question about fread and fwrite

sandhurst

我真在谢谢一个小coding,巴数字放在两维数组里，然后把数组写在文件中，但是，当我读取的时候，我只能拿到第一行，别的拿不到,为什么？
#include <stdio.h>;

int main()
{
int row, col,i;
int sum =0;
FILE *my_stream;
char my_filename[] = "my_numbers";
size_t object_size = sizeof(int);
size_t object_count = 16;
size_t op_return;
int **tmp;

tmp = (int **)malloc((4) * sizeof(int *));
for (i=0; i <=3; i++)
tmp[i] = (int *)malloc((4) *sizeof(int));

for (row = 0; row <= 3; row++)
{
for (col = 0; col <=3; col++)
{
sum++;
tmp[row][col]=sum;
}
printf ("\n";
}
printf ("Initial values of array:\n";
for (row = 0; row <= 3; row++)
{
for (col = 0; col <=3; col++)
printf ("%d ",tmp[row][col]);
printf ("\n";
}

my_stream = fopen (my_filename, "w";
op_return = fwrite (tmp, object_size, object_count, my_stream);
fclose (my_stream);

for (row = 0; row <= 3; row++)
for (col = 0; col <=3; col++)
tmp[row][col] = 0;

printf ("Now reading data back in...\n";
my_stream = fopen (my_filename, "r";
op_return = fread (tmp, object_size, object_count, my_stream);

for (row = 0; row <= 3; row++)
{
for (col = 0; col<=3; col++)
{
printf ("%d ", tmp[row][col]);
}
printf ("\n";
}
fclose (my_stream);
return 0;
}

lenovo

c语言中数组下标从0开始的。
好好看教材吧。
代码写的很怪异。

sandhurst

你说的这点我知道，问题不在这里。我没有给零行和零列的元素付值而已。即使你把row=1改为row=0,结果是一样的。

HappyDuck

问题出在fwrite上。
op_return = fwrite (tmp, object_size, object_count, my_stream);
是把从tmp开始的连续的count个size大小的内存内容拷贝到文件里面。

而tmp要模拟二维数组, 本身首先申请了4×4大小的空间
后面每个tmp都再指向一块4×4大小的空间 所以总共是加起来超过16×4的
而且更重要的是这些空间并不是连续的

请看更改后的程序：打印了tmp[0-15]的内容后，发现最后面有1 2 3 4
就再打印tmp[12]的地址，发现正好是tmp[0]的内容！

1. 
   
2. #include <stdio.h>;
   
3. #include <stdlib.h>;
   
4. 
   
5. int main()
   
6. {
   
7. int row, col,i;
   
8. int sum =0;
   
9. FILE *my_stream;
   
10. char my_filename[] = "my_numbers";
    
11. size_t object_size = sizeof(int);
    
12. size_t object_count = 16;
    
13. size_t op_return;
    
14. int **tmp;
    
15. 
    
16. tmp = (int **)malloc((4) * sizeof(int *));
    
17. for (i=0; i <=3; i++)
    
18. tmp[i] = (int *)malloc((4) *sizeof(int));
    
19. 
    
20. for (row = 0; row <= 3; row++)
    
21. {
    
22.    for (col = 0; col <=3; col++)
    
23.    {
    
24.       sum++;
    
25.       tmp[row][col]=sum;
    
26.    }
    
27. }
    
28. 
    
29. fflush(stdout);
    
30. 
    
31. //++++++++++++++++++++++++++++
    
32. for(i = 0; i<16; i++)
    
33. {
    
34.   printf("%X \n", tmp[i]);
    
35. }
    
36. printf("%X \n", &tmp[12]);
    
37. //+++++++++++++++++++++++++++++++
    
38. printf ("Initial values of array:\n");
    
39. for (row = 0; row <= 3; row++)
    
40. {
    
41.    for (col = 0; col <=3; col++)
    
42.      printf ("%d ",tmp[row][col]);
    
43.    printf ("\n");
    
44. }
    
45. 
    
46. my_stream = fopen (my_filename, "w");
    
47. op_return = fwrite (tmp, object_size, object_count, my_stream);
    
48. printf("write op_return: %d\n", op_return);
    
49. fclose (my_stream);
    
50. 
    
51. for (row = 0; row <= 3; row++)
    
52. for (col = 0; col <=3; col++)
    
53. tmp[row][col] = 0;
    
54. 
    
55. printf ("Now reading data back in...\n");
    
56. my_stream = fopen (my_filename, "r");
    
57. op_return = fread (tmp, object_size, object_count, my_stream);
    
58. printf("read op_return: %d\n", op_return);
    
59. for (row = 0; row <= 3; row++)
    
60. {
    
61. 
    
62.     for (col = 0; col<=3; col++)
    
63.     {
    
64.       printf ("%d ", tmp[row][col]);
    
65.     }
    
66.     printf ("\n");
    
67. }  
    
68. fflush(stdout);
    
69. 
    
70. //+++++++++++++++++++++++++++++++
    
71. for(i = 0; i<16; i++)
    
72. {
    
73.   printf("%X \n", tmp[i]);
    
74. }
    
75. printf("%X \n", &tmp[12]);
    
76. //+++++++++++++++++++++++++++++++
    
77. fclose (my_stream);
    
78. 
    
79. getchar();
    
80. return 0;
    
81. }
    

复制代码

所以fwrite和fread 只是把tmp[0-15]的内容拷贝到文件里面，然后
再读出来，恰好（或者某种必然，编译器分配?），1 2 3 4写进去又
读出来了， 其他的元素在这个范围外面，没有被改动