Is returning a pointer to a static local variable safe?

Question

I\'m working with some code that widely uses the idiom of returning a pointer to a static local variable. eg:

char* const GetString()
{
  static char sTest[         


        

Answer 1

It depends on what you mean by safe. There are a couple of problems that I can see immediately:

1. You've returned a char * const, which will allow callers to change the string at this location. Potential buffer overrun. Or did you mean a const char *?
2. You might have a problem with reentrance, or with concurrency.

To explain the second, consider this:

const char * const format_error_message(int err)
{
    static char error_message[MAXLEN_ERROR_MESSAGE];
    sprintf(error_message, "Error %#x occurred", err);
    return error_message;
}

If you call it like this:

int a = do_something();
int b = do_something_else();

if (a != 0 && b != 0)
{
    fprintf(stderr,
        "do_something failed (%s) AND do_something_else failed (%s)\n",
        format_error_message(a), format_error_message(b));
} 

...what's going to be printed?

Same for threading.