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


#include <netdb.h>
int getprotoent_r( struct protoent *restrict result_buf,
  char *restrict buf,
  size_t buflen,
  struct protoent **restrict result);
int getprotobyname_r( const char *restrict name,
  struct protoent *restrict result_buf,
  char *restrict buf,
  size_t buflen,
  struct protoent **restrict result);
int getprotobynumber_r( int proto,
  struct protoent *restrict result_buf,
  char *restrict buf,
  size_t buflen,
  struct protoent **restrict result);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getprotoent_r(), getprotobyname_r(), getprotobynumber_r():
Since glibc 2.19:
_DEFAULT_SOURCE Glibc 2.19 and earlier:


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 applications.)

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


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.



(getprotoent_r()) No more records in database.


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


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

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


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


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];

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



getprotoent(3), protocols(5)


This page is part of release 5.11 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−pages/.

  Copyright 2008, Linux Foundation, written by Michael Kerrisk

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