getservent_r(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

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

SYNOPSIS         top

       #include <netdb.h>

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

   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

DESCRIPTION         top

       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.

RETURN VALUE         top

       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.

ERRORS         top

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

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌───────────────────────────────┬───────────────┬────────────────┐
       │Interface                      Attribute     Value          │
       ├───────────────────────────────┼───────────────┼────────────────┤
       │getservent_r(),                │ Thread safety │ MT-Safe locale │
       │getservbyname_r(),             │               │                │
       │getservbyport_r()              │               │                │
       └───────────────────────────────┴───────────────┴────────────────┘

CONFORMING TO         top

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

EXAMPLES         top

       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;

           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);
       }

SEE ALSO         top

       getservent(3), services(5)

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

GNU                            2021-03-22                GETSERVENT_R(3)

Pages that refer to this page: getservent(3)nss(5)