Allocating memory dynamically without knowing the size of the string which is going to be entered

Question

Below is the function which returns a character pointer to a string which was initialized using getc(stdin)- character by character.

Is there any flaw in memory allocation method? Is this an efficient way when we don't know the size of the string which is going to be entered, if not please explain.
Using getc(stdin) --will it lead to buffer overflow???()
If I must not use getc(stdin), what will help me achieve my goal in a more efficient way?

Code so far:

char *getstring()
{
  char *str = NULL, *tmp = NULL;
  int size = 0, index = 0;
  int ch = -1;
  int length=0;
  while (ch!=0) 
  {
    ch = getc(stdin);

    if (ch == '\n')
    {   
        ch = 0;
    }

    if (size <= index) 
    {
        size += 15;
        tmp = (char*)realloc(str, size);
        if (!tmp) 
        {
            free(str);
            str = NULL;    
        }
        str = tmp;
    }
    str[index++] = ch;
  }

  if(size==index)
  {
    return str;
  }
  else if(index<size)
  {
    length=strlen(str);
    tmp = (char*)realloc(str,(length+1));
    str[index]='\0';
    //cout<<"length:"<<length;
    str=tmp;
  }

  return str;
}

str is NULL if realloc fails, str[index++] = ch; => NULL[index++] = ch; — BLUEPIXY
– BLUEPIXY, Commented May 26, 2015 at 15:52

Jonathon Reinhart · Accepted Answer · 2015-05-26 15:46:41Z

1

Don't re-invent the wheel: use getline(3).

Example (from same URL):

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

   int
   main(void)
   {
       FILE *stream;
       char *line = NULL;
       size_t len = 0;
       ssize_t read;

       stream = fopen("/etc/motd", "r");
       if (stream == NULL)
           exit(EXIT_FAILURE);

       while ((read = getline(&line, &len, stream)) != -1) {
           printf("Retrieved line of length %zu :\n", read);
           printf("%s", line);
       }

       free(line);
       fclose(stream);
       exit(EXIT_SUCCESS);
   }

answered May 26, 2015 at 15:46

Jonathon Reinhart

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5 Comments

MortalMan Over a year ago

might want to fix the int main line. :)

Jonathan Leffler Over a year ago

Note that POSIX systems should not require _GNU_SOURCE to provide access to getline(). You might need to set #define _XOPEN_SOURCE 700 or #define _POSIX_C_SOURCE 200809L before the first header is included to see the declaration of getline(), though.

MortalMan Over a year ago

@MartinTörnwall it's on two lines. not a big deal

Jonathon Reinhart Over a year ago

@user3711622 For declarations with a lot of tokens before the actual function name, or complex return types, it can be nice to put the function name on a new line - makes it easier to spot when looking through a large body of code. That said, I did not write said example.

Martin Törnwall Over a year ago

That, and you can regexp-search for ^<function name>. While I've rarely seen it in other languages, this is a very common formatting pattern in C.

Weather Vane · Accepted Answer · 2015-05-26 16:28:31Z

0

"Is there any flaw in memory allocation method?" Yes, when the reallocation fails, you free the previous memory pointer, assign NULL to it, then overwrite it with tmp, which has the value NULL, and then plough on using a NULL pointer.

if (size <= index) 
{
    size += 15;
    tmp = (char*)realloc(str, size);
    if (!tmp)                   // <--- NULL!
    {
        free(str);
        str = NULL;    
    }
    str = tmp;                  // <--- NULL!
}
str[index++] = ch;              // <--- NULL!

But if realloc fails you have little choice but to abandon the task in hand gracefully.

Also if you allow for the 0-terminator at the same time, you'll save several messy lines of code and another reallocation. And you don't always insert the '\0'.

if (size <= index+1)            // allow room for string terminator

edited May 26, 2015 at 16:28

answered May 26, 2015 at 16:22

Weather Vane

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1 Comment

Weather Vane Over a year ago

@BLUEPIXY only just noticed your comment.

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