Show the name of libcurl's resolver.

ShowLibcurlResolver.c

/* Show the name of libcurl's resolver.

Usage: ShowLibcurlResolver

"Is there a way to programmatically know if the version being used was compiled
with the threaded resolver?"

curl-library mailing list thread:
'segfaulting in Curl_num_addresses on OS X and Ubuntu'
http://curl.haxx.se/mail/lib-2015-08/0002.html

Copyright (C) 2015 Jay Satiro <raysatiro@yahoo.com>
http://curl.haxx.se/docs/copyright.html

https://gist.github.com/jay/d3323a81345e75462b70
*/

#include <stdio.h>
#include <stdlib.h>

/* http://curl.haxx.se/download.html */
#include <curl/curl.h>


#undef FALSE
#define FALSE 0
#undef TRUE
#define TRUE 1

typedef enum {
  CURLRESOLVER_SYNCH,
  CURLRESOLVER_CARES,
  CURLRESOLVER_THREADED,
  CURLRESOLVER_LAST /* not for use, only a marker for last-in-list */
} curl_resolvertype;

/* Get the type of the resolver used by libcurl.
The return value is a value from curl_resolvertype.
*/
curl_resolvertype curl_resolver_type()
{
  curl_version_info_data *ver = curl_version_info(CURLVERSION_NOW);

  if(!(ver->features & CURL_VERSION_ASYNCHDNS))
    return CURLRESOLVER_SYNCH;
  else if(!ver->age || ver->ares_num)
    return CURLRESOLVER_CARES;
  else
    return CURLRESOLVER_THREADED;
}

/* Get the name of the resolver used by libcurl.
The return value is a pointer to a zero terminated string that must not be
freed by the caller. If the type is not recognized the return value is NULL.
*/
const char *curl_resolver()
{
  switch(curl_resolver_type()) {
  case CURLRESOLVER_SYNCH: return "synchronous";
  case CURLRESOLVER_CARES: return "c-ares";
  case CURLRESOLVER_THREADED: return "threaded";
  default: return NULL;
  }
}

int main(int argc, char *argv[])
{
  const char *resolver_name = NULL;

  (void)argv;

  if(argc > 1) {
    fprintf(stderr, "Usage: ShowLibcurlResolver\n"
                    "Output: synchronous, c-ares or threaded.\n"
                    "If libcurl init fails no resolver name is output.\n");
    return EXIT_FAILURE;
  }

  if(curl_global_init(CURL_GLOBAL_ALL)) {
    fprintf(stderr, "Fatal: The initialization of libcurl has failed.\n");
    return EXIT_FAILURE;
  }

  if(atexit(curl_global_cleanup)) {
    fprintf(stderr, "Fatal: atexit failed to register curl_global_cleanup.\n");
    curl_global_cleanup();
    return EXIT_FAILURE;
  }

  resolver_name = curl_resolver();
  if(!resolver_name) {
    fprintf(stderr, "Fatal: Libcurl resolver type not recognized.\n");
    return EXIT_FAILURE;
  }

  printf("%s\n", resolver_name);
  return EXIT_SUCCESS;
}