1-ng-unpause-from-main.cpp

#include <curl/curl.h>
#include <stddef.h>
#include <unistd.h>

static const char* STATE_0 = "started";
static const char* STATE_1 = "paused";
static const char* STATE_2 = "resumed";

static size_t writeFunction(char* ptr, size_t blockSize, size_t numberOfBlocks, void* userData)
{
    CURL* handle = static_cast<CURL*>(userData);
    long receivedSize = (blockSize * numberOfBlocks);

    // pause transfer when this function is called at the first time
    const char* state;
    curl_easy_getinfo(handle, CURLINFO_PRIVATE, &state);
    if (state == STATE_0) {
        curl_easy_setopt(handle, CURLOPT_PRIVATE, STATE_1);
        printf("> %p: pause\n", handle);
        return CURL_WRITEFUNC_PAUSE;
    }

    printf("> %p: data received (%ld)\n", handle, receivedSize);
    return receivedSize;
}

int main()
{
    // initialize multi handle with multiplexing = ON
    CURLM* multiHandle = curl_multi_init();
    curl_multi_setopt(multiHandle, CURLMOPT_PIPELINING, CURLPIPE_MULTIPLEX);

    // initialize 4 easy handles for getting file1~4.dat
    const char* urls[4] = {
        "https://localhost:8080/file1.dat",
        "https://localhost:8080/file2.dat",
        "https://localhost:8080/file3.dat",
        "https://localhost:8080/file4.dat"
    };
    CURL* handles[4];
    for (int i = 0; i < 4; i++) {
        handles[i] = curl_easy_init();
        curl_easy_setopt(handles[i], CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2TLS);
        curl_easy_setopt(handles[i], CURLOPT_PIPEWAIT, 1L);
        curl_easy_setopt(handles[i], CURLOPT_SSL_ENABLE_ALPN, 1L);
        curl_easy_setopt(handles[i], CURLOPT_SSL_ENABLE_NPN, 0L);
        curl_easy_setopt(handles[i], CURLOPT_HTTPGET, 1L);
        curl_easy_setopt(handles[i], CURLOPT_SSL_VERIFYPEER, 0L);
        curl_easy_setopt(handles[i], CURLOPT_SSL_VERIFYHOST, 0L);
        curl_easy_setopt(handles[i], CURLOPT_WRITEFUNCTION, writeFunction);
        curl_easy_setopt(handles[i], CURLOPT_WRITEDATA, (void*)handles[i]);
        curl_easy_setopt(handles[i], CURLOPT_URL, urls[i]);
        curl_easy_setopt(handles[i], CURLOPT_PRIVATE, STATE_0);

        curl_multi_add_handle(multiHandle, handles[i]);
    }

    // transfer is started here, and will be paused when
    // receiving header is completed and receiving body is started
    int runningHandles = 0;
    int seconds = 0;
    printf("> --------------: start\n");
    do {
        curl_multi_perform(multiHandle, &runningHandles);
        sleep(1);
        for (int i = 0; i < 4; i++) {
            curl_easy_setopt(handles[i], CURLOPT_PRIVATE, STATE_1);
            printf("> %p: resume\n", handles[i]);
            curl_easy_pause(handles[i], CURLPAUSE_CONT);
        }
    } while (runningHandles > 0);

    printf("> --------------: cleanup\n");
    for (int i = 0; i < 4; i++)
        curl_easy_cleanup(handles[i]);
    curl_multi_cleanup(multiHandle);

    return 0;
}

2-good-unpause-from-progress-func.cpp

#include <curl/curl.h>
#include <stddef.h>
#include <unistd.h>

static const char* STATE_0 = "started";
static const char* STATE_1 = "paused";
static const char* STATE_2 = "resumed";

bool shouldResume { false };

static size_t writeFunction(char* ptr, size_t blockSize, size_t numberOfBlocks, void* userData)
{
    CURL* handle = static_cast<CURL*>(userData);
    long receivedSize = (blockSize * numberOfBlocks);

    // pause transfer when this function is called at the first time
    const char* state;
    curl_easy_getinfo(handle, CURLINFO_PRIVATE, &state);
    if (state == STATE_0) {
        curl_easy_setopt(handle, CURLOPT_PRIVATE, STATE_1);
        printf("> %p: pause\n", handle);
        return CURL_WRITEFUNC_PAUSE;
    }

    printf("> %p: data received (%ld)\n", handle, receivedSize);
    return receivedSize;
}

static int progressCallback(void* clientp, double, double, double, double)
{
    if (!shouldResume)
        return 0;

    // resume transfer from callback
    CURL* handle = static_cast<CURL*>(clientp);
    const char* state;
    curl_easy_getinfo(handle, CURLINFO_PRIVATE, &state);
    if (state == STATE_1) {
        curl_easy_setopt(handle, CURLOPT_PRIVATE, STATE_2);
        printf("> %p: resume\n", handle);
        curl_easy_pause(handle, CURLPAUSE_CONT);
    }
    return 0;
}

int main()
{
    // initialize multi handle with multiplexing = ON
    CURLM* multiHandle = curl_multi_init();
    curl_multi_setopt(multiHandle, CURLMOPT_PIPELINING, CURLPIPE_MULTIPLEX);

    // initialize 4 easy handles for getting file1~4.dat
    const char* urls[4] = {
        "https://localhost:8080/file1.dat",
        "https://localhost:8080/file2.dat",
        "https://localhost:8080/file3.dat",
        "https://localhost:8080/file4.dat"
    };
    CURL* handles[4];
    for (int i = 0; i < 4; i++) {
        handles[i] = curl_easy_init();
        curl_easy_setopt(handles[i], CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2TLS);
        curl_easy_setopt(handles[i], CURLOPT_PIPEWAIT, 1L);
        curl_easy_setopt(handles[i], CURLOPT_SSL_ENABLE_ALPN, 1L);
        curl_easy_setopt(handles[i], CURLOPT_SSL_ENABLE_NPN, 0L);
        curl_easy_setopt(handles[i], CURLOPT_HTTPGET, 1L);
        curl_easy_setopt(handles[i], CURLOPT_SSL_VERIFYPEER, 0L);
        curl_easy_setopt(handles[i], CURLOPT_SSL_VERIFYHOST, 0L);
        curl_easy_setopt(handles[i], CURLOPT_WRITEFUNCTION, writeFunction);
        curl_easy_setopt(handles[i], CURLOPT_WRITEDATA, (void*)handles[i]);
        curl_easy_setopt(handles[i], CURLOPT_NOPROGRESS, 0L);
        curl_easy_setopt(handles[i], CURLOPT_PROGRESSFUNCTION, progressCallback);
        curl_easy_setopt(handles[i], CURLOPT_PROGRESSDATA, (void*)handles[i]);
        curl_easy_setopt(handles[i], CURLOPT_URL, urls[i]);
        curl_easy_setopt(handles[i], CURLOPT_PRIVATE, STATE_0);

        curl_multi_add_handle(multiHandle, handles[i]);
    }

    // transfer is started here, and will be paused when
    // receiving header is completed and receiving body is started
    int runningHandles = 0;
    int seconds = 0;
    printf("> --------------: start\n");
    do {
        curl_multi_perform(multiHandle, &runningHandles);
        sleep(1);
        seconds++;
        // turn ON the resume flag 10 seconds after the start of transfer
        if (seconds == 10)
            shouldResume = true;
    } while (runningHandles > 0);

    printf("> --------------: cleanup\n");
    for (int i = 0; i < 4; i++)
        curl_easy_cleanup(handles[i]);
    curl_multi_cleanup(multiHandle);

    return 0;
}