How does the auto-free()ing work when I use functions like mktemp()?

Greetings,

I'm using mktemp() (iPhone SDK) and this function returns a char * to the new file name wh开发者_运维百科ere all "X" are replaced by random letters.

What confuses me is the fact that the returned string is automatically free()d. How (and when) does that happen? I doubt it has something to do with the Cocoa event loop. Is it automatically freed by the kernel?

Thanks in advance!

mktemp just modifies the buffer you pass in, and returns the same poiinter you pass in, there's no extra buffer to be free'd.

That's at least how the OSX manpage describes it(I couldn't find documentation for IPhone) , and the posix manpage (although the example in the posix manpage looks to be wrong, as it pass in a pointer to a string literal - possibly an old remnant, the function is also marked as legacy - use mkstemp instead. The OSX manpage specifically mention that as being an error).

So, this is what will happen:

char template[] = "/tmp/fooXXXXXX";
char *ptr;
if((ptr = mktemp(template)) == NULL) {
   assert(ptr == template); //will be true, 
                           // mktemp just return the same pointer you pass in
}

If it's like the cygwin function of the same name, then it's returning a pointer to an internal static character buffer that will be overwritten by the next call to mktemp(). On cygwin, the mktemp man page specifically mentions _mktemp_r() and similar functions that are guaranteed reentrant and use a caller-provided buffer.