leaks in my_strcat with malloc

Question

I'm trying to recode the function strcat featuring an auto-malloc inside it. I can see in few tests that there are some data leaks (I don't know the correct word) in my function.

Here's my code :

char    *my_strcat(char *a, char *b)
{
  char  *result;
  int   i;
  int   j;
  int   la;
  int   lb;

  la = -1;
  lb = -1;
  while (a[++la] != '\0');
  while (b[++lb] != '\0');
  result = malloc(sizeof(char) * (la + lb) + 1);
  i = -1;
  while (a[++i] != '\0')
    result[i] = a[i];
  j = -1;
  while (b[++j] != '\0')
    result[i + j] = b[j];
  result[i + j] = '\0';
  return (result);
}

So when I try to use my my_strcat in a while loop with some one-length strings I get a :

a.out: malloc.c:2392: sysmalloc: Assertion `(old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)' failed.
Aborted (core dumped)

I'm trying to find where my memory leak comes from. But I can't figure out.

Here's the calling part :

while (!is_line_ended(read_result))
{
  read(fd, read_result, READ_SIZE);
  result = my_strcat(result, read_line(read_result, fd));
}

Here's MY_STRLEN :

# define MY_STRLEN(s) (sizeof(s)/sizeof(s[0]))

Please notice that I'm limited to one file and one header. I'm limited to 5 functions by file, and I'm not allowed to use string functions.

Uh, I don't see them :/ It's maybe because I copy-pasted from emacs — Orionss, Jan 12 '17 at 14:51
The error occurs in `malloc()` therefore the code after the `malloc()` cannot be relevant. Therefore the most likely issue is that `MY_STRLEN()` has bugs. — slim, Jan 12 '17 at 14:58
I'm reading a text with a variable READ_SIZE macro I use for my read(). When I use a long Lorem Ipsum, using a 4096 READ_SIZE, I get the correct text. When I use a 256 READ_SIZE, I get memory leaks and when I use a 1 READ_SIZE I just get a Core Dumped. — Orionss, Jan 12 '17 at 14:58
`read` doesn't add a `NUL` char, so `read_result` isn't NUL-terminated. — Klas Lindbäck, Jan 12 '17 at 15:01
I put the informations you asked. And nobody told me that read doesn't insert a NULL char. — Orionss, Jan 12 '17 at 15:02
Memory leaks are because you don't free the memory returned by the function. The reported crash is caused by writing out of bounds of allocated memory. They're different problems with different fixes. — Jonathan Leffler, Jan 12 '17 at 15:03
`MY_STRLEN()` is broken. You cannot use sizeof on a pointer to get the size of some elsewhere allocated memory. That is what `strlen` is made for. — Gerhardh, Jan 12 '17 at 15:04
To summarize the comments: Please read an introduction to `malloc()` and string functions in C. — Amin Negm-Awad, Jan 12 '17 at 15:05
I have to use only one file. I'm limited at 5 functions/file, I can't use the standard C library function except malloc, sizeof and read. — Orionss, Jan 12 '17 at 15:05
Note that `sizeof` is an operator and not a function. It is part of the core language, not the library. — Jonathan Leffler, Jan 12 '17 at 15:06
Then you might just implement `strlen` functionality within your function. It's just a one-liner. — Gerhardh, Jan 12 '17 at 15:07
For a pointer , `sizeof` returns address length that is 4 (or 8 for 64bit OS) always. you have to write `my_strlen` too :D — HamidReza, Jan 12 '17 at 15:08
The problem came from my MY_STRLEN macro, I fixed it, thanks for help guys ! — Orionss, Jan 12 '17 at 15:09
I closed this as a duplicate to a question explaining why you can't use `sizeof` as done in this macro, since that was the culprit. — Lundin, Jan 12 '17 at 15:11
One more comment on the allocation part: `malloc(sizeof(char) * (la + lb) + 1);`: This only works because `sizeof(char)` is 1 by definition. Otherwise you would need to move the `+1` into the brackets due to math operator precedence. — Gerhardh, Jan 12 '17 at 15:29

score 1 · Accepted Answer · answered Jan 12 '17 at 15:09

1

The problem is your macro # define MY_STRLEN(s) (sizeof(s)/sizeof(s[0])):

sizeof(s) does not return the string length of s but rather the size of s's type, i.e. sizeof(char*).

Use strlen(s) instead of your macro; and if - for any reason - you are not allowed to use strlen, you could use the following function:

int my_strlen(const char* s) {
  int len=0;
  while (*s++ != '\0')
    len++;
  return len;
}

answered Jan 12 '17 at 15:09

Stephan Lechner

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Thanks, I know that but I couldn't use one more function. ut I solved it I post the code – Orionss Jan 12 '17 at 15:11

leaks in my_strcat with malloc

1 Answers1