Big Sky

追記 最新版はgithubで作ってます。

mattn's nicodown at master — GitHub
http://github.com/mattn/nicodown/tree/master

適当だけど書いてみた。
タイトル取って来る所はlibxml使うの面倒臭かったのでXMLパーサ使わずベタで(実態参照文字あると変になるので気を付けて)。Windowsの場合だけWin32 APIでシフトJISにファイル名を変換しています。
荒いコードなので色々直し所がありますが、サンプルって事で。
//#define CURL_STATICLIB
#include <curl/curl.h>

#define HEX_DIGITS "0123456789ABCDEF"
#define IS_QUOTED(x) (*x == '%' && strchr(HEX_DIGITS, *(x+1)) && strchr(HEX_DIGITS, *(x+2)))

static char* response_data = NULL;  /* response data from server. */
static size_t response_size = 0;    /* response size of data */

static void
curl_handle_init() {
    response_data = NULL;
    response_size = 0;
}

static void
curl_handle_term() {
    if (response_data) free(response_data);
}

static size_t
curl_handle_returned_data(char* ptr, size_t size, size_t nmemb, void* stream) {
    if (!response_data)
        response_data = (char*)malloc(size*nmemb);
    else
        response_data = (char*)realloc(response_data, response_size+size*nmemb);
    if (response_data) {
        memcpy(response_data+response_size, ptr, size*nmemb);
        response_size += size*nmemb;
    }
    return size*nmemb;
}

int
main(int argc, char* argv[]) {
    CURLcode res;
    CURL* curl;
    char error[256];
    char name[256];
    char data[1024];
    char* buf = NULL;
    char* ptr = NULL;
    char* tmp = NULL;
    FILE* fp = NULL;
    int status = 0;

    // usage
    if (argc != 4) {
        fputs("usage: nicodown [usermail] [password] [video_id]", stderr);
        goto leave;
    }

    // default filename
    sprintf(name, "%s.flv", argv[3]);

    curl = curl_easy_init();
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0);
    curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, &error);
    curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1);
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, curl_handle_returned_data);
    curl_easy_setopt(curl, CURLOPT_COOKIEJAR, "cookies.jar");
    curl_easy_setopt(curl, CURLOPT_COOKIEFILE, "cookies.txt");

    // login
    sprintf(data, "mail=%s&password=%s&next_url=/watch/%s", argv[1], argv[2], argv[3]);
    curl_handle_init();
    curl_easy_setopt(curl, CURLOPT_URL, "https://secure.nicovideo.jp/secure/login?site=niconico");
    curl_easy_setopt(curl, CURLOPT_POST, 1);
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, data);
    res = curl_easy_perform(curl);
    if (res != CURLE_OK) {
        fprintf(stderr, error);
        goto leave;
    }
    buf = malloc(response_size + 1);
    strcpy(buf, response_data);
    if (strstr(buf, "id=\"login_bar\"")) {
        printf("%s\n", buf);
        free(buf);
        fprintf(stderr, "failed to login\n");
        goto leave;
    }
    free(buf);
    curl_handle_term();

    // get video url, and get filename
    sprintf(data, "http://www.nicovideo.jp/api/getthumbinfo?v=%s", argv[3]);
    curl_handle_init();
    curl_easy_setopt(curl, CURLOPT_URL, data);
    curl_easy_setopt(curl, CURLOPT_POST, 0);
    res = curl_easy_perform(curl);
    if (res != CURLE_OK) {
        fprintf(stderr, error);
        goto leave;
    }
    buf = malloc(response_size + 1);
    strcpy(buf, response_data);
    ptr = strstr(buf, "<title>");
    if (ptr) {
        ptr += 7;
        tmp = strstr(ptr, "</title>");
        if (*tmp) {
            *tmp = 0;
            strcpy(name, ptr);
        }
#ifdef _WIN32
        {
            UINT codePage;
            size_t wcssize;
            wchar_t* wcsstr;
            size_t mbssize;
            char* mbsstr;

            codePage = CP_UTF8;
            wcssize = MultiByteToWideChar(codePage, 0, name, -1,  NULL, 0);
            wcsstr = (wchar_t*)malloc(sizeof(wchar_t) * (wcssize + 1));
            wcssize = MultiByteToWideChar(codePage, 0, ptr, -1, wcsstr, wcssize + 1);
            wcsstr[wcssize] = 0;
            codePage = GetACP();
            mbssize = WideCharToMultiByte(codePage, 0, (LPCWSTR)wcsstr,-1,NULL,0,NULL,NULL);
            mbsstr = (char*)malloc(mbssize+1);
            mbssize = WideCharToMultiByte(codePage, 0, (LPCWSTR)wcsstr, -1, mbsstr, mbssize, NULL, NULL);
            mbsstr[mbssize] = 0;
            sprintf(name, "%s.flv", mbsstr);
            free(mbsstr);
            free(wcsstr);
        }
#endif
    }
    free(buf);
    printf("downloading %s\n", name);
    curl_handle_term();

    // get video url
    sprintf(data, "http://www.nicovideo.jp/api/getflv?v=%s", argv[3]);
    curl_handle_init();
    curl_easy_setopt(curl, CURLOPT_URL, data);
    curl_easy_setopt(curl, CURLOPT_POST, 0);
    res = curl_easy_perform(curl);
    if (res != CURLE_OK) {
        fprintf(stderr, error);
        goto leave;
    }
    buf = malloc(response_size + 1);
    strcpy(buf, response_data);
    ptr = strstr(response_data, "url=");
    if (!ptr) {
        free(buf);
        fprintf(stderr, "failed to get video info\n");
        goto leave;
    }
    tmp = strstr(ptr, "&");
    if (tmp) *tmp = 0;
    tmp = ptr;
    while(*tmp) {
        if (IS_QUOTED(tmp)) {
            char num = 0;
            sscanf(tmp+1, "%02x", &num);
            *tmp = num;
            strcpy(tmp + 1, tmp + 3);
        }
        tmp++;
    }
    strcpy(data, ptr + 4);
    printf("URL: %s\n", data);
    free(buf);
    curl_handle_term();

    // download video
    fp = fopen(name, "wb");
    if (!fp) {
        fprintf(stderr, "failed to open file\n");
        goto leave;
    }
    curl_handle_init();
    curl_easy_setopt(curl, CURLOPT_URL, data);
    curl_easy_setopt(curl, CURLOPT_POST, 0);
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, fwrite);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
    res = curl_easy_perform(curl);
    fclose(fp);
    if (res != CURLE_OK) {
        fprintf(stderr, error);
        goto leave;
    }

leave:
    curl_handle_term();
    curl_easy_cleanup(curl);

    return 0;
}
確認はWindowsでしかしてませんが、きっとUN*Xでも動くはず。perlやpythonやruby使わない派の方は雛形として持ってって下さい。