getservent_r(3)

💖 getservent_r — get service entry (reentrant) 💖

heyyy bestie, pull up a chair 🫶 this one's from the Library Functions Manual, and it's the glow-up version of the classic service-lookup trio. Same energy, but reentrant — meaning it plays nice when multiple threads wanna do lookups at the same time. icon behavior, honestly 👑

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📚 LIBRARY

Standard C library (libc, -lc) 🎀

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✨ SYNOPSIS

#include <netdb.h>

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

🌈 Feature Test Macro Requirements for glibc (peep feature_test_macros(7) for the lore)

getservent_r(), getservbyname_r(), getservbyport_r():

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

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🦋 DESCRIPTION

So here's the tea ☕: getservent_r(), getservbyname_r(), and getservbyport_r() 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. no cap, that's the whole scope 💅

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. slay, no shared state, no drama.

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 size. 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 size 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. that's it, that's the vibe check 🎯

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🎀 RETURN VALUE

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.

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💀 ERRORS

• ENOENT — (getservent_r()) No more records in database. that's a wrap, bestie, nothing left to read 🙈
• ERANGE — buf is too small. Try again with a larger buffer (and increased size). literally just give it more room 🫶

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👑 ATTRIBUTES

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

• getservent_r(), getservbyname_r(), getservbyport_r() — Thread safety: MT-Safe locale ✨

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🌈 VERSIONS

Functions with similar names exist on some other systems, though typically with different calling signatures. so like… don't assume the doppelgänger behaves the same, mkay? ⚠️

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⭐ STANDARDS

GNU. periodt 💅

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🧚 EXAMPLES

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 size; 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)  (size=87)
s_name=echo; s_proto=tcp; s_port=7; aliases=
$ ./a.out 77777 tcp
getservbyport_r() returned: 0 (success)  (size=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

#define streq(...)  (strcmp(__VA_ARGS__) == 0)

int
main(int argc, char *argv[])
{
    int size, 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 [size]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    port = htons(atoi(argv[1]));
    protop = (streq(argv[2], "null") ||
              streq(argv[2], "NULL")) ?  NULL : argv[2];

    size = 1024;
    if (argc > 3)
        size = atoi(argv[3]);

    if (size > 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, size, &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.  */

            size++;

            if (size > MAX_BUF) {
                printf("Exceeded buffer limit (%d)\n", MAX_BUF);
                exit(EXIT_FAILURE);
            }
        }
    } while (s == ERANGE);

    printf("getservbyport_r() returned: %s  (size=%d)\n",
           (s == 0) ? "0 (success)" : (s == ENOENT) ? "ENOENT" :
           strerror(s), size);

    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);
}

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🔥 SEE ALSO

getservent(3), services(5) 💜

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and that's the whole page bestie!! 💅✨ ty for reading, go forth and look up services reentrantly 🦋 this was Linux man-pages 6.20, dated 2026-02-22, and it lives at getservent_r(3) — mwah, take care of her 🌸💖👑