arrays - C pointer dereference error -


suppose have following code (example):

#include <stdio.h> #include <stdlib.h> #include <stdbool.h>  void ex(void *ret, int ret_len, int choice){     if(choice==1){         int *ret = ret;                       for(int i=0; i<ret_len; i++){             *(ret+i) = i;         }      } else {         int **ret = ret;                     for(int i=0; i<ret_len; i++){             for(int j=0; j<ret_len; j++){                 *(*(ret+i)+j) = i*j;             }         }      } }  int main() {     int m[10];     ex(m,10,1);     printf("%i",m[3]);     return 0; }

the goal of function take pointer preallocated memory of 1 or 2 dimensional array.

when compiled , executed code works when choice==1, otherwise, if run

#include <stdio.h> #include <stdlib.h> #include <stdbool.h>  void ex(void *ret, int ret_len, int choice){     if(choice==1){         int *ret = ret;                       for(int i=0; i<ret_len; i++){             *(ret+i) = i;         }      } else {         int **ret = ret;                     for(int i=0; i<ret_len; i++){             for(int j=0; j<ret_len; j++){                 *(*(ret+i)+j) = i*j;             }         }      } }  int main() {     int m[100];     ex(m,10,2);     printf("%i", m[3][5]);     return 0; }

no output ever produced , code seems run forever (note choice not 1 , m proper size)

i under impression casting in ex should change shape of array , allow main index if 2d neither indexing if 1d nor 2d work.

having looked @ comments on question , answers have gathered requirements.

you need generic function takes array of choice dimensions , initializes values.

this can suit purpose. 

void ex(void *_ret, int ret_len, int choice){     if(choice==1){         int *ret = _ret;                       int i;         for(i=0; i<ret_len; i++){             ret[i] = i;         }      } else {         int (*ret)[ret_len] = _ret;                     int i, j;         for(i=0; i<ret_len; i++){             for(j=0; j<ret_len; j++){                 ret[i][j] = i*j;             }         }      } }

now how can call it

int main(void) {     int first[10];     ex(first, 10, 1);     int second[20][20];     ex(second, 20, 2);     printf("first[4] = %d\n", first[4]);     printf("second[3][4] = %d\n", second[3][4]); }

you can see demo here


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