在使用C語言進行編程時，有時需要接收上傳的JSON格式的圖片。下面通過一個示例代碼來介紹如何實現這一功能。

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <curl/curl.h>
#define HTTP_FORM_BOUNDARY "--------------------------145817612720890910647490"
#define UPLOAD_IMAGE_URL "http://example.com/upload"
#define IMAGE_PATH "image.jpg"
static size_t read_callback(void *ptr, size_t size, size_t nmemb, void *stream) {
FILE *file = (FILE*)stream;
if (!file) return 0;
return fread(ptr, size, nmemb, file);
}
int main(int argc, const char* argv[]) {
CURL *curl = curl_easy_init();
CURLcode res;
FILE *file = fopen(IMAGE_PATH, "rb");
if (!curl || !file) return -1;
curl_easy_setopt(curl, CURLOPT_URL, UPLOAD_IMAGE_URL);
curl_easy_setopt(curl, CURLOPT_POST, 1);
struct curl_slist *headers = NULL;
headers = curl_slist_append(headers, "Content-Type: multipart/form-data; boundary=" HTTP_FORM_BOUNDARY);
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
fseek(file, 0, SEEK_END);
long fileSize = ftell(file);
rewind(file);
char *payload = calloc(sizeof(char), fileSize + 2048);
sprintf(payload, "--" HTTP_FORM_BOUNDARY "\r\nContent-Disposition: form-data; name=\"file\"; filename=\"" IMAGE_PATH "\"\r\nContent-Type: image/jpeg\r\n\r\n");
int offset = strlen(payload);
fseek(file, 0, SEEK_SET);
if (fileSize >fread(payload + offset, 1, fileSize, file)) {
fclose(file);
free(payload);
return -3;
}
offset += fileSize;
sprintf(payload + offset, "\r\n--" HTTP_FORM_BOUNDARY "--\r\n");
curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE_LARGE, offset);
curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);
curl_easy_setopt(curl, CURLOPT_READDATA, file);
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_callback);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, stdout);
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
res = curl_easy_perform(curl);
curl_easy_cleanup(curl);
free(payload);
fclose(file);
return res;
}

這是一個使用CURL庫來實現的上傳圖片的示例，其中關鍵部分是如何拼接請求的payload。首先，設置請求的Content-Type為multipart/form-data，并指定分隔符。接著，讀取圖片的內容并將其拼接到payload中。最后，在curl_easy_setopt中指定讀取函數和寫入函數，然后使用curl_easy_perform執行請求即可。