raytracer using pthreads

int main(int a, char *args[]) {
    int i;
    pthread_t threads[8];
    unsigned int* pixmap = malloc(1024*1024*sizeof(int));

    v_threadargs.height = 1024.0f;
    v_threadargs.width = 1024.0f;
    v_threadargs.pixmap = pixmap;

    for (i = 0; i < 8; i++) {
        v_threadargs.threadid = i;
        pthread_create(&threads[i], NULL, render, (void *) &v_threadargs);
    }

    for (i = 0; i < 8; i++)
        pthread_join(threads[i], NULL);

    writetga(pixmap, 1024, 1024, "ray8out.tga");
    free(pixma开发者_如何学编程p);

    pthread_exit(NULL);
    return 0;
}

void *render(void *r_threadargs) {
    int i,j, threadid, start;
    float height, width;
    int *pixmap;

    struct s_threadargs *p_threadargs;
    p_threadargs = (struct s_threadargs *) r_threadargs;
    height = p_threadargs -> height;
    width = p_threadargs -> width;
    pixmap = p_threadargs -> pixmap;
    threadid = p_threadargs -> threadid;

    stepy = viewplaney0;
    deltax = (viewplanex1 - viewplanex0) / width;
    deltay = (viewplaney1 - viewplaney0) / height;
    stepy += deltay;

    float *viewer = (float[3]){0.0f, 0.0f, -7.0f};

    if (threadid == 1)
        start = 0;
    else
        start = threadid * height/8;

    for (i = start; i < (threadid + 1)*(height/8); i++) {
        stepx = viewplanex0;
        for (j = 0; j < width; j++) {
            float *color = (float[3]){0.0f, 0.0f, 0.0f};
            float *raydir = (float[3]){stepx - viewer[0], stepy - viewer[1], 0 - viewer[2]};
            float maxdist = 100000.0f;
            normalize(raydir);
            trace(raydir, viewer, color,  0,maxdist);
            int r = (int)(roundf(color[0]*255.0f));
            if (r > 255) { r = 255; }
            int g = (int)(roundf(color[1]*255.0f));
            if (g > 255) { g = 255; }
            int b = (int)(roundf(color[2]*255.0f));
            if (b > 255) { b = 255; }
            pixmap[j+i*(int)width] = (r << 16) | (g << 8) | (b);
            stepx += deltax;
        }
        stepy += deltay;
    }
}

I am trying to implement this raytracer program using 8 threads (pthreads using gcc), but output image "ray8out.tga" is not correct. Program is executing correcly without any error but something missing in logic. It will be pleasure if someone please could help me, where is the error?

You're passing the same threadargs structure to every thread; there's no guarentee that they'll start running before you change the threadid value. You should allocate a new threadargs structure with malloc for each thread (and free it from within the new thread itself).