getprotoent_r(3) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       #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

DESCRIPTION         top

       The getprotoent_r(), getprotobyname_r(), and getprotobynumber_r()
       functions 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 differences 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

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

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 (getprotobyname_r(),
       getprotobynumber_r()), or end of input (getprotoent_r()) result is
       set to NULL.

ERRORS         top

       ENOENT (getprotoent_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

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

CONFORMING TO         top

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

EXAMPLES         top

       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);


SEE ALSO         top

       getprotoent(3), protocols(5)

COLOPHON         top

       This page is part of release 5.08 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                              2020-06-09                 GETPROTOENT_R(3)

Pages that refer to this page: endprotoent(3)getprotobyname(3)getprotobynumber(3)getprotoent(3)setprotoent(3)