getservbyport_r man page on Archlinux

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

       getservent_r,  getservbyname_r,	getservbyport_r	 -  get	 service entry

       #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():

       The getservent_r(), getservbyname_r(), and getservbyport_r()  functions
       are  the	 reentrant  equivalents	 of, respectively, getservent(3), get‐
       servbyname(3), and getservbyport(3).  They differ in the way  that  the
       servent	structure  is  returned, and in the function calling signature
       and return value.  This manual page describes just the differences from
       the nonreentrant functions.

       Instead of returning a pointer to a statically allocated servent struc‐
       ture 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 structure.  (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

       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 (getservent_r()) result is set to NULL.

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

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

       These functions are GNU extensions.  Functions with similar names exist
       on some other systems, though typically with different  calling	signa‐

       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

       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]);

	   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);

	   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");

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


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

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


       getservent(3), services(5)

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

GNU				  2010-09-10		       GETSERVENT_R(3)

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