demo:

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

#define PERM 0
char * name = "curl.c";

int main()
{
  CURL *curl;
  CURLcode rcode;
  char *url = "192.168.1.100";
  char *head_t = "66666666";
  char heads[1024];
  struct curl_httppost *formpost=NULL;
  struct
 
 curl_httppost *lastptr=NULL;
  struct curl_slist *headers=NULL; /* init to NULL is important */
  sprintf(heads,"head-sky:%s",head_t);

  headers = curl_slist_append(headers, heads);

  rcode = curl_global_init(CURL_GLOBAL_NOTHING);
  if (CURLE_OK != rcode)
    goto err1;

  curl = curl_easy_init();
  if
 
 (NULL == curl)
    goto err2;
#if PERM == 1
  /*設定傳送單個檔案*/
  curl_formadd(&formpost,
    &lastptr,
    CURLFORM_COPYNAME,"sendfile",
    CURLFORM_FILE, name,
    CURLFORM_END);
#else
    /*設定傳送多個檔案*/
    struct curl_forms array[3];
    array[0].option = CURLFORM_FILE;
    array[0].value = "curl.c"
 
;
    array[1].option = CURLFORM_FILE;
    array[1].value = "1.png";
    array[2].option = CURLFORM_END;
        curl_formadd(&formpost,
          &lastptr,
          CURLFORM_COPYNAME, "sendfile",
           CURLFORM_ARRAY, array,
          CURLFORM_END);
#endif

  curl_formadd(&formpost,
    &lastptr,
    CURLFORM_COPYNAME, "filename",
    CURLFORM_COPYCONTENTS, name,
    CURLFORM_END);


  curl_formadd(&formpost,
    &lastptr,
    CURLFORM_COPYNAME, "submit",
    CURLFORM_COPYCONTENTS, "send",
    CURLFORM_END);

  //設定傳送http頭部
  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
  //設定url
  curl_easy_setopt(curl, CURLOPT_URL, url);
  //設定http以post方式請求
  curl_easy_setopt(curl, CURLOPT_HTTPPOST, formpost);
  //設定超時時間
  curl_easy_setopt(curl, CURLOPT_TIMEOUT, 10L);

  //傳送資料
  rcode = curl_easy_perform(curl);

  curl_formfree(formpost);
  curl_easy_cleanup(curl);
  curl_slist_free_all(headers);
  curl_global_cleanup();
  return 0;
err2:
    curl_global_cleanup();
err1:
    return;
}

除錯時可使用網路除錯助手，建立一個tcp server端，使用80埠，接收http請求。
如此在除錯助手上即可看到傳送的檔案及頭部。