The read_callback()
是 CURL 在需要获取要上传到服务器的数据时调用的函数。想象一下read_callback()
is 相似的东西 to fread()
。当调用时,它会执行任何所需的 voodoo-mumbo-jumbo,但最终可上传的数据必须存储在*ptr
buffer,这是curl的内部缓冲区。对于您的内存缓冲区memcpy()
会做的很好,就像身体一样read_callback()
,所以你不需要真正的fread()
at all.
size * nmemb
告诉您curl 为单个数据块保留了多大的缓冲区。最后void*
是一个由 CURLOPT_READDATA 选项设置的指针 - 它是一个“do-what-ever-you-need-with-it”类型的指针,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,例如现在的进展。
您可以使用它作为示例:
#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);
}