically with different calling signatures.

EXAMPLE
       The program below uses getservbyport_r() to retrieve the service record for the	port  and
       protocol  named	in  its  first	command-line argument.	If a third (integer) command-line
       argument is supplied, it is used as the initial value  for  buflen;  if	getservbyport_r()
       fails  with the error ERANGE, the program retries with larger buffer sizes.  The following
       shell session shows a couple of sample runs:

	   $ ./a.out 7 tcp 1
	   ERANGE! Retrying with larger buffer
	   getservbyport_r() returned: 0 (success)  (buflen=87)
	   s_name=echo; s_proto=tcp; s_port=7; aliases=
	   $ ./a.out 77777 tcp
	   getservbyport_r() returned: 0 (success)  (buflen=1024)
	   Call failed/record not found

   Program source

       #define _GNU_SOURCE
       #include <ctype.h>
       #include <netdb.h>
       #include <stdlib.h>
       #include <stdio.h>
       #include <errno.h>
       #include <string.h>

       #define MAX_BUF 10000

       int
       main(int argc, char *argv[])
       {
	   int buflen, erange_cnt, port, s;
	   struct servent result_buf;
	   struct servent *result;
	   char buf[MAX_BUF];
	   char *protop;
	   char **p;

	   if (argc < 3) {
	       printf("Usage: %s port-num proto-name [buflen]\n", argv[0]);
	       exit(EXIT_FAILURE);
	   }

	   port = htons(atoi(argv[1]));
	   protop = (strcmp(argv[2], "null") == 0 ||
		  strcmp(argv[2], "NULL") == 0) ?  NULL : argv[2];

	   buflen = 1024;
	   if (argc > 3)
	       buflen = atoi(argv[3]);

	   if (buflen > MAX_BUF) {
	       printf("Exceeded buffer limit (%d)\n", MAX_BUF);
	       exit(EXIT_FAILURE);
	   }

	   erange_cnt = 0;
	   do {
	       s = getservbyport_r(port, protop, &result_buf,
			    buf, buflen, &result);
	       if (s == ERANGE) {
		   if (erange_cnt == 0)
		       printf("ERANGE! Retrying with larger buffer\n");
		   erange_cnt++;

		   /* Increment a byte at a time so we can see exactly
		      what size buffer was required */

		   buflen++;

		   if (buflen > MAX_BUF) {
		       printf("Exceeded buffer limit (%d)\n", MAX_BUF);
		       exit(EXIT_FAILURE);
		   }
	       }
	   } while (s == ERANGE);

	   printf("getservbyport_r() returned: %s  (buflen=%d)\n",
		   (s == 0) ? "0 (success)" : (s == ENOENT) ? "ENOENT" :
		   strerror(s), buflen);

	   if (s != 0 || result == NULL) {
	       printf("Call failed/record not found\n");
	       exit(EXIT_FAILURE);
	   }

	   printf("s_name=%s; s_proto=%s; s_port=%d; aliases=",
		       result_buf.s_name, result_buf.s_proto,
		       ntohs(result_buf.s_port));
	   for (p = result_buf.s_aliases; *p != NULL; p++)
	       printf("%s ", *p);
	   printf("\n");

	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       getservent(3), services(5)

COLOPHON
       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

GNU					    2010-09-10				  GETSERVENT_R(3)
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