Trying to use realloc(), getting core dumped

Question

I'm trying to write a little program which uses realloc(), getchar() and some pointer arithmetic to store an array of characters in the memory.

I have a function called "inputArray" (in convert.c) which receives a pointer to a char (which is NULL to begin with, declared in main.c), then gets reallocated with one char until getchar() gets a '\n' char. the functions seems to work ok, but then when I try to print the string back in main.c, i get a "segmentation fault (core dumped)" error. I've been looking for hours, can't find where the problem is. Thanks!

main.c:

# include "convert.h"

int main()
{
  char * string = NULL;
  inputArray(string);
  printf("%s", string);    
  free(string);
  return 0;
}

convert.c:

#include "convert.h"

void inputArray(char * array)
{
    /*pointer to the array*/
    char * ptr = NULL;

    /*stores the char*/
    char c = 0;

    /*counter used for pointer arithmetic*/
    int count = 0;

    /*loop for getting chars in array*/
    while ((c = getchar()) != '\n')
    {
      array = realloc(array, sizeof(char));
      ptr = array + count;
      *ptr = c;
      ++count;
    }

    /*add the null char to the end of the string*/
    array = realloc(array, sizeof(char));
    ptr += count;
    *ptr = '\0';
}

convert.h:

#include <stdio.h>
#include <stdlib.h>

void inputArray(char * array);

Search for, and read about *emulating pass by reference in c*. — Some programmer dude, Dec 07 '16 at 15:47
`sizeof(char)` is always going to be 1..... (well, perhaps 2 if you're using double-width characters -- but it's a fixed size, which is not what you want). — gilez, Dec 07 '16 at 15:50

Vlad from Moscow · Accepted Answer · 2016-12-07T16:12:10.147

The size of the new allocated array is incorrect. You have to allocate count + 1 characters.

array = realloc(array, ( count + 1 ) * sizeof(char));

Take into account that it is more safe to use a temporary pointer to reallocate the memory. Otherwise the original address of the previously allocated memory will be lost.

Also these statements

array = realloc(array, sizeof(char));
ptr += count;

are wrong. You should at least write

array = realloc(array, count * sizeof(char));
ptr = array + count - 1;

Also the function should be declared like

char * inputArray(char * array);

and it must to return the new pointer to the caller.

And in main you have to write

string = inputArray(string);

Otherwise the function should accept the argument by reference that is the parameter should be declared like

void inputArray(char ** array);

and be processed correspondingly in the function.

Abhinav Upadhyay · Answer 2 · 2016-12-07T16:22:53.110

2

You are missing one level of indirection in the inputArray function. It should be declared like

void inputArray(char **array)

and it should be realloc'd like this (you also need to increase the size of the array by multiplying with count + 1)

*array = realloc(*array, (count + 1) * sizeof(char));

Call it like this:

 inputArray(&string);

edited Dec 07 '16 at 16:22

answered Dec 07 '16 at 15:50

Abhinav Upadhyay

2,477
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@BeyelerStudios Oops, result of a hurry :-) – Abhinav Upadhyay Dec 07 '16 at 15:54
@BeyelerStudios Thank you :-) – Abhinav Upadhyay Dec 07 '16 at 16:11

Trying to use realloc(), getting core dumped

2 Answers2