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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | VERSIONS | STANDARDS | EXAMPLES | SEE ALSO | COLOPHON |
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getservent_r(3) Library Functions Manual getservent_r(3)
getservent_r, getservbyname_r, getservbyport_r - get service entry
(reentrant)
Standard C library (libc, -lc)
#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 (see
feature_test_macros(7)):
getservent_r(), getservbyname_r(), getservbyport_r():
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 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.
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.
ENOENT (getservent_r()) No more records in database.
ERANGE buf is too small. Try again with a larger buffer (and
increased size).
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────┼───────────────┼────────────────┤
│ getservent_r(), │ Thread safety │ MT-Safe locale │
│ getservbyname_r(), │ │ │
│ getservbyport_r() │ │ │
└───────────────────────────────┴───────────────┴────────────────┘
Functions with similar names exist on some other systems, though
typically with different calling signatures.
GNU.
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
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 = (strcmp(argv[2], "null") == 0 ||
strcmp(argv[2], "NULL") == 0) ? 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);
}
getservent(3), services(5)
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man-pages@man7.org
Linux man-pages 6.15 2025-06-28 getservent_r(3)
Pages that refer to this page: getservent(3), nss(5)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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