Simple question. I have a char pointer in a struct. When I try to free it, I get the following error at run time:
a.out(1186) malloc: *** error for object 0x1ffc: Non-aligned pointer being freed
*** set a breakpoint in malloc_error_break to debug
Why?
Code:typedef struct foo {
char *str;
} foo;
void
foo_free(foo *f) {
free(f->str);
}
int
main(int argc, char **argv) {
foo *f = malloc(sizeof(foo));
f->str = "foo";
foo_free(f);
return 0;
}
you should only free pointer returned by malloc, you cannot free pointer just because you like to write free here and there in the code
f->str was not allocated with malloc - so you are not allowed to free it
Plus string literals should be const: http://cpwiki.sourceforge.net/Common...kes_and_errors
Who says that the only thing which will ever be assigned to "str" is a literal?Quote:
Originally Posted by Elysia
Change it accordingly. If you're going to assign a string literal, make it const.
If you're going to use strcpy, then don't. The point is that the lesson needs to be taught. And often when someone assigns a string literals to non-const, they don't know what they're doing.
Perhaps the faq should be updated with more information about string literals, though.
So this code is invalid according to you (not complete):
Yes, I'm sure it CAN be rewritten to use const wherever needed, but I can certainly see how you would want to do similar work in a function SOMETIMES.Code:int main()
{
char *res = "Unknown";
char buffer[100];
int somestuff = somefunc();
switch(somestuff)
{
case 1:
case 2:
sprintf(buffer, "Result = %d", somestuff);
res = buffer;
if ((somestuff = someotherfunc()))
strcat(res, " extra stuff");
break;
default:
// ...
}
printf("Res=%s\n", res);
return 0;
}
--
Mats
As I said, the point was more to learn about string literals.
The point was that usually, when using char* to store a string literal, most newbies lack understanding of string literals and how they work. Yes, it's valid, but that doesn't mean you should use it, unless you find a good reason for doing it.
I'm trying to get them a good idea of what string literals are, so they do not commit typical mistakes.
But surely, the issue here is freeing something that didn't come from malloc, not about const.
--
Mats
Yes, yes, absolutely. That is the error, but a little lesson about string literals doesn't hurt either :)
I also understand it as that the OP is not understanding string literals correctly and therefore tried to free it.
I'd say the OP doesn't understand memory management. String literals is just one of many circumstances when there is a poitner to non-malloc'ed memory.Quote:
Originally Posted by Elysia
--
Mats
That too. String literals is just one part of the whole package.