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

#include <netdb.h>
int getservent_r(struct servent *restrict result_buf,
 char buf[restrict .buflen], size_t buflen,
 struct servent **restrict result);
int getservbyname_r(const char *restrict name,
 const char *restrict proto,
 struct servent *restrict result_buf,
 char buf[restrict .buflen], size_t buflen,
 struct servent **restrict result);
int getservbyport_r(int port,
 const char *restrict proto,
 struct servent *restrict result_buf,
 char buf[restrict .buflen], size_t buflen,
 struct servent **restrict result);

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

    Since glibc 2.19:
        _DEFAULT_SOURCE
    glibc 2.19 and earlier:
        _BSD_SOURCE || _SVID_SOURCE

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.

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.

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 <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);
}

getservent(3), services(5)