动态分配2d数组在C差异与一个简单的2d阵列Mpi [英] Dynamic Allocation 2d array in C difference with a simple 2d array Mpi
问题描述
float ** masterbuf;
masterbuf = arralloc（sizeof（float），2，M，N）;
当我使用
[code] float masterbuf [M] [N];
程序给出的图像看起来不错。
问题是当我使用动态分配时，图像左侧会丢失一些像素。所以这些丢失的像素会产生黑线。就像图像已经移动了2像素。我不会对图像进行任何其他操作，只需读取并再次打印。
我用于写入图像的功能是：
void pgmwrite（char * filename，void * vx，int nx，int ny）
{
FILE * fp;
int i，j，k，gray;
float xmin，xmax，tmp，fval;
float thresh = 255.0;
float * x =（float *）vx;
if（NULL ==（fp = fopen（filename，w）））
{
fprintf（stderr，pgmwrite：无法创建<％s> \\ n，filename）;
exit（-1）;
}
printf（将％d x％d图片写入文件：％s\\\
，nx，ny，filename）;
/ *
*查找数组的最大和最小绝对值
* /
xmin = fabs（x [0]）;
xmax = fabs（x [0]）;对于（i = 0; i< nx * ny; i ++）
{
if（fabs（x [i]）
}
if（xmin == xmax）xmin = xmax-1.0;
fprintf（fp，P2\\\
）;
fprintf（fp，＃由pgmwrite\\\
编写）;
fprintf（fp，％d％d\\\
，nx，ny）;
fprintf（fp，％d\\\
，（int）thresh）;
k = 0; （i = 0; i
（j = ny-1; j> = 0; j--）
{ $ b {
/ *
*访问x [i] [j]的值
* /
tmp = x [j + ny * i];
/ *
*适当地缩放值，它位于0和thresh之间
* /
fval = thresh *（（fabs（tmp） -xmin）/（XMAX-XMIN））+ 0.5;
gray =（int）fval;
fprintf（fp，％3d，灰色）;
if（0 ==（k + 1）％16）fprintf（fp，\\\
）;
k ++;
}
}
if（0！= k％16）fprintf（fp，\\\
）;
fclose（fp）;
}
你对masterbuf的两个定义可能都创建2D数组，但它们不会以相同的方式执行。功能 arralloc（）为数据创建空间，指针 - 不仅仅是简单的静态数组定义的数据。这意味着在pgmwrite（）中，而x [i] [j]将返回相同的结果，无论使用哪种方法，x [i]将意味着两个不同的东西，因为指针的参与。 >
值得注意的是，您将被编译器给出一个线索，如果您更改 void * vx 原型为 float * vx 。由于您立即无条件地将此void *转换为浮点数*，所以无论如何，执行此操作会更好。
（第二个)另外，如果有兴趣，请查看此回复。它显示了如何使用二维来索引到单个malloc'd块中，而不使用arralloc（）。
I have a MPI program for image processing (pgm file) in MPI C and I use Dynamic allocation for a 2D Array as follows.
float **masterbuf;
masterbuf = arralloc(sizeof(float), 2, M, N);
When I use
float masterbuf[M][N];
the image that the program gives looks fine.
The problem is that when I use dynamic allocation the image loses some pixels in its left side. So these missing pixels create a black line. It's like the image has been shifted 2 pixels right. I don't do any other operations to the image, just read it and print it again.
The function that I use to write the image is:
void pgmwrite(char *filename, void *vx, int nx, int ny)
{
FILE *fp;
int i, j, k, grey;
float xmin, xmax, tmp, fval;
float thresh = 255.0;
float *x = (float *) vx;
if (NULL == (fp = fopen(filename,"w")))
{
fprintf(stderr, "pgmwrite: cannot create <%s>\n", filename);
exit(-1);
}
printf("Writing %d x %d picture into file: %s\n", nx, ny, filename);
/*
* Find the max and min absolute values of the array
*/
xmin = fabs(x[0]);
xmax = fabs(x[0]);
for (i=0; i < nx*ny; i++)
{
if (fabs(x[i]) < xmin) xmin = fabs(x[i]);
if (fabs(x[i]) > xmax) xmax = fabs(x[i]);
}
if (xmin == xmax) xmin = xmax-1.0;
fprintf(fp, "P2\n");
fprintf(fp, "# Written by pgmwrite\n");
fprintf(fp, "%d %d\n", nx, ny);
fprintf(fp, "%d\n", (int) thresh);
k = 0;
for (j=ny-1; j >=0 ; j--)
{
for (i=0; i < nx; i++)
{
/*
* Access the value of x[i][j]
*/
tmp = x[j+ny*i];
/*
* Scale the value appropriately so it lies between 0 and thresh
*/
fval = thresh*((fabs(tmp)-xmin)/(xmax-xmin))+0.5;
grey = (int) fval;
fprintf(fp, "%3d ", grey);
if (0 == (k+1)%16) fprintf(fp, "\n");
k++;
}
}
if (0 != k%16) fprintf(fp, "\n");
fclose(fp);
}
Your two definitions of masterbuf may both create 2D arrays, but they don't do so in the same way. The function arralloc() creates space for data and pointers--not just data as with the simple static array definition. What this works out to mean is that in pgmwrite(), while x[i][j] will return the same result regardless of the method used, x[i] will mean two different things because of the pointer involvement.
It's worth noting that you'll be given a clue by the compiler as to the problem should you change void *vx in the prototype to float *vx. Since you're immediately and unconditionally casting this void * to a float *, it'd be much better practice to do this anyhow.
(2nd edit:) Also, if interested, check out this response. It shows how to index using two dimensions into a single malloc'd block, without arralloc().
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