如何在 C++ makefile 中包含库

Question

我正在尝试使用 curl 中的示例代码，但我遇到了一个问题，我认为这是因为我的 makefile 需要更新：

#include <stdio.h>
#include <string.h>
#include "curl-7.64.0/include/curl/curl.h"

/*
 * For an SMTP example using the multi interface please see smtp-multi.c.
 */ 

/* The libcurl options want plain addresses, the viewable headers in the mail
 * can very well get a full name as well.
 */ 
#define FROM_ADDR    "<sender@example.org>"
#define TO_ADDR      "<addressee@example.net>"
#define CC_ADDR      "<info@example.org>"

#define FROM_MAIL "Sender Person " FROM_ADDR
#define TO_MAIL   "A Receiver " TO_ADDR
#define CC_MAIL   "John CC Smith " CC_ADDR

static const char *payload_text[] = {
  "Date: Mon, 29 Nov 2010 21:54:29 +1100\r\n",
  "To: " TO_MAIL "\r\n",
  "From: " FROM_MAIL "\r\n",
  "Cc: " CC_MAIL "\r\n",
  "Message-ID: <dcd7cb36-11db-487a-9f3a-e652a9458efd@"
  "rfcpedant.example.org>\r\n",
  "Subject: SMTP example message\r\n",
  "\r\n", /* empty line to divide headers from body, see RFC5322 */ 
  "The body of the message starts here.\r\n",
  "\r\n",
  "It could be a lot of lines, could be MIME encoded, whatever.\r\n",
  "Check RFC5322.\r\n",
  NULL
};

struct upload_status {
  int lines_read;
};

static size_t payload_source(void *ptr, size_t size, size_t nmemb, void *userp)
{
  struct upload_status *upload_ctx = (struct upload_status *)userp;
  const char *data;

  if((size == 0) || (nmemb == 0) || ((size*nmemb) < 1)) {
    return 0;
  }

  data = payload_text[upload_ctx->lines_read];

  if(data) {
    size_t len = strlen(data);
    memcpy(ptr, data, len);
    upload_ctx->lines_read++;

    return len;
  }

  return 0;
}

int main(void)
{
  CURL *curl;
  CURLcode res = CURLE_OK;
  struct curl_slist *recipients = NULL;
  struct upload_status upload_ctx;

  upload_ctx.lines_read = 0;

  curl = curl_easy_init();
  if(curl) {
    /* This is the URL for your mailserver */ 
    curl_easy_setopt(curl, CURLOPT_URL, "smtp://mail.example.com");

    /* Note that this option isn't strictly required, omitting it will result
     * in libcurl sending the MAIL FROM command with empty sender data. All
     * autoresponses should have an empty reverse-path, and should be directed
     * to the address in the reverse-path which triggered them. Otherwise,
     * they could cause an endless loop. See RFC 5321 Section 4.5.5 for more
     * details.
     */ 
    curl_easy_setopt(curl, CURLOPT_MAIL_FROM, FROM_ADDR);

    /* Add two recipients, in this particular case they correspond to the
     * To: and Cc: addressees in the header, but they could be any kind of
     * recipient. */ 
    recipients = curl_slist_append(recipients, TO_ADDR);
    recipients = curl_slist_append(recipients, CC_ADDR);
    curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);

    /* We're using a callback function to specify the payload (the headers and
     * body of the message). You could just use the CURLOPT_READDATA option to
     * specify a FILE pointer to read from. */ 
    curl_easy_setopt(curl, CURLOPT_READFUNCTION, payload_source);
    curl_easy_setopt(curl, CURLOPT_READDATA, &upload_ctx);
    curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);

    /* Send the message */ 
    res = curl_easy_perform(curl);

    /* Check for errors */ 
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n",
              curl_easy_strerror(res));

    /* Free the list of recipients */ 
    curl_slist_free_all(recipients);

    /* curl won't send the QUIT command until you call cleanup, so you should
     * be able to re-use this connection for additional messages (setting
     * CURLOPT_MAIL_FROM and CURLOPT_MAIL_RCPT as required, and calling
     * curl_easy_perform() again. It may not be a good idea to keep the
     * connection open for a very long time though (more than a few minutes
     * may result in the server timing out the connection), and you do want to
     * clean up in the end.
     */ 
    curl_easy_cleanup(curl);
  }

  return (int)res;
}

我在第 75 行“/main.cpp:75: undefined reference to `curl_easy_init'”中遇到了我的第一个错误。这是目前我的makefile：

CXX = g++
CXXFLAGS = -c -g -std=c++11 -Wall -W -Werror -pedantic
LDFLAGS =

email: main.o
    $(CXX) $(LDFLAGS) main.o

main.o : main.cpp
    $(CXX) $(CXXFLAGS) main.cpp

clean :
    rm -f core $(PROG) *.o

如何更改 makefile 以正确编译库？而且，下载 curl 并设置好路径后，我还需要做什么才能使用该库吗？

编辑：我尝试将我的 makefile 更改为：

CXX = g++
CXXFLAGS = -g -std=c++11 -Wall -W -Werror -pedantic
INCLUDE = -Iinclude -I/usr/local/include -I/usr/include
LDFLAGS =
LDLIBS = -lcurl

email: main.o
    $(CXX) $(INCLUDE) $(LDFLAGS) $(CXXFLAGS) $(LDLIBS) main.o

main.o : main.cpp
    $(CXX) $(CXXFLAGS) main.cpp

clean :
    rm -f core $(PROG) *.o

但是，我收到错误“找不到 -lcurl”。我是否需要执行与 .cpp 文件中包含的方式类似的操作？

Answer 1

添加期望找到 curl 库 (libcurl.so) 的目录路径

LDFLAGS         = "-L/the/path/to/the/curl/lib"

有一个 curl 的示例 makefile https://github.com/curl/curl/blob/master/docs/examples/Makefile.example