Thread: 2D arrays and double pointers

Hi All,

The following code prints out values of a 2 D array in main as well as in the function printvalues. However, fd->mychar[i] = newptr[i]; throws an error. I understand that a ** myarray is not the same as myarray [][] and I am trying to figure out a solution to this. My problem is, in my real program (I reproduce the same problem below with minimal code), I am modifying pieces of pre-existing code of a 10,000+ lines, so the argument to printvalues(char **) cannot be changed, it has to be a double pointer, as this is how its used in several places in the code (and I dont want to break anything), and, structure mystruct is large with multiple 2D arrays such as mychar, and since I want to always ensure a contiguous block of memory is allocated to mystruct, I cannot use pointers like so: char *mychar [20].

With these restrictions, is it still possible to make this work? How? Thanks!

Code:

typedef struct mystruct{
  char mychar [20][20];
}mystruct_t;

void printvalues ( char ** newptr){
  int i;
  mystruct_t * fd;
  for (i=0;i<3;i++){
   printf("My value is %s\n", newptr[i]); // prints fine if following 2 lines commented
    fd->mychar[i] = newptr[i];               // error: incompatible types in assignment

    printf("My value is %s and in struct %s\n", newptr[i], fd->mychar[i]);
  }
}
int main (int argc, char **argv){
 int i;
 char * abc[5] = {"123", "456", "789"};

 for (i=0;i<3;i++){
  printf("My value is %s\n", abc[i]);
 }
 printvalues(abc);
}

Thanks Quzah,

This code segfaults :-(

Code:

typedef struct mystruct{
    char mychar [5][4];
}mystruct_t;

void printvalues ( char ** newptr){
   int i,j;
   mystruct_t * fd;
   for (i=0;i<3;i++){
      for(j=0;j<3;j++){
      fd->mychar[i][j] = newptr[i][j];
      printf("%s:%s\t", newptr[i][j], fd->mychar[i][j]);
      }
      printf("\n");
   }
}
int main (int argc, char **argv){
   int i;
   char  abc[5][4] = {"123", "456", "789"};

   for (i=0;i<3;i++){
      printf("My value in main is %s\n", abc[i]);
   }
   printf("");
   printvalues((char ** )abc);
}

When you type "newptr[i]" it is interpreting the value as a pointer. But it isn't, since it's just the character data "12" or whatever.