curl编译安装以及断点续传下载原理

根据百度百科的解释：CURL是一个利用URL语法在命令行下工作的文件传输工具，1997年首次发行。它支持文件上传和下载，所以是综合传输工具，但按传统，习惯称cURL为下载工具。cURL还包含了用于程序开发的libcurl。

cURL是一个利用URL语法在命令行下工作的文件传输工具，1997年首次发行。它支持文件上传和下载，所以是综合传输工具，但按传统，习惯称cURL为下载工具。cURL还包含了用于程序开发的libcurl。

cURL支持的通信协议有FTP、FTPS、HTTP、HTTPS、TFTP、SFTP、Gopher、SCP、Telnet、DICT、FILE、LDAP、LDAPS、IMAP、POP3、SMTP和RTSP。

curl还支持SSL认证、HTTP POST、HTTP PUT、FTP上传, HTTP form based upload、proxies、HTTP/2、cookies、用户名+密码认证(Basic, Plain, Digest, CRAM-MD5, NTLM, Negotiate and Kerberos)、file transfer resume、proxy tunneling。

系统环境：

下载curl

curl - Download

下载后，解压：

配置

./configure --prefix=/home/caozilong/Workspace/curl/install --with-openssl

编译

执行make进行编译：

安装

 make install，安装进INSTALL目录：

 基于libcurl开发下载应用

​
/*
demo_curl.c
curl demo : 结合 lbcurl example 下的ftpsget.c progressfunc.c
            实现下载网络文件，下载进度显示，断点下载

*/

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
 *
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at https://curl.se/docs/copyright.html.
 *
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 ***************************************************************************/

#include <stdio.h>
#include <sys/stat.h>

#include <curl/curl.h>

/* <DESC>
 * Get a single file from an FTPS server.
 * </DESC>
 */

struct FtpFile {
  const char *filename;
  FILE *stream;
};

static size_t my_fwrite(void *buffer, size_t size, size_t nmemb,
                        void *stream)
{
  struct FtpFile *out = (struct FtpFile *)stream;
  if(!out->stream) {
    /* open file for writing */
    out->stream = fopen(out->filename, "ab+");
    if(!out->stream)
      return -1; /* failure, can't open file to write */
  }
  return fwrite(buffer, size, nmemb, out->stream);
}
struct myprogress {
  curl_off_t lastruntime; /* type depends on version, see above */
  CURL *curl;
};

/* this is how the CURLOPT_XFERINFOFUNCTION callback works */
static int xferinfo(void *p,
                    curl_off_t dltotal, curl_off_t dlnow,
                    curl_off_t ultotal, curl_off_t ulnow)
{
  struct myprogress *myp = (struct myprogress *)p;
  CURL *curl = myp->curl;
  curl_off_t curtime = 0;

  curl_easy_getinfo(curl, CURLINFO_TOTAL_TIME_T, &curtime);

  /* under certain circumstances it may be desirable for certain functionality
     to only run every N seconds, in order to do this the transaction time can
     be used */
  if((curtime - myp->lastruntime) >= 3000000) {
    myp->lastruntime = curtime;
    fprintf(stderr, "TOTAL TIME: %" CURL_FORMAT_CURL_OFF_T ".%06ld\r\n",
            (curtime / 1000000), (long)(curtime % 1000000));
  }

  fprintf(stderr, "UP: %" CURL_FORMAT_CURL_OFF_T " of %" CURL_FORMAT_CURL_OFF_T
          "  DOWN: %" CURL_FORMAT_CURL_OFF_T " of %" CURL_FORMAT_CURL_OFF_T
          "\r\n",
          ulnow, ultotal, dlnow, dltotal);

  return 0;
}

int main(void)
{
  CURL *curl;
  CURLcode res;
  struct FtpFile ftpfile = {
    "yourfile.bin", /* name to store the file as if successful */
    NULL
  };
  struct myprogress prog;

  curl_global_init(CURL_GLOBAL_DEFAULT);

  curl = curl_easy_init();
  if(curl) {
    /*
     * You better replace the URL with one that works! Note that we use an
     * FTP:// URL with standard explicit FTPS. You can also do FTPS:// URLs if
     * you want to do the rarer kind of transfers: implicit.
     */
    curl_easy_setopt(curl, CURLOPT_URL,
                     "https://curl.se/download/curl-7.83.1.tar.gz");

    curl_off_t local_file_len = -1;
    struct stat file_info;
    int use_resume = 0;
    if(stat(ftpfile.filename , &file_info) == 0) 
    {
        local_file_len =  file_info.st_size;
        use_resume  = 1;
    }

    /* resuming upload at this position, possibly beyond 2GB */
    curl_easy_setopt(curl, CURLOPT_RESUME_FROM_LARGE, use_resume?local_file_len:0);

    /* Define our callback to get called when there's data to be written */
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, my_fwrite);
    /* Set a pointer to our struct to pass to the callback */
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &ftpfile);

    /* We activate SSL and we require it for both control and data */
    curl_easy_setopt(curl, CURLOPT_USE_SSL, CURLUSESSL_ALL);

    /* Switch on full protocol/debug output */
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

    prog.lastruntime = 0;
    prog.curl = curl;
    curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0L);
    curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, xferinfo);
    /* pass the struct pointer into the progress function */
    curl_easy_setopt(curl, CURLOPT_PROGRESSDATA, &prog);

    res = curl_easy_perform(curl);

    /* always cleanup */
    curl_easy_cleanup(curl);

    if(CURLE_OK != res) {
      /* we failed */
      fprintf(stderr, "curl told us %d\n", res);
    }
  }

  if(ftpfile.stream)
    fclose(ftpfile.stream); /* close the local file */

  curl_global_cleanup();

  return 0;
}

编译应用

gcc -I ../install/include/ -L ../install/lib/ main.c -lcurl

将下载的yourfile.bin和curl-7.83.1.tar.gz对比，二者完全一致。

经过调试，程序是双线程程序，但是进度打印是在主线程中完成的

断点续传的原理：

检查文件是否存在，如果存在，说明是之前下载过一部分，没有下载完成，这个时候设置use_resume FLAG，并且调用CURLOPT_RESUME_FROM_LARGE来设置断点续传模式：

CURL具备更多的SSL支持

curl可以使用十个不同的SSL/TLS库之一来构建，它可以提供更多控制权并更广泛地支持协议详细信息。

测试用例

编译测试:

make test

---

结束！