realloc() invalid pointer error when increasing size of string in a function

Question

When i run code it show realloc() invalid pointer error.

Is anything wrong in input() function?

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<time.h>
char *input(void)
{
    int n = 1;
    char *str = malloc(sizeof(char));
    *str = '\0';
    while((*str=getchar())!='\n')
    {
        n++;
        str = realloc(str,sizeof(char)*n);
        str++;
    }
    return str;
}

int main(int argc, char const *argv[])
{
    char *str = input();
    printf("%s",str);
    free(str);
    return 0;
}

Paul Ogilvie · Accepted Answer · 2018-11-03 15:11:05Z

4

You make a few errors:

you return the end of the string, not the beginning.
realloc needs the original address (see Thomas' answer)
realloc may return a new address
you do not terminate the string.

The following fixes these errors and includes some suggestions:

char *input(void)
{
    size_t i=0;
    int c;
    char *str = malloc(1);
    if (!str) return 0;
    while((c=getchar())!=EOF && c!='\n')
    {
        str[i]= c;
        if ((newstr = realloc(str,i+1))==0)
            break;          // out of memory: return what we have
        str= newstr;
        i++;
    }
    str[i]= '\0';
    return str;
}

edited Nov 3, 2018 at 15:11

answered Nov 3, 2018 at 10:31

Paul Ogilvie

25.3k4 gold badges25 silver badges43 bronze badges

Sign up to request clarification or add additional context in comments.

11 Comments

Thomas Padron-McCarthy Over a year ago

I would suggest two changes: (1) Skip sizeof(char), which is always 1, and at least to me it is a code smell. (2) Use an integer as index into the allocated string instead of an extra pointer, to avoid the pointer arithmetic.

alk Over a year ago

The code shown will fail miserably if EOF is detected before a new-line had been read.

alk Over a year ago

This if (newstr != str) might invoke undefined behaviour, asstr might point to invalid memory after realloc().

Paul Ogilvie Over a year ago

@alk, fixed EOF. I am not sure about the undefined behavior comment. The str pointer (a variable) is compared with another pointer (variable), no matter if the memory pointed to is still valid. If it is undefined behavior, then it is not possible to check if the new pointer is different from the old pointer and the assignment must always be made.

alk Over a year ago

"it is not possible to check if the new pointer is different from the old pointer" that's true. But one never needs to do this.

|

Thomas Padron-McCarthy · Accepted Answer · 2018-11-03 10:25:47Z

2

After doing str++, the pointer no longer points to the start of the allocated string. realloc needs the original pointer, not one that points to somewhere inside the allocated data.

edited Nov 3, 2018 at 10:25

answered Nov 3, 2018 at 10:21

Thomas Padron-McCarthy

27.7k8 gold badges55 silver badges77 bronze badges

Comments

alk · Accepted Answer · 2018-11-03 12:28:39Z

An approach with no redundant calls and complete error checking:

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

char *input(void)
{
  size_t n = 0;
  char * str = NULL;

  do
  {
    ++n;

    {
      void * pv = realloc(str, (n + 1) * sizeof *str);
      if (NULL == pv)
      {
        perror("realloc() failed");

        break;
      }

      str = pv;
    }

    {
      int result = getchar();

      if (EOF == result)
      {
        if (ferror(stdin))
        {
          fprintf(stderr, "getchar() failed\n");
        }

        --n;

        break;
      }

      str[n - 1] = result;
    }
  } while ('\n' != str[n - 1]);

  if (NULL != str)
  {
    str[n] = '\0';
  }

  return str;
}

int main(int argc, char const *argv[])
{
  int result = EXIT_SUCCESS; /* Be optimistic. */

  char * str = input();
  if (NULL == str)
  {
    result = EXIT_FAILURE;

    fprintf(stderr, "input() failed\n");
  }
  else
  {
    printf("input is: '%s'\n", str);
  }

  free(str);

  return result;
}

Collectives™ on Stack Overflow

realloc() invalid pointer error when increasing size of string in a function

3 Answers 3

11 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

11 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related