c - 用 calloc 初始化的随机数填充指针数组的问题

Question

1. 我的程序应该创建 10 个 vector ，每个 vector 的随机维度在 1 到 10 之间，每个维度都填充有 0 到 24 之间的随机数。

   int * makeVector(int dimension) {
     int * Vector = NULL;
   
     Vector = (int*) calloc(dimension - 1, sizeof(int));
     if (NULL == Vector) exit(1);
   
     srand(time(0));
   
     for (int i = 0; i < (dimension - 1); i++) {
       Vector[i] = rand() % 25;
     }
   
     return Vector; 
   }
   
   int main() {
     srand(time(0));
   
     for (int i = 1; i <= 10; i++) {
       printf("-----%d. Vector-----\n", i);
       fflush(stdout);
       int dimension = (rand() % 10) + 1;
       int *Vector = NULL;
   
       memcpy(Vector, makeVector(dimension), dimension);
   
       for (int k = 1; k <= dimension; k++) {
         printf("%d. Dimension: %d\n", k, Vector[k - 1]);
       }
       if (Vector) {
         free(Vector);
         Vector = NULL;
       }
     }
     return 0;
   }
   

2. 我的问题:没有出现警告，也没有出现错误!尽管 makeVector 函数中的 For-Loop 似乎根本不起作用。

   for (int i = 0; i < (dimension - 1); i++) {
        Vector[i] = rand() % 25;
   }
   

int i 第一次只等于 0，没有任何反应。 For 循环跳过，程序终止，退出值:-1.073.741.819

PS:我对编程还很陌生......可能会错误地解释调试器。

提前谢谢您!

Answer 1

您的主要问题是:

memcpy(Vector, makeVector(dimension), dimension);

而Vector是一个空指针，因此行为未定义

做

Vector = makeVector(dimension);

<小时/>

另一个问题是重做srand(time(0));是makeVector，执行时间几乎没有，所以你重新初始化随机生成再次给你相同的值(时间变化几乎没有变化)。

执行期间仅执行一次srand(time(0));一次

<小时/>

在

for (int k = 1; k <= dimension; k++) {
  printf("%d. Dimension: %d\n", k, Vector[k - 1]);
}

您可以访问 vector 的未初始化的最后一个条目，因为在 makeVector 中:

for (int i = 0; i < (dimension - 1); i++) {
    Vector[i] = rand() % 25;
 }

必须是

for (int k = 1; k < dimension; k++) {
  printf("%d. Dimension: %d\n", k, Vector[k - 1]);
}

或者当然修改初始化循环以不丢失最后一个条目

<小时/>

修改后编译执行:

pi@raspberrypi:/tmp $ gcc -pedantic -Wextra m.c
pi@raspberrypi:/tmp $ ./a.out
-----1. Vector-----
1. Dimension: 18
2. Dimension: 21
3. Dimension: 22
4. Dimension: 21
5. Dimension: 2
6. Dimension: 0
7. Dimension: 24
-----2. Vector-----
1. Dimension: 20
2. Dimension: 24
3. Dimension: 13
-----3. Vector-----
1. Dimension: 3
-----4. Vector-----
1. Dimension: 5
2. Dimension: 24
3. Dimension: 10
4. Dimension: 4
5. Dimension: 20
-----5. Vector-----
1. Dimension: 19
2. Dimension: 7
3. Dimension: 24
4. Dimension: 24
5. Dimension: 16
6. Dimension: 11
-----6. Vector-----
1. Dimension: 10
2. Dimension: 14
3. Dimension: 17
4. Dimension: 9
5. Dimension: 9
-----7. Vector-----
1. Dimension: 9
2. Dimension: 8
3. Dimension: 19
4. Dimension: 9
5. Dimension: 7
-----8. Vector-----
1. Dimension: 6
2. Dimension: 8
3. Dimension: 23
-----9. Vector-----
1. Dimension: 14
2. Dimension: 13
3. Dimension: 12
4. Dimension: 13
-----10. Vector-----
1. Dimension: 18
2. Dimension: 10
3. Dimension: 3
4. Dimension: 12
5. Dimension: 20
pi@raspberrypi:/tmp $ 

在valgrind下执行

pi@raspberrypi:/tmp $ valgrind ./a.out
==29590== Memcheck, a memory error detector
==29590== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==29590== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==29590== Command: ./a.out
==29590== 
-----1. Vector-----
1. Dimension: 21
2. Dimension: 16
3. Dimension: 3
4. Dimension: 11
5. Dimension: 11
6. Dimension: 8
7. Dimension: 13
8. Dimension: 8
9. Dimension: 23
-----2. Vector-----
1. Dimension: 11
2. Dimension: 3
3. Dimension: 5
4. Dimension: 21
5. Dimension: 18
6. Dimension: 4
-----3. Vector-----
1. Dimension: 20
2. Dimension: 15
3. Dimension: 4
4. Dimension: 17
5. Dimension: 0
6. Dimension: 2
7. Dimension: 21
-----4. Vector-----
1. Dimension: 24
2. Dimension: 6
3. Dimension: 20
4. Dimension: 10
5. Dimension: 1
6. Dimension: 19
-----5. Vector-----
1. Dimension: 19
2. Dimension: 24
3. Dimension: 19
-----6. Vector-----
1. Dimension: 7
2. Dimension: 10
-----7. Vector-----
1. Dimension: 5
2. Dimension: 6
3. Dimension: 4
4. Dimension: 10
5. Dimension: 11
6. Dimension: 2
7. Dimension: 4
-----8. Vector-----
1. Dimension: 0
2. Dimension: 1
-----9. Vector-----
1. Dimension: 6
2. Dimension: 18
3. Dimension: 12
4. Dimension: 10
5. Dimension: 16
-----10. Vector-----
==29590== 
==29590== HEAP SUMMARY:
==29590==     in use at exit: 0 bytes in 0 blocks
==29590==   total heap usage: 11 allocs, 11 frees, 1,212 bytes allocated
==29590== 
==29590== All heap blocks were freed -- no leaks are possible
==29590== 
==29590== For counts of detected and suppressed errors, rerun with: -v
==29590== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)