在网上搜了很久相关的问题，但是好像没有一个在linux下跑得比较完整的例子，不过经过自己一番搜索和总结，终于做出来了，哈哈，看下面的代码吧。

这个例子可以保存成bmp或者jpeg格式的图片。

下面的结构是保存bmp图片是用到的结构：

//把内存对齐定义为2个字节，
//这个可以避免BITMAPFILEHEADER
//出现4字节的对齐而使bmp位图的头出错
#pragma pack(2)

//下面两个结构是位图的结构
typedef struct BITMAPFILEHEADER
{
    u_int16_t bfType;
    u_int32_t bfSize;
    u_int16_t bfReserved1;
    u_int16_t bfReserved2;
    u_int32_t bfOffBits;
}BITMAPFILEHEADER;

typedef struct BITMAPINFOHEADER
{
    u_int32_t biSize;
    u_int32_t biWidth;
    u_int32_t biHeight;
    u_int16_t biPlanes;
    u_int16_t biBitCount;
    u_int32_t biCompression;
    u_int32_t biSizeImage;
    u_int32_t biXPelsPerMeter;
    u_int32_t biYPelsPerMeter;
    u_int32_t biClrUsed;
    u_int32_t biClrImportant;
}BITMAPINFODEADER;

下面两段代码是把yuv420的数据转成rgb24的代码：

下面的是用ffmpeg的库转的。

//===================把yuv帧数据转为rgb===========
    unsigned char *rgbBuf = new unsigned char[width*height*3];

    struct SwsContext* img_convert_ctx = 0;
    int linesize[4] = {3*width, 0, 0, 0};

    img_convert_ctx = sws_getContext(width, height,
        PIX_FMT_YUV420P,
        width,
        height,
        PIX_FMT_RGB24, SWS_FAST_BILINEAR, 0, 0, 0);
    if (img_convert_ctx != 0)
    {
        sws_scale(img_convert_ctx, pict->data, pict->linesize, 0, height, (uint8_t**)&rgbBuf, linesize);
        sws_freeContext(img_convert_ctx);
    saveFrame(rgbBuf, currentFrameNum_/(frameNum_/pictureNum), width, height);//生成图片
    }
    delete [] rgbBuf;
    //==============================================

下面给出自己在网上找到的把yuv420转成rgb24的代码：

const int Table_fv1[256]={ -180, -179, -177, -176, -174, -173, -172, -170, -169, -167, -166, -165, -163, -162, -160, -159, -158, -156, -155, -153, -152, -151, -149, -148, -146, -145, -144, -142, -141, -139, -138, -137, -135, -134, -132, -131, -130, -128, -127, -125, -124, -123, -121, -120, -118, -117, -115, -114, -113, -111, -110, -108, -107, -106, -104, -103, -101, -100, -99, -97, -96, -94, -93, -92, -90, -89, -87, -86, -85, -83, -82, -80, -79, -78, -76, -75, -73, -72, -71, -69, -68, -66, -65, -64, -62, -61, -59, -58, -57, -55, -54, -52, -51, -50, -48, -47, -45, -44, -43, -41, -40, -38, -37, -36, -34, -33, -31, -30, -29, -27, -26, -24, -23, -22, -20, -19, -17, -16, -15, -13, -12, -10, -9, -8, -6, -5, -3, -2, 0, 1, 2, 4, 5, 7, 8, 9, 11, 12, 14, 15, 16, 18, 19, 21, 22, 23, 25, 26, 28, 29, 30, 32, 33, 35, 36, 37, 39, 40, 42, 43, 44, 46, 47, 49, 50, 51, 53, 54, 56, 57, 58, 60, 61, 63, 64, 65, 67, 68, 70, 71, 72, 74, 75, 77, 78, 79, 81, 82, 84, 85, 86, 88, 89, 91, 92, 93, 95, 96, 98, 99, 100, 102, 103, 105, 106, 107, 109, 110, 112, 113, 114, 116, 117, 119, 120, 122, 123, 124, 126, 127, 129, 130, 131, 133, 134, 136, 137, 138, 140, 141, 143, 144, 145, 147, 148, 150, 151, 152, 154, 155, 157, 158, 159, 161, 162, 164, 165, 166, 168, 169, 171, 172, 173, 175, 176, 178 };
const int Table_fv2[256]={ -92, -91, -91, -90, -89, -88, -88, -87, -86, -86, -85, -84, -83, -83, -82, -81, -81, -80, -79, -78, -78, -77, -76, -76, -75, -74, -73, -73, -72, -71, -71, -70, -69, -68, -68, -67, -66, -66, -65, -64, -63, -63, -62, -61, -61, -60, -59, -58, -58, -57, -56, -56, -55, -54, -53, -53, -52, -51, -51, -50, -49, -48, -48, -47, -46, -46, -45, -44, -43, -43, -42, -41, -41, -40, -39, -38, -38, -37, -36, -36, -35, -34, -33, -33, -32, -31, -31, -30, -29, -28, -28, -27, -26, -26, -25, -24, -23, -23, -22, -21, -21, -20, -19, -18, -18, -17, -16, -16, -15, -14, -13, -13, -12, -11, -11, -10, -9, -8, -8, -7, -6, -6, -5, -4, -3, -3, -2, -1, 0, 0, 1, 2, 2, 3, 4, 5, 5, 6, 7, 7, 8, 9, 10, 10, 11, 12, 12, 13, 14, 15, 15, 16, 17, 17, 18, 19, 20, 20, 21, 22, 22, 23, 24, 25, 25, 26, 27, 27, 28, 29, 30, 30, 31, 32, 32, 33, 34, 35, 35, 36, 37, 37, 38, 39, 40, 40, 41, 42, 42, 43, 44, 45, 45, 46, 47, 47, 48, 49, 50, 50, 51, 52, 52, 53, 54, 55, 55, 56, 57, 57, 58, 59, 60, 60, 61, 62, 62, 63, 64, 65, 65, 66, 67, 67, 68, 69, 70, 70, 71, 72, 72, 73, 74, 75, 75, 76, 77, 77, 78, 79, 80, 80, 81, 82, 82, 83, 84, 85, 85, 86, 87, 87, 88, 89, 90, 90 };
const int Table_fu1[256]={ -44, -44, -44, -43, -43, -43, -42, -42, -42, -41, -41, -41, -40, -40, -40, -39, -39, -39, -38, -38, -38, -37, -37, -37, -36, -36, -36, -35, -35, -35, -34, -34, -33, -33, -33, -32, -32, -32, -31, -31, -31, -30, -30, -30, -29, -29, -29, -28, -28, -28, -27, -27, -27, -26, -26, -26, -25, -25, -25, -24, -24, -24, -23, -23, -22, -22, -22, -21, -21, -21, -20, -20, -20, -19, -19, -19, -18, -18, -18, -17, -17, -17, -16, -16, -16, -15, -15, -15, -14, -14, -14, -13, -13, -13, -12, -12, -11, -11, -11, -10, -10, -10, -9, -9, -9, -8, -8, -8, -7, -7, -7, -6, -6, -6, -5, -5, -5, -4, -4, -4, -3, -3, -3, -2, -2, -2, -1, -1, 0, 0, 0, 1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4, 5, 5, 5, 6, 6, 6, 7, 7, 7, 8, 8, 8, 9, 9, 9, 10, 10, 11, 11, 11, 12, 12, 12, 13, 13, 13, 14, 14, 14, 15, 15, 15, 16, 16, 16, 17, 17, 17, 18, 18, 18, 19, 19, 19, 20, 20, 20, 21, 21, 22, 22, 22, 23, 23, 23, 24, 24, 24, 25, 25, 25, 26, 26, 26, 27, 27, 27, 28, 28, 28, 29, 29, 29, 30, 30, 30, 31, 31, 31, 32, 32, 33, 33, 33, 34, 34, 34, 35, 35, 35, 36, 36, 36, 37, 37, 37, 38, 38, 38, 39, 39, 39, 40, 40, 40, 41, 41, 41, 42, 42, 42, 43, 43 };
const int Table_fu2[256]={ -227, -226, -224, -222, -220, -219, -217, -215, -213, -212, -210, -208, -206, -204, -203, -201, -199, -197, -196, -194, -192, -190, -188, -187, -185, -183, -181, -180, -178, -176, -174, -173, -171, -169, -167, -165, -164, -162, -160, -158, -157, -155, -153, -151, -149, -148, -146, -144, -142, -141, -139, -137, -135, -134, -132, -130, -128, -126, -125, -123, -121, -119, -118, -116, -114, -112, -110, -109, -107, -105, -103, -102, -100, -98, -96, -94, -93, -91, -89, -87, -86, -84, -82, -80, -79, -77, -75, -73, -71, -70, -68, -66, -64, -63, -61, -59, -57, -55, -54, -52, -50, -48, -47, -45, -43, -41, -40, -38, -36, -34, -32, -31, -29, -27, -25, -24, -22, -20, -18, -16, -15, -13, -11, -9, -8, -6, -4, -2, 0, 1, 3, 5, 7, 8, 10, 12, 14, 15, 17, 19, 21, 23, 24, 26, 28, 30, 31, 33, 35, 37, 39, 40, 42, 44, 46, 47, 49, 51, 53, 54, 56, 58, 60, 62, 63, 65, 67, 69, 70, 72, 74, 76, 78, 79, 81, 83, 85, 86, 88, 90, 92, 93, 95, 97, 99, 101, 102, 104, 106, 108, 109, 111, 113, 115, 117, 118, 120, 122, 124, 125, 127, 129, 131, 133, 134, 136, 138, 140, 141, 143, 145, 147, 148, 150, 152, 154, 156, 157, 159, 161, 163, 164, 166, 168, 170, 172, 173, 175, 177, 179, 180, 182, 184, 186, 187, 189, 191, 193, 195, 196, 198, 200, 202, 203, 205, 207, 209, 211, 212, 214, 216, 218, 219, 221, 223, 225 };


bool YV12_To_RGB24(const unsigned char* pYV12, unsigned char* &pRGB24, const int& width, const int& height)
{
    if(NULL == pYV12)
        return false;

    long buffsize = width * 24 * height;

    pRGB24 = new unsigned char[buffsize];

    if(!pYV12 || !pRGB24)
        return false;

    const long nYLen = long(width*height);
    const int nHfWidth = (width>>1);

    if(nYLen<1 || nHfWidth<1)
        return false;

    // Y data
    const unsigned char* yData = pYV12;
    // v data
    const unsigned char* vData = &yData[nYLen];
    // u data
    const unsigned char* uData = &vData[nYLen>>2];

    if(!uData || !vData)
        return false;

    int rgb[3];
    int i, j, m, n, x, y, py, rdif, invgdif, bdif;
    m = -width;
    n = -nHfWidth;

    bool addhalf = true;
    for(y=0; y<height;y++) {
        m += width;
        if( addhalf ){
            n+=nHfWidth;
            addhalf = false;
        } else {
            addhalf = true;
        }
        for(x=0; x<width;x++)  {
            i = m + x;
            j = n + (x>>1);

            py = yData[i];

            // search tables to get rdif invgdif and bidif
            rdif = Table_fv1[vData[j]];    // fv1
            invgdif = Table_fu1[uData[j]] + Table_fv2[vData[j]]; // fu1+fv2
            bdif = Table_fu2[uData[j]]; // fu2

            rgb[2] = py+rdif;    // R
            rgb[1] = py-invgdif; // G
            rgb[0] = py+bdif;    // B

            //j = nYLen - width - m + x;//Õâžö»áÍŒÏñµßµ¹
            j = m + x;
            i = (j<<1) + j;

            // copy this pixel to rgb data
            for(j=0; j<3; j++){
                if(rgb[j]>=0 && rgb[j]<=255){
                    pRGB24[i + j] = rgb[j];
                }else{
                    pRGB24[i + j] = (rgb[j] < 0)? 0 : 255;
                }
            }
        }
    }
    return true;
}

下面是实际执行生成图片的代码，输入rgb24的数据。注释了的是生成bmp的代码，没注释的是生成jpeg图片的代码。

void ipcToFile::saveFrame(uint8_t *pRGBBuffer, int iFrame, int width, int height)
{
    /*BITMAPFILEHEADER bmpheader;
    BITMAPINFOHEADER bmpinfo;
    FILE *fp;

    unsigned int uiTmp, uiTmp2;
    unsigned char *ucTmp = NULL;
    unsigned char ucRGB;
    int i;

    uiTmp = (width*3+3)/4*4*height;
    uiTmp2 = width*height*3;

    char szFilename[1024];
    sprintf(szFilename, "test%d.bmp",  iFrame);//图片名字为视频名+号码
    fp=fopen(szFilename, "wb");
    if(fp==NULL)
            return;

    //文件标识"BM"（即0x4D42）表示位图
    bmpheader.bfType = 0x4D42;
    //保留。设置为0
    bmpheader.bfReserved1 = 0;
    //保留。设置为0
    bmpheader.bfReserved2 = 0;
    //从文件开始到位图数据的偏移量（单位：字节）
    bmpheader.bfOffBits = sizeof(bmpheader) + sizeof(bmpinfo);
    //整个文件的大小（单位：字节）
    bmpheader.bfSize = bmpheader.bfOffBits + uiTmp;

    //信息头长度（单位：字节）。典型值为28
    bmpinfo.biSize = 0x28;
    //位图宽度（单位：像素）
    bmpinfo.biWidth = width;
    //位图高度（单位：像素）。若其为正，表示倒向的位图。若为负，表示正向的位图
    bmpinfo.biHeight = height;
    //位图的面数（为1）
    bmpinfo.biPlanes = 1;
    //每个像素的位数
    bmpinfo.biBitCount = 24;
    //压缩说明。0(BI_RGB)表示不压缩
    bmpinfo.biCompression = 0;
    //用字节数表示的位图数据的大小（为4的位数）
    bmpinfo.biSizeImage = uiTmp;
    //水平分辨率（单位：像素/米）
    bmpinfo.biXPelsPerMeter = 0;
    //垂直分辨率（单位：像素/米）
    bmpinfo.biYPelsPerMeter = 0;
    //位图使用的颜色数
    bmpinfo.biClrUsed = 0;
    //重要的颜色数
    bmpinfo.biClrImportant = 0;

    fwrite(&bmpheader,sizeof(bmpheader),1,fp);
    fwrite(&bmpinfo,sizeof(bmpinfo),1,fp);

    //把图像数据倒置
    uint8_t tmp[width*3];//临时数据
    for(int i = 0; i < height/2; i++)
    {
        memcpy(tmp, &(pRGBBuffer[width*i*3]), width*3);
        memcpy(&(pRGBBuffer[width*i*3]), &(pRGBBuffer[width*(height-1-i)*3]), width*3);
        memcpy(&(pRGBBuffer[width*(height-1-i)*3]), tmp, width*3);
    }

    fwrite(pRGBBuffer,width*height*3,1,fp);
    fclose(fp);*/

    struct jpeg_compress_struct jcs;

    // 声明错误处理器，并赋值给jcs.err域
    struct jpeg_error_mgr jem;
    jcs.err = jpeg_std_error(&jem);

    jpeg_create_compress(&jcs);

    char szFilename[1024];
    sprintf(szFilename, "test%d.jpg", iFrame);//图片名字为视频名+号码
    FILE *fp = fopen(szFilename, "wb");
    if(fp == NULL)
            return;

    jpeg_stdio_dest(&jcs, fp);

    jcs.image_width = width;    // 为图的宽和高，单位为像素
    jcs.image_height = height;
    jcs.input_components = 3;   // 在此为1,表示灰度图， 如果是彩色位图，则为3
    jcs.in_color_space = JCS_RGB; //JCS_GRAYSCALE表示灰度图，JCS_RGB表示彩色图像

    jpeg_set_defaults(&jcs);
    jpeg_set_quality (&jcs, 80, true);

    jpeg_start_compress(&jcs, TRUE);

    JSAMPROW row_pointer[1];   // 一行位图
    int row_stride = jcs.image_width * 3;//每一行的字节数,如果不是索引图,此处需要乘以3

    // 对每一行进行压缩
    while (jcs.next_scanline < jcs.image_height) {
        row_pointer[0] = &(pRGBBuffer[jcs.next_scanline * row_stride]);
        jpeg_write_scanlines(&jcs, row_pointer, 1);
    }

    jpeg_finish_compress(&jcs);
    jpeg_destroy_compress(&jcs);

    fclose(fp);
}