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getservent_r(3) — Linux manual page

NAME \|LIBRARY \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ERRORS \|ATTRIBUTES \|VERSIONS \|STANDARDS \|EXAMPLES \|SEE ALSO \|COLOPHON

getservent_r(3)          Library Functions Manualgetservent_r(3)

NAME top

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

LIBRARY top

       Standard C library (libc,-lc)

SYNOPSIS top

#include <netdb.h>int getservent_r(size_t size;struct servent *restrictresult_buf,charbuf[restrictsize], size_tsize,struct servent **restrictresult);int getservbyname_r(size_t size;const char *restrictname,const char *restrictproto,struct servent *restrictresult_buf,charbuf[restrictsize], size_tsize,struct servent **restrictresult);int getservbyport_r(size_t size;intport,const char *restrictproto,struct servent *restrictresult_buf,charbuf[restrictsize], size_tsize,struct servent **restrictresult);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):getservent_r(),getservbyname_r(),getservbyport_r():           Since glibc 2.19:               _DEFAULT_SOURCE           glibc 2.19 and earlier:               _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION top

       Thegetservent_r(),getservbyname_r(), andgetservbyport_r()       functions are the reentrant equivalents of, respectively,getservent(3),getservbyname(3), andgetservbyport(3).  They       differ in the way that theservent 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 allocatedservent       structure as the function result, these functions copy the       structure into the location pointed to byresult_buf.       Thebuf array is used to store the string fields pointed to by the       returnedservent structure.  (The nonreentrant functions allocate       these strings in static storage.)  The size of this array is       specified insize.  Ifbuf is too small, the call fails with the       errorERANGE, 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 toresult_buf; otherwise,*result is set       to NULL.

RETURN VALUE top

       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.

ERRORS top

ENOENT(getservent_r()) No more records in database.ERANGEbuf is too small.  Try again with a larger buffer (and              increasedsize).

ATTRIBUTES top

       For an explanation of the terms used in this section, seeattributes(7).       ┌───────────────────────────────┬───────────────┬────────────────┐       │Interface│Attribute│Value│       ├───────────────────────────────┼───────────────┼────────────────┤       │getservent_r(),               │ Thread safety │ MT-Safe locale │       │getservbyname_r(),            │               │                │       │getservbyport_r()             │               │                │       └───────────────────────────────┴───────────────┴────────────────┘

VERSIONS top

       Functions with similar names exist on some other systems, though       typically with different calling signatures.

STANDARDS top

       GNU.

EXAMPLES top

       The program below usesgetservbyport_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 forsize; ifgetservbyport_r()       fails with the errorERANGE, 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 foundProgram 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);       }

COLOPHON top

       This page is part of theman-pages (Linux kernel and C library       user-space interface documentation) project.  Information about       the project can be found at        ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report       for this manual page, see       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.       This page was obtained from the tarball man-pages-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  If you discover any rendering problems in this HTML       version of the page, or you believe there is a better or more up-       to-date source for the page, or you have corrections or       improvements to the information in this COLOPHON (which isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-06-28getservent_r(3)

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