man3/getservent_r.3

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  26 .TH GETSERVENT_R 3  2019-03-06 "GNU" "Linux Programmer's Manual"
  27 .SH NAME
  28 getservent_r, getservbyname_r, getservbyport_r \- get
  29 service entry (reentrant)
  30 .SH SYNOPSIS
  31 .nf
  32 .B #include <netdb.h>
  33 .PP
  34 .BI "int getservent_r(struct servent *" result_buf ", char *" buf ,
  35 .BI "                size_t " buflen ", struct servent **" result );
  36 .PP
  37 .BI "int getservbyname_r(const char *" name ", const char *" proto ,
  38 .BI "                struct servent *" result_buf ", char *" buf ,
  39 .BI "                size_t " buflen ", struct servent **" result );
  40 .PP
  41 .BI "int getservbyport_r(int " port ", const char *" proto ,
  42 .BI "                struct servent *" result_buf ", char *" buf ,
  43 .BI "                size_t " buflen ", struct servent **" result );
  44 .PP
  45 .fi
  46 .in -4n
  47 Feature Test Macro Requirements for glibc (see
  48 .BR feature_test_macros (7)):
  49 .ad l
  50 .in
  51 .PP
  52 .BR getservent_r (),
  53 .BR getservbyname_r (),
  54 .BR getservbyport_r ():
  55     Since glibc 2.19:
  56         _DEFAULT_SOURCE
  57     Glibc 2.19 and earlier:
  58         _BSD_SOURCE || _SVID_SOURCE
  59 .ad b
  60 .SH DESCRIPTION
  61 The
  62 .BR getservent_r (),
  63 .BR getservbyname_r (),
  64 and
  65 .BR getservbyport_r ()
  66 functions are the reentrant equivalents of, respectively,
  67 .BR getservent (3),
  68 .BR getservbyname (3),
  69 and
  70 .BR getservbyport (3).
  71 They differ in the way that the
  72 .I servent
  73 structure is returned,
  74 and in the function calling signature and return value.
  75 This manual page describes just the differences from
  76 the nonreentrant functions.
  77 .PP
  78 Instead of returning a pointer to a statically allocated
  79 .I servent
  80 structure as the function result,
  81 these functions copy the structure into the location pointed to by
  82 .IR result_buf .
  83 .PP
  84 The
  85 .I buf
  86 array is used to store the string fields pointed to by the returned
  87 .I servent
  88 structure.
  89 (The nonreentrant functions allocate these strings in static storage.)
  90 The size of this array is specified in
  91 .IR buflen .
  92 If
  93 .I buf
  94 is too small, the call fails with the error
  95 .BR ERANGE ,
  96 and the caller must try again with a larger buffer.
  97 (A buffer of length 1024 bytes should be sufficient for most applications.)
  98 .\" I can find no information on the required/recommended buffer size;
  99 .\" the nonreentrant functions use a 1024 byte buffer -- mtk.
 100 .PP
 101 If the function call successfully obtains a service record, then
 102 .I *result
 103 is set pointing to
 104 .IR result_buf ;
 105 otherwise,
 106 .I *result
 107 is set to NULL.
 108 .SH RETURN VALUE
 109 On success, these functions return 0.
 110 On error, they return one of the positive error numbers listed in errors.
 111 .PP
 112 On error, record not found
 113 .RB ( getservbyname_r (),
 114 .BR getservbyport_r ()),
 115 or end of input
 116 .RB ( getservent_r ())
 117 .I result
 118 is set to NULL.
 119 .SH ERRORS
 120 .TP
 121 .B ENOENT
 122 .RB ( getservent_r ())
 123 No more records in database.
 124 .TP
 125 .B ERANGE
 126 .I buf
 127 is too small.
 128 Try again with a larger buffer
 129 (and increased
 130 .IR buflen ).
 131 .SH ATTRIBUTES
 132 For an explanation of the terms used in this section, see
 133 .BR attributes (7).
 134 .ad l
 135 .TS
 136 allbox;
 137 lbw18 lb lb
 138 l l l.
 139 Interface       Attribute       Value
 140 T{
 141 .BR getservent_r (),
 142 .BR getservbyname_r (),
 143 .BR getservbyport_r ()
 144 T}      Thread safety   MT-Safe locale
 145 .TE
 146 .ad
 147 .SH CONFORMING TO
 148 These functions are GNU extensions.
 149 Functions with similar names exist on some other systems,
 150 though typically with different calling signatures.
 151 .SH EXAMPLE
 152 The program below uses
 153 .BR getservbyport_r ()
 154 to retrieve the service record for the port and protocol named
 155 in its first command-line argument.
 156 If a third (integer) command-line argument is supplied,
 157 it is used as the initial value for
 158 .IR buflen ;
 159 if
 160 .BR getservbyport_r ()
 161 fails with the error
 162 .BR ERANGE ,
 163 the program retries with larger buffer sizes.
 164 The following shell session shows a couple of sample runs:
 165 .PP
 166 .in +4n
 167 .EX
 168 .RB "$" " ./a.out 7 tcp 1"
 169 ERANGE! Retrying with larger buffer
 170 getservbyport_r() returned: 0 (success)  (buflen=87)
 171 s_name=echo; s_proto=tcp; s_port=7; aliases=
 172 .RB "$" " ./a.out 77777 tcp"
 173 getservbyport_r() returned: 0 (success)  (buflen=1024)
 174 Call failed/record not found
 175 .EE
 176 .in
 177 .SS Program source
 178 \&
 179 .EX
 180 #define _GNU_SOURCE
 181 #include <ctype.h>
 182 #include <netdb.h>
 183 #include <stdlib.h>
 184 #include <stdio.h>
 185 #include <errno.h>
 186 #include <string.h>
 187
 188 #define MAX_BUF 10000
 189
 190 int
 191 main(int argc, char *argv[])
 192 {
 193     int buflen, erange_cnt, port, s;
 194     struct servent result_buf;
 195     struct servent *result;
 196     char buf[MAX_BUF];
 197     char *protop;
 198     char **p;
 199
 200     if (argc < 3) {
 201         printf("Usage: %s port\-num proto\-name [buflen]\en", argv[0]);
 202         exit(EXIT_FAILURE);
 203     }
 204
 205     port = htons(atoi(argv[1]));
 206     protop = (strcmp(argv[2], "null") == 0 ||
 207               strcmp(argv[2], "NULL") == 0) ?  NULL : argv[2];
 208
 209     buflen = 1024;
 210     if (argc > 3)
 211         buflen = atoi(argv[3]);
 212
 213     if (buflen > MAX_BUF) {
 214         printf("Exceeded buffer limit (%d)\en", MAX_BUF);
 215         exit(EXIT_FAILURE);
 216     }
 217
 218     erange_cnt = 0;
 219     do {
 220         s = getservbyport_r(port, protop, &result_buf,
 221                      buf, buflen, &result);
 222         if (s == ERANGE) {
 223             if (erange_cnt == 0)
 224                 printf("ERANGE! Retrying with larger buffer\en");
 225             erange_cnt++;
 226
 227             /* Increment a byte at a time so we can see exactly
 228                what size buffer was required */
 229
 230             buflen++;
 231
 232             if (buflen > MAX_BUF) {
 233                 printf("Exceeded buffer limit (%d)\en", MAX_BUF);
 234                 exit(EXIT_FAILURE);
 235             }
 236         }
 237     } while (s == ERANGE);
 238
 239     printf("getservbyport_r() returned: %s  (buflen=%d)\en",
 240             (s == 0) ? "0 (success)" : (s == ENOENT) ? "ENOENT" :
 241             strerror(s), buflen);
 242
 243     if (s != 0 || result == NULL) {
 244         printf("Call failed/record not found\en");
 245         exit(EXIT_FAILURE);
 246     }
 247
 248     printf("s_name=%s; s_proto=%s; s_port=%d; aliases=",
 249                 result_buf.s_name, result_buf.s_proto,
 250                 ntohs(result_buf.s_port));
 251     for (p = result_buf.s_aliases; *p != NULL; p++)
 252         printf("%s ", *p);
 253     printf("\en");
 254
 255     exit(EXIT_SUCCESS);
 256 }
 257 .EE
 258 .SH SEE ALSO
 259 .BR getservent (3),
 260 .BR services (5)