Trouble printing an array of strings after realloc? - c

Question

The code below reads characters and splits them into C-style strings when a delimiter is encountered, then it stores the words (white-space-separated sequences of characters) in string array till a sentinel is encountered; updates size of string array:
#include <stdio.h> // printf()
#include <stdlib.h> // malloc(); realloc()
#include <string.h> // strcmp()
#include <stddef.h> // size_t
void print_array(char* arr[ ], size_t size); // forward declaration to use in to_array()
char* get_word(char delimiter)
{
size_t size = 8;
size_t index = 0;
int c = 0;
char* word = 0;
char* expand_word = 0;
word = (char*) malloc(sizeof(char) * size);
if (word == NULL)
{
perror("get_word::bad malloc!\n");
exit(-1);
}
while ((c = getchar()) != EOF && c != delimiter && c != '\n')
{
if (index >= size)
{
size *= 2;
expand_word = (char*) realloc(word, sizeof(char) * size);
if (expand_word == NULL)
{
perror("get_word::bad realloc!\n");
exit(-1);
}
word = expand_word;
}
word[index++] = c;
}
word[index] = 0;
return word;
}
//-------------------------------------------------------------------------------------
void to_array(char* arr[ ], size_t* size, char* sentinel)
{
size_t index = 0;
char* word = 0;
char** expand_arr = 0;
char delimiter = ' ';
while ((word = get_word(delimiter)) && strcmp(word, sentinel) != 0)
{
if (index >= (*size))
{
(*size) *= 2;
expand_arr = (char**) realloc(arr, sizeof(char*) * (*size));
if (expand_arr == NULL)
{
perror("to_array::bad realloc!\n");
exit(-1);
}
arr = expand_arr;
}
arr[index++] = word;
}
(*size) = index;
// print_array(arr, *size); // <---- here, all words printed OK.
// getchar();
}
//-------------------------------------------------------------------------------------
void print_array(char* arr[ ], size_t size)
{
size_t i = 0;
printf("{ ");
for (i; i < size; ++i)
{
printf("%s", arr[i]);
if (i < size - 1)
{
printf(", ");
}
}
printf(" }\n");
}
//-------------------------------------------------------------------------------------
int main()
{
size_t size = 4;
char** arr = 0;
char* sentinel = "quit";
arr = (char**) malloc(sizeof(char*) * size);
if (arr == NULL)
{
perror("array of strings::bad malloc!\n");
exit(-1);
}
printf("Type a sentence and get each word as an array element:\n");
to_array(arr, &size, sentinel);
printf("Words:\n");
print_array(arr, size); // <--------- here, error!
getchar();
}
When trying to print the string array, I get:
Access violation reading location 0xcd007361.
Why I can't print the strings in arr at the end?
P.S.: I guess that the problem comes from the pointer arithmetic and the reallocation of the char** arr within function to_array(). (If previous right) I'm not sure what would be the standard way to deal with it?

Answer 1

Problem: the first parameter in void to_array(), i.e. char* arr[ ] passes a copy of a pointer to array of char. Every change on the pointer made inside the function does not affect the actual pointer to char array outside, specifically the function realloc() may move the initial memory block to a new location, which would invalidate the pointer passed as first parameter.
Solution: either to modify the function void to_array() to return the modified arr, or to modify the first parameter of the function to char** arr[ ]. The latter was chosen and the modified code looks like this:
void to_array(char** arr[ ], size_t* size, char* quit)
{
size_t index = 0;
char* word = 0;
char** expand_arr = 0;
char sentinel = ' ';
while ((word = get_word(sentinel)) && strcmp(word, quit) != 0)
{
if (index >= (*size))
{
(*size) *= 2;
expand_arr = (char**) realloc((*arr), sizeof(char*) * (*size));
if (expand_arr == NULL)
{
perror("to_array::bad realloc!\n");
exit(-1);
}
(*arr) = expand_arr;
}
(*arr)[index++] = word;
}
(*size) = index;
}
then the function call must be done as:
to_array(&arr, &size, quit);