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

       For an explanation of the terms used in this section, see attributes(7).
       ┌──────────────────────────────────────────────────────────────────────┬───────────────┬────────────────┐
       │ Interface                                                            │ Attribute     │ Value          │
       ├──────────────────────────────────────────────────────────────────────┼───────────────┼────────────────┤
       │ getservent_r(), getservbyname_r(), getservbyport_r()                 │ Thread safety │ MT-Safe locale │
       └──────────────────────────────────────────────────────────────────────┴───────────────┴────────────────┘

       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 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 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 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 NULL.

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

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

       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.out7tcp1
           ERANGE! Retrying with larger buffer
           getservbyport_r() returned: 0 (success)  (buflen=87)
           s_name=echo; s_proto=tcp; s_port=7; aliases=
           $ ./a.out77777tcp
           getservbyport_r() returned: 0 (success)  (buflen=1024)
           Call failed/record not found

   Programsource

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

           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 (char **p = result_buf.s_aliases; *p != NULL; p++)
               printf("%s ", *p);
           printf("\n");

           exit(EXIT_SUCCESS);
       }

       Standard C library (libc, -lc)

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

       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.

getservent(3), services(5)

Linux man-pages 6.9.1                              2024-06-15                                    getservent_r(3)

       GNU.

#include<netdb.h>intgetservent_r(structservent*restrictresult_buf,charbuf[restrict.buflen],size_tbuflen,structservent**restrictresult);intgetservbyname_r(constchar*restrictname,constchar*restrictproto,structservent*restrictresult_buf,charbuf[restrict.buflen],size_tbuflen,structservent**restrictresult);intgetservbyport_r(intport,constchar*restrictproto,structservent*restrictresult_buf,charbuf[restrict.buflen],size_tbuflen,structservent**restrictresult);

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

       getservent_r(), getservbyname_r(), getservbyport_r():
           Since glibc 2.19:
               _DEFAULT_SOURCE
           glibc 2.19 and earlier:
               _BSD_SOURCE || _SVID_SOURCE

       Functions with similar names exist on  some  other  systems,  though  typically  with  different  calling
       signatures.