C指针指针问题

Question

struct instruction { 
    int value; 
}; 

int n; // Number of instructions 

struct instruction **instructions; //Required to use ** and dynamic array 

说我想存储n个指令并在每条指令中存储一个值。 我该怎么做**说明？ 所以我希望能够稍后从特定的指令中调用一个值。

非常感谢

到目前为止，我尝试了这些，一些scanfs和动态数组创建。 所以它需要计数器的数量，然后需要线程数（pthreads），然后在每个线程内需要多少条指令。我正试图找到一种方法来在每个线程中存储指令。 **结构给出

int 
main(void) { 

     scanf(" %d", &ncounters); //Ask for number of counters 

     int i; 

     if(counters = malloc(sizeof(struct counter)*ncounters)){ 
      for(i=0; i < ncounters ;i++){ 
      counters[i].counter = 0; 
      } 
     } 

     scanf(" %d", &nthreads); //Ask for number of threads 

     if(ninstructions = (int*) malloc(nthreads*sizeof(int))){ 
      for(i=0; i < nthreads ;i++){ 
      ninstructions[i] = 0; 
      } 
     } 

     for(i=0; i < nthreads ;i++){ 

      scanf(" %d", &ninstructions[i]); //Ask for number of instructions within threads[i] 
     // Things got messy from here ... 
      instructions = malloc(sizeof(struct instruction*)*ninstructions[i]); 

      for(int j=0; j < ninstructions[j] ;j++){ 
      instructions[j] = malloc(sizeof(struct instruction)); 
      int x; 
      printf("Enter rep for thread %d inst %d.\n",i+1 ,j+1); 
      scanf(" %d", &x); 
      instructions[i][j].repetitions = x; 
      } 

     } 

     printf(" Instruction x: %d.\n", instructions[0][0].repetitions); 

//============================================================================= 
// Just testing with printf 
     printf("Number of counters: %d.\n", ncounters); 
     printf("Number of threads: %d.\n", nthreads); 

     for(i=0; i < nthreads; i++){ 
      printf("Thread %d has %d instructions.\n", i+1, ninstructions[i]); 
     } 

//============================================================================= 

     free(instructions); 
     free(ninstructions); 
     free(counters); 
    return 0; 
} 

我终于得到了一些进展，但在指令存储部分出现分段错误。

struct counter *counters; 

struct instruction{ 
    struct counter *counter; 
    int repetitions; 
    void (*work_fn)(long long*); 
}; 

for(i=0; i < nthreads ;i++){ 

    scanf(" %d", &ninstructions[i]); //Ask for number of instructions within threads[i] 

    instructions = malloc(nthreads*sizeof(struct instruction *)); 

    instructions[i] = malloc(ninstructions[i]*sizeof(struct instruction)); 
    for(int j=0;j < ninstructions[i] ;j++){ 
    int Srepetition; 
    char Sfunction; 
    int Scounter; 

    scanf(" %d %c %d", &Scounter, &Sfunction, &Srepetition); 

    //Problem seems to be here "Segmentation fault" .............. 

    instructions[i][j].repetitions = Srepetition; 
    instructions[i][j].counter = (counters+Scounter); 

    if(Sfunction == 'I'){ 
     instructions[i][j].work_fn(&increment); 
    }else if(Sfunction == 'D'){ 
     instructions[i][j].work_fn(&decrement); 
    }else if(Sfunction == '2'){ 
     instructions[i][j].work_fn(&mult2); 
    }else{ 
     printf("error\n"); 
    } 

    printf("Thread: %d Instruction: %d.\n", i+1, j+1); 
    } 
} 

Answer 1

只需一个*就足以创造一个动态数组，**你会创建一个矩阵。

struct instruction *instructions = malloc(n * sizeof(struct instruction)); 

/* setting some random values */ 
for (int i=0;i<n;i++) 
    instructions[i]->value = i; 

/* accessing values */ 
for (int i=0;i<n;i++) 
    printf("instruction %d value %d\n",i,instructions[i]->value); 

/* don't forget to free */ 
free(instructions); 

请参阅关于dynamic arrays in C的参考资料来调查更多。例如this one

编辑

...如果你真正需要的矩阵，这是等效代码：

int n,z; // for number cols and rows 

struct instruction **instructions = malloc(n * sizeof(struct instruction *)); 

/* setting some random values */ 
for (int i=0;i<n;i++) { 
    instructions[i] = malloc(z * sizeof(struct instruction)); 
    for (int j=0;j<z;j++) 
     instructions[i][j]->value = i * j; 
} 
/* accessing values */ 
for (int i=0;i<n;i++) { 
    for (int j=0;j<z;j++) 
      printf("instruction %d,%d value %d\n",i,j,instructions[i][j]->value); 
} 

/* don't forget to free */ 
for (int i=0;i<n;i++) 
    free(instructions[i]); 
free(instructions);