C: How to free the initial section of an allocated string?

Question

How can I free a string up to a certain point? For example if I have:

char *s = (char *)malloc(sizeof(char) * 21);
s[20] = '\0';
int i = 0;
while (i < 21)
{
    s[i] = i + 'a';
    i++;
}

And then I cut off the string at some point and store that half:

*(s + 10) = '\0';
char *m = s + 11;

Is there a way to free s up to s + 10 or the first \0?

No. You can free the end with realloc. What do you mean by free anyway? — Antti Haapala -- Слава Україні, Feb 23 '17 at 20:09
Unrelated to the question, but note that you [shouldn't cast the result of malloc](http://stackoverflow.com/q/605845/3425536). — Emil Laine, Feb 23 '17 at 20:10
In `while (s[i] != '\0')` you are making an assumption of what the allocated memory contains. Anything up to the byte you specifically set with `s[20] = '\0';` could be `0`. You have *undefined behaviour*. — Weather Vane, Feb 23 '17 at 20:12

Jean-François Fabre · Accepted Answer · 2017-02-23T21:01:10.307

4

Since s has been allocated by the system, you can perform a realloc on it to shorten the size:

s = realloc(s, 11);

But you cannot free the start of an allocated zone without freeing the rest, that you cannot. Use memmove & realloc to shift the data and reduce size afterwards.

Attempting to free a pointer which hasn't been allocated, or an allocated pointer plus an offset results in undefined behaviour.

You could do (as you suggested yourself, but with problems fixed :)):

char *c = strdup(s + 10);
free(s);
s = c;

so now s points to the end of the string.

alternative without strdup but using standard functions only (with the risk of forgetting the null-termination char):

char *c = malloc(strlen(s) - 10 + 1);
strcpy(c,s + 10);
free(s);
s = c;

and the memmove solution I was hinting about at first (avoids to allocate/free but offset/size is trickier to compute):

int offset = 10;
int size = strlen(s) - offset;
memmove(s,s+offset,size);
s[size]='\0';  // null-terminate
s = realloc(s,size+1);  // adjust size after null-termination

edited Feb 23 '17 at 21:01

answered Feb 23 '17 at 20:12

Jean-François Fabre

137,073
23
153
219

Is something like this going to work? `char *c = strdup(s + 10); free(&s); s = strdup(c); free(&c);` – Cosmin Serdean Feb 23 '17 at 20:31
probably but drop the `&` in `free`. I'll edit my answer to include your code, it solves it (and since you accepted it :)) – Jean-François Fabre Feb 23 '17 at 20:37
1

@CosminSerdean `char *c = strdup(s + 10); free(s); s = c;` is all you need. – Andrew Henle Feb 23 '17 at 20:38
@AndrewHenle that's telepathy, I figured out that much in my edit. – Jean-François Fabre Feb 23 '17 at 20:39
Note that `strdup()` is not defined by the standard. – pmg Feb 23 '17 at 20:42
@pmg you're right, but that's pretty common. 3 upvotes means that I'm morally compelled to edit for an alternative :) – Jean-François Fabre Feb 23 '17 at 20:43
@Jean-FrançoisFabre Indeed. Your edit appeared right when I entered my comment. – Andrew Henle Feb 23 '17 at 21:28
yes, I usually credit people if I lift some of their code :) maybe the `memmove` approach is more performant because you don't have to allocate & free, just realloc. – Jean-François Fabre Feb 23 '17 at 21:31
@pmg *Note that `strdup()` is not defined by the standard.* Which standard? [`strdup()` has been part of of the POSIX standard](http://pubs.opengroup.org/onlinepubs/009695399/functions/strdup.html) since probably 2001 or so, judging from the [dates for POSIX issue 6 on Wikipedia](https://en.wikipedia.org/wiki/POSIX). – Andrew Henle Feb 23 '17 at 21:32
@AndrewHenle: the [ISO/IEC 9899 Standard](http://www.open-std.org/jtc1/sc22/wg14/www/standards). – pmg Feb 23 '17 at 21:40

Emil Laine · Answer 2 · 2017-02-23T20:19:45.663

2

Is there a way to free s up to s + 10 or the first \0?

No. Not in the standard library at least.

There's realloc but it isn't guaranteed to "free s up to s + 10 or the first \0".

edited Feb 23 '17 at 20:19

answered Feb 23 '17 at 20:09

Emil Laine

41,598
9
101
157

C: How to free the initial section of an allocated string?

2 Answers2