这read_callback()
是 CURL 在需要获取将上传到服务器的数据时调用的函数。想象一下,read_callback()
这与fread()
. 当被调用时,它会执行任何需要的 voodoo-mumbo-jumbo,但最终可上传的数据必须存储在*ptr
缓冲区中,这是 curl 的内部缓冲区。因为您的内存缓冲区memcpy()
将与 body of 一样好read_callback()
,所以您根本不需要真正fread()
的。
size * nmemb
告诉您 curl 为单个数据块保留了多大的缓冲区。最后void*
一个是由 CURLOPT_READDATA 选项设置的指针 - 它是一种随心所欲的指针,因此它可以指向包含您正在上传的数据和一些附加信息的结构例如当前的进展。
您可以将此用作示例:
#include <gdk-pixbuf/gdk-pixbuf.h>
#include <stdlib.h>
#include <curl/curl.h>
#include <string.h>
struct transfer
{
gchar *buf;
gsize total;
size_t uploaded;
};
static size_t read_callback(void *ptr, size_t size, size_t nmemb, void *data)
{
struct transfer * tr = data;
size_t left = tr->total - tr->uploaded;
size_t max_chunk = size * nmemb;
size_t retcode = left < max_chunk ? left : max_chunk;
memcpy(ptr, tr->buf + tr->uploaded, retcode); // <-- voodoo-mumbo-jumbo :-)
tr->uploaded += retcode; // <-- save progress
return retcode;
}
int main()
{
GdkPixbuf * gbuffer = NULL;
GError * error = NULL;
gchar * buffer;
gsize size;
g_type_init();
gbuffer = gdk_pixbuf_new_from_file("g.png", &error);
gdk_pixbuf_save_to_buffer(gbuffer, &buffer, &size, "jpeg", &error, NULL);
struct transfer tr = {buffer, size, 0};
CURL *easyhandle = curl_easy_init();
curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, read_callback);
curl_easy_setopt(easyhandle, CURLOPT_READDATA, &tr); // <-- this will be *data in read_callback()
curl_easy_setopt(easyhandle, CURLOPT_UPLOAD, 1L);
curl_easy_setopt(easyhandle, CURLOPT_URL, "http://example.com/upload.php");
CURLcode rc = curl_easy_perform(easyhandle);
}