Thread: About free()

In relation to the rule "Never use something you free()d", I have the following question:

Is this allowed?

Code:

char* foo;
foo = malloc(2);//2 bytes

<snip, do whatever with foo>

free(foo);
foo = malloc(10);

Or should I set foo to NULL before malloc()ing it again?

You're fine in that assignment. The only thing you would have a problem with is if you try to dereference 'foo' before you malloc some new space, or make it point elsewhere. Of course you'll always want to test to make sure your allocation didn't fail. If it did, your pointer will be set to NULL.

Code:

foo = malloc( 10 ); /* foo is NULL on malloc failing... */

Quzah.

Originally Posted by quzah

The only thing you would have a problem with is if you try to dereference 'foo' before you malloc some new space, or make it point elsewhere.
Quzah.

I am not sure I understand your second point. Do you mean

Code:

char* foo;
char* bar;

foo = malloc(2);
bar = malloc(2);

free(foo);

foo = bar;//Making it point elsewhere

is not correct? Isn't it just like saying foo = NULL;?

Code:

char* foo;
char* bar;

foo = malloc(2);
bar = malloc(2);

free(foo);

foo[0]='\0';  // this is wrong
foo = bar;//Making it point elsewhere
foo[0]='\0';  // safe again

Kurt

Edit:
I think there is nothing mysterious about the use of pointers. I think that it is obvoious that you must not do what I have marked as wrong in the example above.
Trouble is that you cannot do much to prevent such mistakes. ( other than being careful ).
Another example of what could go wrong is:

Code:

char* foo;
char* bar;

foo = malloc(2);
bar = malloc(2);

foo = bar;    // Making it point elsewhere, original foo is lost now
free(foo);

bar[0]='\0';  // wrong bar was freed via foo

I agree with you - but now, I am even wiser about it. Thank you for your help, everybody.