freeing of char**

[gregg]

Hello all,


I have a char **, which points to a number of strings (char*).
When comes the moment to free it, i have to free all the (char*) inside.
Yet when i do this:

int i;

for ( i=0; *(myChar+i) != NULL; i++ )
free (*(myChar+i)); /* to free the (char*) inside
free(myChar);


well, on execution the system says 'junk pointer, too low to make sense'
(and the pointers aren't freed)

of course, when i write (*(myChar+i) = NULL), it's all okay
(except the memory hasn't been freed ?!)

so how should i free the (char**) to make it work, and also be clean ?

thx
gregg

 

[Dan Pop]

I have a char **, which points to a number of strings (char*).
When comes the moment to free it, i have to free all the (char*) inside.
Yet when i do this:

int i;

for ( i=0; *(myChar+i) != NULL; i++ )
free (*(myChar+i)); /* to free the (char*) inside */
free(myChar);


well, on execution the system says 'junk pointer, too low to make sense'
(and the pointers aren't freed)

Are you sure that *(myChar+i) (usually written myChar) has been
initialised with a malloc (or friends) call? Are you sure that your
array of pointers is terminated by a null pointer?

Only pointer values obtained from malloc and friends need to be freed.
It is a major mistake to free any other pointer values.

So, without seeing how myChar and myChar have been initialised, there
is no way to telling what you're doing wrong. This is an initialisation
scenario compatible with your freeing code (error checking omitted for
brevity):

char **myChar = malloc(10 * sizeof *myChar);
for (i = 0; i < 3; i++) myChar = malloc(25);
myChar[3] = NULL;

Dan

 

[CBFalconer]

I have a char **, which points to a number of strings (char*).
When comes the moment to free it, i have to free all the (char*)
inside. Yet when i do this:

int i;

for ( i=0; *(myChar+i) != NULL; i++ )
free (*(myChar+i)); /* to free the (char*) inside
free(myChar);

well, on execution the system says 'junk pointer, too low to
make sense' (and the pointers aren't freed)

of course, when i write (*(myChar+i) = NULL), it's all okay
(except the memory hasn't been freed ?!)

so how should i free the (char**) to make it work, and also
be clean ?

You should post complete compilable programs, cut to the minimum
from the original, for questions like this. Your description is
missing the essentials, i.e. how was myChar declared and
initialized, and how was *myChar initialized.

 

[Mike Wahler]

Hello all,


I have a char **, which points to a number of strings (char*).
When comes the moment to free it, i have to free all the (char*) inside.
Yet when i do this:

int i;

for ( i=0; *(myChar+i) != NULL; i++ )
free (*(myChar+i)); /* to free the (char*) inside
free(myChar);


well, on execution the system says 'junk pointer, too low to make sense'
(and the pointers aren't freed)

of course, when i write (*(myChar+i) = NULL), it's all okay
(except the memory hasn't been freed ?!)

so how should i free the (char**) to make it work, and also be clean ?

Regardless of the types of objects, only 'free()' what memory
you 'malloc()' or 'realloc()'. Your code snipped doesn't show
whether you've done any allocations or not.

-Mike

 

[Christopher Benson-Manica]

for ( i=0; *(myChar+i) != NULL; i++ )

Nice try, but no. First of all, just because a pointer isn't NULL
doesn't mean you should attempt to free it (you may not have even
malloc()'ed it!). Secondly, you seem to be treating your char** like
a char* - i.e., that it is NULL terminated. If it were, you'd be
fine. However, unless you've made specific arrangements (i.e., myChar
points to enough space for a pointer to NULL in addition to all the
char pointers), this will almost assuredly fail. It's a much better
plan to just know how many character pointers you allocated, and then
free() exactly that many.

free (*(myChar+i)); /* to free the (char*) inside

Why not just free( myChar ); ? It looks cleaner and accomplishes
the same thing.

of course, when i write (*(myChar+i) = NULL), it's all okay
(except the memory hasn't been freed ?!)

Quite correct - setting a pointer to NULL in no way frees any memory
it might have been pointing at.

so how should i free the (char**) to make it work, and also be clean ?

Keep track of how many you have (say j):

for( i=0 ; i<j ; i++)
free( myChar );
free( myChar );

 

[gregg]

You should post complete compilable programs, cut to the minimum
from the original, for questions like this. Your description is
missing the essentials, i.e. how was myChar declared and
initialized, and how was *myChar initialized.

This is the function:

char ** resetResults (char ** res) {

int i;

if ( res != NULL ) { /* if not NULL, has to be freed */
for ( i=0; *(res+i) != NULL; i++ ) {
free(*(res+i));
}
free(res);
}
/* new allocation, shall contains one char* to NULL */
res=(char**)realloc(NULL, sizeof(char*));

if ( res != NULL ) return res;
else exit(RESULT_ALLOC_PB);
}


The purpose is to stock an indefinite number of strings (ie. char*).
The end is marked with NULL pointer (a bit like **argv).

Now comes a moment when a new search is to be made: thus the strings
contained in char** must be freed, and a new char** containing one NULL
pointer (meaning it is empty) has to be created.

That's my point: i want to free all strings, then the char** which
contained them. and then create a fresh new one.
(To keep memory clean, i don't want to use *(res+i)= NULL, which works,
but does not free memory !)

 

[Eric Sosman]

This is the function:

char ** resetResults (char ** res) {

int i;

if ( res != NULL ) { /* if not NULL, has to be freed */
for ( i=0; *(res+i) != NULL; i++ ) {
free(*(res+i));
}
free(res);
}
/* new allocation, shall contains one char* to NULL */
res=(char**)realloc(NULL, sizeof(char*));

Add: *res = NULL;

 

[Eric Sosman]

Add: *res = NULL;

Ah, yes, well, what I *really* meant, of COURSE,
was add the assignment after checking that `res' itself
is non-NULL. (Harrumph.) While you're at it, replace
the peculiar realloc() with the simpler malloc() and
clean things up a trifle:

/* new allocation, shall contains one char* to NULL */
res = malloc(sizeof *res);
if (res != NULL) {
*res = NULL;
return res;
}
exit (RESULT_ALLOC_PB);

 

[CBFalconer]

This is the function:

char ** resetResults (char ** res) {

int i;

if ( res != NULL ) { /* if not NULL, has to be freed */
for ( i=0; *(res+i) != NULL; i++ ) {
free(*(res+i));
}
free(res);
}
/* new allocation, shall contains one char* to NULL */
res=(char**)realloc(NULL, sizeof(char*));

if ( res != NULL ) return res;
else exit(RESULT_ALLOC_PB);
}

The purpose is to stock an indefinite number of strings (ie. char*).
The end is marked with NULL pointer (a bit like **argv).

That is still not a complete compilable program. I, and many
others, will not bother to look at it. Dan and Mike have given
you good advice.