动态分配二维数组用C差异用一个简单的二维数组Mpi的 [英] Dynamic Allocation 2d array in C difference with a simple 2d array Mpi
问题描述
我在MPIÇ图像处理（PGM文件）一个MPI程序，我使用动态分配一个二维数组如下：
浮动** masterbuf;
masterbuf = arralloc（的sizeof（浮动）; 2，M，N）;
当我使用
浮动masterbuf [M] [N];
该程序使图像看起来罚款。
的问题是，当我使用动态分配的图像失去一些像素在其左侧。因此，这些丢失的像素创建一条黑线。这就像图像被移位2个像素的权利。我没有做任何其他操作的形象，只是读它，并再次打印。
这是我用写的图像的功能是：
无效pgmwrite（字符*文件名，无效* VX，诠释NX，诠释NY）
{
FILE * FP; INT I，J，K，灰色; 浮XMIN，XMAX，TMP，未来值;
浮脱粒= 255.0; 浮动* X =（浮点*）VX; 如果（NULL ==（FP =的fopen（文件名，在w）））
{
fprintf中（标准错误，pgmwrite：无法创建＆LT;％S＆GT; \\ N，文件名）;
出口（-1）;
} 的printf（写％深x％d张图片到文件：％s \\ n，NX，NY，文件名）; / *
*查找最大和最小数组的绝对值
* / XMIN =晶圆厂（X [0]）;
XMAX =晶圆厂（X [0]）; 对于（i = 0; I＆LT; NX * NY;我++）
{
如果（晶圆厂（X [I]）LT; XMIN）XMIN =晶圆厂（X [I]）;
如果（晶圆厂（X [I]）GT; XMAX）XMAX =晶圆厂（X [I]）;
} 如果（XMIN == XMAX）XMIN = XMAX-1.0; fprintf中（FP，P2 \\ n）;
fprintf中（FP＃写的pgmwrite \\ n）;
fprintf中（FP，％D \\ n，NX，NY）;
fprintf中（FP，％d个\\ N（INT）脱粒）; K = 0; 为（J = NY-1; J＆GT; = 0; j--）
{
对于（i = 0; I＆LT; NX;我++）
{
/ *
*访问x的值[I] [J]。
* / TMP =×〔J + NY * I] / *
*适当缩放值，因此在0到脱粒之间
* / 未来值= THRESH *（（晶圆厂（TMP）-xmin）/（XMAX-XMIN））+ 0.5;
灰色=（int）的未来值; fprintf中（FP，％3D，灰色）; 如果（0 ==第（k + 1）％16）fprintf中（FP，\\ n）; ķ++;
}
} 如果（0 = K％16！）fprintf中（FP，\\ n）;
FCLOSE（FP）;
}
您masterbuf的两个定义可以都创建2D阵列，但它们不以同样的方式这样做。该功能 arralloc（）以创造空间数据指针 - 不仅仅是数据与单纯的静态数组的定义。这是什么工程以意思是说，在pgmwrite（），而X [I] [j]的将返回相同的结果而不管所使用的方法，其中x [Ⅰ]将意味着，因为指针的参与两个不同的东西。
这是值得注意的，你会得到编译器的线索，你应该改变无效* VX 中的原型问题浮* VX 。既然你立即和无条件地铸造了这一空白*为float *，它会是更好的做法，无论如何做到这一点。
（第2)另外，如果有兴趣，看看这个响应。它展示了如何使用二维成一个单一的malloc分配块索引，而不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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