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

       getprotoent_r,  getprotobyname_r,  getprotobynumber_r  -  get  protocol
       entry (reentrant)

       #include <netdb.h>

       int getprotoent_r(struct protoent *result_buf, char *buf,
                       size_t buflen, struct protoent **result);

       int getprotobyname_r(const char *name,
                       struct protoent *result_buf, char *buf,
                       size_t buflen, struct protoent **result);

       int getprotobynumber_r(int proto,
                       struct protoent *result_buf, char *buf,
                       size_t buflen, struct protoent **result);

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

       getprotoent_r(), getprotobyname_r(), getprotobynumber_r():
           Since glibc 2.19:
           Glibc 2.19 and earlier:
               _BSD_SOURCE || _SVID_SOURCE

       The getprotoent_r(), getprotobyname_r(), and getprotobynumber_r() func-
       tions  are  the reentrant equivalents of, respectively, getprotoent(3),
       getprotobyname(3), and getprotobynumber(3).  They  differ  in  the  way
       that  the  protoent  structure is returned, and in the function calling
       signature and return value.  This manual page describes just  the  dif-
       ferences from the nonreentrant functions.

       Instead  of  returning  a  pointer  to  a statically allocated protoent
       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 protoent  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  protocol  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 (getprotobyname_r(), getprotobynumber_r()),
       or end of input (getprotoent_r()) result is set to NULL.

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

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

       For   an   explanation   of   the  terms  used  in  this  section,  see

       |Interface            | Attribute     | Value          |
       |getprotoent_r(),     | Thread safety | MT-Safe locale |
       |getprotobyname_r(),  |               |                |
       |getprotobynumber_r() |               |                |

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

       The program below uses  getprotobyname_r()  to  retrieve  the  protocol
       record for the protocol named in its first command-line argument.  If a
       second (integer) command-line argument is supplied, it is used  as  the
       initial  value  for  buflen; if getprotobyname_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 tcp 1
           ERANGE! Retrying with larger buffer
           getprotobyname_r() returned: 0 (success)  (buflen=78)
           p_name=tcp; p_proto=6; aliases=TCP
           $ ./a.out xxx 1
           ERANGE! Retrying with larger buffer
           getprotobyname_r() returned: 0 (success)  (buflen=100)
           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, s;
           struct protoent result_buf;
           struct protoent *result;
           char buf[MAX_BUF];
           char **p;

           if (argc < 2) {
               printf("Usage: %s proto-name [buflen]\n", argv[0]);

           buflen = 1024;
           if (argc > 2)
               buflen = atoi(argv[2]);

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

           erange_cnt = 0;
           do {
               s = getprotobyname_r(argv[1], &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("getprotobyname_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("p_name=%s; p_proto=%d; aliases=",
                       result_buf.p_name, result_buf.p_proto);
           for (p = result_buf.p_aliases; *p != NULL; p++)
               printf("%s ", *p);


       getprotoent(3), protocols(5)

       This  page  is  part of release 4.15 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

GNU                               2017-09-15                  GETPROTOENT_R(3)
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