path: root/man3/getservent_r.3

'\" t
.\" Copyright 2008, Linux Foundation, written by Michael Kerrisk
.\"	<mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH getservent_r 3 2023-07-20 "Linux man-pages 6.05.01"
.SH NAME
getservent_r, getservbyname_r, getservbyport_r \- get
service entry (reentrant)
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <netdb.h>
.PP
.BI "int getservent_r(struct servent *restrict " result_buf ,
.BI "                 char " buf "[restrict ." buflen "], size_t " buflen ,
.BI "                 struct servent **restrict " result );
.BI "int getservbyname_r(const char *restrict " name ,
.BI "                 const char *restrict " proto ,
.BI "                 struct servent *restrict " result_buf ,
.BI "                 char " buf "[restrict ." buflen "], size_t " buflen ,
.BI "                 struct servent **restrict " result );
.BI "int getservbyport_r(int " port ,
.BI "                 const char *restrict " proto ,
.BI "                 struct servent *restrict " result_buf ,
.BI "                 char " buf "[restrict ." buflen "], size_t " buflen ,
.BI "                 struct servent **restrict " result );
.PP
.fi
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR getservent_r (),
.BR getservbyname_r (),
.BR getservbyport_r ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    glibc 2.19 and earlier:
        _BSD_SOURCE || _SVID_SOURCE
.fi
.SH DESCRIPTION
The
.BR getservent_r (),
.BR getservbyname_r (),
and
.BR getservbyport_r ()
functions are the reentrant equivalents of, respectively,
.BR getservent (3),
.BR getservbyname (3),
and
.BR getservbyport (3).
They differ in the way that the
.I servent
structure is returned,
and in the function calling signature and return value.
This manual page describes just the differences from
the nonreentrant functions.
.PP
Instead of returning a pointer to a statically allocated
.I servent
structure as the function result,
these functions copy the structure into the location pointed to by
.IR result_buf .
.PP
The
.I buf
array is used to store the string fields pointed to by the returned
.I servent
structure.
(The nonreentrant functions allocate these strings in static storage.)
The size of this array is specified in
.IR buflen .
If
.I buf
is too small, the call fails with the error
.BR ERANGE ,
and the caller must try again with a larger buffer.
(A buffer of length 1024 bytes should be sufficient for most applications.)
.\" I can find no information on the required/recommended buffer size;
.\" the nonreentrant functions use a 1024 byte buffer -- mtk.
.PP
If the function call successfully obtains a service record, then
.I *result
is set pointing to
.IR result_buf ;
otherwise,
.I *result
is set to NULL.
.SH RETURN VALUE
On success, these functions return 0.
On error, they return one of the positive error numbers listed in errors.
.PP
On error, record not found
.RB ( getservbyname_r (),
.BR getservbyport_r ()),
or end of input
.RB ( getservent_r ())
.I result
is set to NULL.
.SH ERRORS
.TP
.B ENOENT
.RB ( getservent_r ())
No more records in database.
.TP
.B ERANGE
.I buf
is too small.
Try again with a larger buffer
(and increased
.IR buflen ).
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR getservent_r (),
.BR getservbyname_r (),
.BR getservbyport_r ()
T}	Thread safety	MT-Safe locale
.TE
.sp 1
.SH VERSIONS
Functions with similar names exist on some other systems,
though typically with different calling signatures.
.SH STANDARDS
GNU.
.SH EXAMPLES
The program below uses
.BR 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
.IR buflen ;
if
.BR getservbyport_r ()
fails with the error
.BR ERANGE ,
the program retries with larger buffer sizes.
The following shell session shows a couple of sample runs:
.PP
.in +4n
.EX
.RB "$" " ./a.out 7 tcp 1"
ERANGE! Retrying with larger buffer
getservbyport_r() returned: 0 (success)  (buflen=87)
s_name=echo; s_proto=tcp; s_port=7; aliases=
.RB "$" " ./a.out 77777 tcp"
getservbyport_r() returned: 0 (success)  (buflen=1024)
Call failed/record not found
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (getservent_r.c)
.EX
#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]\en", 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)\en", 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\en");
            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)\en", MAX_BUF);
                exit(EXIT_FAILURE);
            }
        }
    } while (s == ERANGE);
\&
    printf("getservbyport_r() returned: %s  (buflen=%d)\en",
           (s == 0) ? "0 (success)" : (s == ENOENT) ? "ENOENT" :
           strerror(s), buflen);
\&
    if (s != 0 || result == NULL) {
        printf("Call failed/record not found\en");
        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("\en");
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR getservent (3),
.BR services (5)