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In relation to the rule "Never use something you free()d", I have the following question:
Is this allowed?
Or should I set foo to NULL before malloc()ing it again?Code:
char* foo;
foo = malloc(2);//2 bytes
<snip, do whatever with foo>
free(foo);
foo = malloc(10);
No. malloc will assign a new value anyway ( NULL if it fails ).Quote:
Originally Posted by Silfer
Kurt
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.Quzah.Code:foo = malloc( 10 ); /* foo is NULL on malloc failing... */
I am not sure I understand your second point. Do you meanQuote:
Originally Posted by quzah
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 = bar;//Making it point elsewhere
Pointing foo to bar in this example is fine because bar hasn't been free'ed yet. But once you free bar (or foo a second time) don't use the other one that now free'ed memory.
KurtCode: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
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.