21世紀C語言之21 :curl

DAY 21

21世紀C語言實作及感想系列第 21 篇

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timloo

2014-10-21 23:42:25

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老牌的網路連線函式庫。

http://curl.haxx.se/libcurl/c/example.html

看範例，

makefile的寫法

CFLAGS=-g -Wall -O3 curl-config --cflags -I/usr/include/libxml2
LDLIBS=curl-config --libs -lxml2 -lpthread
CC=gcc

把網頁的HTML碼，寫到畫面上，

http://curl.haxx.se/libcurl/c/simple.html

make simple 可編譯

#include <stdio.h>
#include <curl/curl.h>
 
int main(void)
{
  CURL *curl;
  CURLcode res;
 
  curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
    /* example.com is redirected, so we tell libcurl to follow redirection */ 
    curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
 
    /* Perform the request, res will get the return code */ 
    res = curl_easy_perform(curl);
    /* Check for errors */ 
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n",
              curl_easy_strerror(res));
 
    /* always cleanup */ 
    curl_easy_cleanup(curl);
  }
  return 0;
}

這樣就把網頁抓回來了!!

curl 的API命名原則，非常一致。

curl_easy_init();





curl_easy_setopt





curl_easy_cleanup(curl);

包裝的很簡單。

另一個範例，抓網頁，並用libxml2解析TAG。

在下載的源碼中，docs/examples/htmltitle.cpp

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <string>
#include <curl/curl.h>
#include <libxml/HTMLparser.h>

//
//  Case-insensitive string comparison
//

#ifdef _MSC_VER
#define COMPARE(a, b) (!stricmp((a), (b)))
#else
#define COMPARE(a, b) (!strcasecmp((a), (b)))
#endif

//
//  libxml callback context structure
//

struct Context
{
  Context(): addTitle(false) { }

  bool addTitle;
  std::string title;
};

//
//  libcurl variables for error strings and returned data

static char errorBuffer[CURL_ERROR_SIZE];
static std::string buffer;

//
//  libcurl write callback function
//

static int writer(char *data, size_t size, size_t nmemb,
                  std::string *writerData)
{
  if (writerData == NULL)
    return 0;

  writerData->append(data, size*nmemb);

  return size * nmemb;
}

//
//  libcurl connection initialization
//

static bool init(CURL *&conn, char *url)
{
  CURLcode code;

  conn = curl_easy_init();

  if (conn == NULL)
  {
    fprintf(stderr, "Failed to create CURL connection\n");

    exit(EXIT_FAILURE);
  }

  code = curl_easy_setopt(conn, CURLOPT_ERRORBUFFER, errorBuffer);
  if (code != CURLE_OK)
  {
    fprintf(stderr, "Failed to set error buffer [%d]\n", code);

    return false;
  }

  code = curl_easy_setopt(conn, CURLOPT_URL, url);
  if (code != CURLE_OK)
  {
    fprintf(stderr, "Failed to set URL [%s]\n", errorBuffer);

    return false;
  }

  code = curl_easy_setopt(conn, CURLOPT_FOLLOWLOCATION, 1L);
  if (code != CURLE_OK)
  {
    fprintf(stderr, "Failed to set redirect option [%s]\n", errorBuffer);

    return false;
  }

  code = curl_easy_setopt(conn, CURLOPT_WRITEFUNCTION, writer);
  if (code != CURLE_OK)
  {
    fprintf(stderr, "Failed to set writer [%s]\n", errorBuffer);

    return false;
  }

  code = curl_easy_setopt(conn, CURLOPT_WRITEDATA, &buffer);
  if (code != CURLE_OK)
  {
    fprintf(stderr, "Failed to set write data [%s]\n", errorBuffer);

    return false;
  }

  return true;
}

//
//  libxml start element callback function
//

static void StartElement(void *voidContext,
                         const xmlChar *name,
                         const xmlChar **attributes)
{
  Context *context = (Context *)voidContext;

  if (COMPARE((char *)name, "TITLE"))
  {
    context->title = "";
    context->addTitle = true;
  }
  (void) attributes;
}

//
//  libxml end element callback function
//

static void EndElement(void *voidContext,
                       const xmlChar *name)
{
  Context *context = (Context *)voidContext;

  if (COMPARE((char *)name, "TITLE"))
    context->addTitle = false;
}

//
//  Text handling helper function
//

static void handleCharacters(Context *context,
                             const xmlChar *chars,
                             int length)
{
  if (context->addTitle)
    context->title.append((char *)chars, length);
}

//
//  libxml PCDATA callback function
//

static void Characters(void *voidContext,
                       const xmlChar *chars,
                       int length)
{
  Context *context = (Context *)voidContext;

  handleCharacters(context, chars, length);
}

//
//  libxml CDATA callback function
//

static void cdata(void *voidContext,
                  const xmlChar *chars,
                  int length)
{
  Context *context = (Context *)voidContext;

  handleCharacters(context, chars, length);
}

//
//  libxml SAX callback structure
//

static htmlSAXHandler saxHandler =
{
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  StartElement,
  EndElement,
  NULL,
  Characters,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  NULL,
  cdata,
  NULL
};

//
//  Parse given (assumed to be) HTML text and return the title
//

static void parseHtml(const std::string &html,
                      std::string &title)
{
  htmlParserCtxtPtr ctxt;
  Context context;

  ctxt = htmlCreatePushParserCtxt(&saxHandler, &context, "", 0, "",
                                  XML_CHAR_ENCODING_NONE);

  htmlParseChunk(ctxt, html.c_str(), html.size(), 0);
  htmlParseChunk(ctxt, "", 0, 1);

  htmlFreeParserCtxt(ctxt);

  title = context.title;
}

int main(int argc, char *argv[])
{
  CURL *conn = NULL;
  CURLcode code;
  std::string title;

  // Ensure one argument is given

  if (argc != 2)
  {
    fprintf(stderr, "Usage: %s <url>\n", argv[0]);

    exit(EXIT_FAILURE);
  }

  curl_global_init(CURL_GLOBAL_DEFAULT);

  // Initialize CURL connection

  if (!init(conn, argv[1]))
  {
    fprintf(stderr, "Connection initializion failed\n");

    exit(EXIT_FAILURE);
  }

  // Retrieve content for the URL

  code = curl_easy_perform(conn);
  curl_easy_cleanup(conn);

  if (code != CURLE_OK)
  {
    fprintf(stderr, "Failed to get '%s' [%s]\n", argv[1], errorBuffer);

    exit(EXIT_FAILURE);
  }

  // Parse the (assumed) HTML code

  parseHtml(buffer, title);

  // Display the extracted title

  printf("Title: %s\n", title.c_str());

  return EXIT_SUCCESS;
}

編譯的方法為，

$ g++ -Wall -I/usr/curl/include -I/usr/include/libxml2 htmltitle.cpp \