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Returning an Array of Strings in a Parameter

Passing the Address of an Array of Strings: In the middle example below, we pass the address of an array of strings, give the array of strings a value, return it through the parameter (since this parameter is a pointer to an array of strings) and print it in the calling program.  On the left is an incorrect method. On the right is a another correct method, which passes just a array of strings, but with its main array of pointers already allocated. Notice that the variable q on the right is not at all the same as the variable q on the left and in the middle.

These examples do not free their allocated storage, as they ought to.

Questions (in the recitation):

  1. What is wrong with the program on the left?
  2. Why does the call to mess_up seem to cause the problem to occur?

Incorrect Method Correct Method (Use for Rec 8) Another Correct Method 
#include <stdio.h>

void create_string(char ***q) {
   char *name [] =
      

      {"Neal",
       "R.",
       "Wagner",
        NULL};
   *q = name;
}

void mess_up() { /* stack */
   int a[20];
   int i;
   for (i = 0; i < 20; i++)
      a[i]= i;
}

void print_string(char **p) {
   while (*p)
      printf("%s ", *(p++));
   printf("
");
}

int main() {
   char **p;
   
   
   create_string(&p);
   print_string(p);
   mess_up();
   print_string(p);
}
 
#include <stdio.h>

void create_string(char ***q) {
   char **name =
      (char **)
       malloc(sizeof(char *)*4);
   *name     = "Neal";
   *(name+1) = "R.";
   *(name+2) = "Wagner"; 
   *(name+3) = NULL;
   *q = name;
}

void mess_up() {
   int a[20];
   int i;
   for (i = 0; i < 20; i++)
      a[i]= i;
}

void print_string(char **p) {
   while (*p)
      printf("%s ", *(p++));
   printf("
");
}

int main() {
   char **p;
   
   
   create_string(&p);
   print_string(p);
   mess_up();
   print_string(p);
}
 
#include <stdio.h>

void create_string(char **q) {
   
   
   
   *q     = "Neal";
   *(q+1) = "R.";
   *(q+2) = "Wagner"; 
   *(q+3) = NULL;
}


void mess_up() {
   int a[20];
   int i;
   for (i = 0; i < 20; i++)
      a[i]= i;
}

void print_string(char **p) {
   while (*p)
      printf("%s ", *(p++));
   printf("
");
}

int main() {
   char **p =
      (char **)
       malloc(sizeof(char *)*4);
   create_string(p);
   print_string(p);
   mess_up();
   print_string(p);
}
 
% cc -o param_wrong param_wrong.c
% param_wrong
Neal R. Wagner
Segmentation fault
      
% cc -o param param.c
% param
Neal R. Wagner
Neal R. Wagner
      
% cc -o param2 param2.c
% param2
Neal R. Wagner
Neal R. Wagner

Created By Dr. Neal Wager : The University of Texas at San Antonio