写个c程序举个例子

好的，以下是一个简单的 C 语言程序，用于获取 DRM 设备的当前帧缓冲对象并将其保存为 BMP 图像文件：

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <linux/fb.h>
#include <linux/drm.h>
#include <drm/drm_fourcc.h>

#define WIDTH 800
#define HEIGHT 600

int main(int argc, char **argv)
{
    // 打开 DRM 设备
    int fd = open("/dev/dri/card0", O_RDWR);
    if (fd < 0) {
        fprintf(stderr, "failed to open DRM device\n");
        exit(EXIT_FAILURE);
    }

    // 获取当前显示的帧缓冲对象
    drmModeRes *resources = drmModeGetResources(fd);
    if (!resources) {
        fprintf(stderr, "failed to get DRM resources\n");
        close(fd);
        exit(EXIT_FAILURE);
    }
    drmModeConnector *connector = NULL;
    drmModeEncoder *encoder = NULL;
    drmModeCrtc *crtc = NULL;
    for (int i = 0; i < resources->count_connectors; i++) {
        connector = drmModeGetConnector(fd, resources->connectors[i]);
        if (connector->connection == DRM_MODE_CONNECTED) {
            for (int j = 0; j < connector->count_encoders; j++) {
                encoder = drmModeGetEncoder(fd, connector->encoders[j]);
                if (encoder->crtc_id) {
                    crtc = drmModeGetCrtc(fd, encoder->crtc_id);
                    break;
                }
                drmModeFreeEncoder(encoder);
            }
        }
        drmModeFreeConnector(connector);
        if (crtc) break;
    }
    if (!crtc) {
        fprintf(stderr, "failed to get current CRTC\n");
        drmModeFreeResources(resources);
        close(fd);
        exit(EXIT_FAILURE);
    }

    // 获取当前帧缓冲对象的信息
    drmModeFB *fb = drmModeGetFB(fd, crtc->buffer_id);
    if (!fb) {
        fprintf(stderr, "failed to get current FB\n");
        drmModeFreeCrtc(crtc);
        drmModeFreeResources(resources);
        close(fd);
        exit(EXIT_FAILURE);
    }
    if (fb->width != WIDTH || fb->height != HEIGHT) {
        fprintf(stderr, "current FB size is not %dx%d\n", WIDTH, HEIGHT);
        drmModeFreeFB(fb);
        drmModeFreeCrtc(crtc);
        drmModeFreeResources(resources);
        close(fd);
        exit(EXIT_FAILURE);
    }

    // 映射帧缓冲对象到用户空间
    uint32_t *buf = NULL;
    struct drm_mode_map_dumb map_dumb = {
        .handle = fb->handle,
    };
    if (ioctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &map_dumb) < 0) {
        fprintf(stderr, "failed to map dumb buffer\n");
        drmModeFreeFB(fb);
        drmModeFreeCrtc(crtc);
        drmModeFreeResources(resources);
        close(fd);
        exit(EXIT_FAILURE);
    }
    buf = mmap(NULL, fb->size, PROT_READ, MAP_SHARED, fd, map_dumb.offset);
    if (buf == MAP_FAILED) {
        fprintf(stderr, "failed to mmap dumb buffer\n");
        drmModeFreeFB(fb);
        drmModeFreeCrtc(crtc);
        drmModeFreeResources(resources);
        close(fd);
        exit(EXIT_FAILURE);
    }

    // 转换像素格式为 RGB565
    uint16_t *rgb_buf = (uint16_t *)malloc(WIDTH * HEIGHT * sizeof(uint16_t));
    for (int i = 0; i < HEIGHT; i++) {
        for (int j = 0; j < WIDTH; j++) {
            uint32_t pixel = buf[i * fb->pitch / 4 + j];
            uint16_t r = (pixel >> 16) & 0xff;
            uint16_t g = (pixel >> 8) & 0xff;
            uint16_t b = pixel & 0xff;
            rgb_buf[i * WIDTH + j] = ((r >> 3) << 11) | ((g >> 2) << 5) | (b >> 3);
        }
    }

    // 保存为 BMP 图像文件
    FILE *fp = fopen("drm_fb.bmp", "wb");
    if (fp) {
        uint8_t bmp_header[54] = {
            0x42, 0x4d, 0x36, 0x20, 0x0c, 0x00, 0x00, 0x00, 0x00, 0x00,
            0x36, 0x00, 0x00, 0x00, 0x28, 0x00, 0x00, 0x00, 0x20, 0x03,
            0x00, 0x00, 0xe0, 0x01, 0x00, 0x00, 0x01, 0x00, 0x10, 0x00,
            0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x20, 0x0c, 0x00,
            0x00, 0x13, 0x0b, 0x00, 0x00, 0x13, 0x0b, 0x00, 0x00, 0x00,
            0x00, 0x00, 0x00,
        };
        uint32_t row_size = WIDTH * 2;
        uint32_t file_size = sizeof(bmp_header) + row_size * HEIGHT;
        uint32_t padding_size = (4 - (row_size % 4)) % 4;
        uint8_t padding[4] = {0};
        bmp_header[2] = file_size;
        bmp_header[6] = file_size >> 8;
        bmp_header[10] = sizeof(bmp_header);
        bmp_header[18] = WIDTH;
        bmp_header[19] = WIDTH >> 8;
        bmp_header[20] = WIDTH >> 16;
        bmp_header[21] = WIDTH >> 24;
        bmp_header[22] = HEIGHT;
        bmp_header[23] = HEIGHT >> 8;
        bmp_header[24] = HEIGHT >> 16;
        bmp_header[25] = HEIGHT >> 24;
        fwrite(bmp_header, 1, sizeof(bmp_header), fp);
        for (int i = 0; i < HEIGHT; i++) {
            fwrite(rgb_buf + (HEIGHT - i - 1) * WIDTH, 1, row_size, fp);
            fwrite(padding, 1, padding_size, fp);
        }
        fclose(fp);
    }

    // 释放资源
    free(rgb_buf);
    munmap(buf, fb->size);
    drmModeFreeFB(fb);
    drmModeFreeCrtc(crtc);
    drmModeFreeResources(resources);
    close(fd);

    return 0;
}

注意，该程序只是一个简单的示例，实际应用中还需要考虑各种异常情况的处理。