要用C語言繪制曲線，可以使用圖形庫如OpenGL或者SDL來實現。下面是一個簡單的使用SDL繪制曲線的示例代碼：

#include <SDL.h>

int main(int argc, char *argv[]) {
    SDL_Init(SDL_INIT_VIDEO);

    SDL_Window *window = SDL_CreateWindow("Curve Drawing",
                                          SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
                                          800, 600, 0);

    SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);

    SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
    SDL_RenderClear(renderer);

    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);

    int x1 = 100, y1 = 100;
    int x2 = 700, y2 = 500;
    int steps = 100;
    for (int i = 0; i < steps; i++) {
        float t = (float)i / steps;
        int x = x1 + t * (x2 - x1);
        int y = y1 + t * (y2 - y1);
        SDL_RenderDrawPoint(renderer, x, y);
    }

    SDL_RenderPresent(renderer);

    bool quit = false;
    SDL_Event event;
    while (!quit) {
        while (SDL_PollEvent(&event)) {
            if (event.type == SDL_QUIT) {
                quit = true;
            }
        }
    }

    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    SDL_Quit();

    return 0;
}

這個示例代碼使用SDL庫來創建窗口和渲染器，然后繪制一條直線。你可以修改x1、y1、x2、y2來繪制不同的曲線，也可以修改steps來控制曲線的平滑度。整個繪制過程是通過在起點和終點之間插值的方式來實現的。最后，通過處理SDL的事件循環來保持窗口顯示，直到用戶關閉窗口。