How far down can I use this pointer? - c

Question

Assuming if (websValidateUrl(wp, path) < 0) is true in function below:
int websDefaultHandler(webs_t wp, char_t *urlPrefix, char_t *webDir, int arg,
char_t *url, char_t *path, char_t *query)
{
websStatType sbuf;
char_t *lpath, *tmp, *date;
int bytes, flags, nchars;
a_assert(websValid(wp));
a_assert(url && *url);
a_assert(path);
a_assert(query);
/*
* Validate the URL and ensure that ".."s don't give access to unwanted files
*/
flags = websGetRequestFlags(wp);
if (websValidateUrl(wp, path) < 0)
{
websError(wp, 500, T("Invalid URL %s"), url); //points to valid string "/index.html"
return 1;
}
}
url is then passed into here, where fmt is iterated with va_start():
void websError(webs_t wp, int code, char_t *fmt, ...)
{
va_list args;
char_t *msg, *userMsg, *buf;
char_t* safeUrl = NULL;
char_t* safeMsg = NULL;
#ifdef qRichErrorPage
static int reEntry = 0;
int errorOk;
#endif
a_assert(websValid(wp));
a_assert(fmt);
websStats.errors++;
/* remove any dangerous characters in the url, and replace the string in the
* wp structure. The webs_t cleanup code will free this memory for us.
*/
safeUrl = websSafeUrl(wp->url);
bfreeSafe(B_L, wp->url);
wp->url = safeUrl;
va_start(args, fmt); //AT this point args is a bad pointer??
userMsg = NULL;
fmtValloc(&userMsg, WEBS_BUFSIZE, fmt, args);
va_end(args);
.
.
.
By the time we get to va_start(args, fmt); args contains some weird characters, not the "/index.html" I was expecting.
This pointer was allocated at the level higher than the first function but shouldn't it still be there as it is valid in websDefaultHandler?
In general what is the best practice for doing something like this? Do I need to allocate memoery again for it in websDefaultHandler before passing it to websError()?
Any help is appreciated.

Answer 1

Once you initialize args with va_start, you should be using va_arg to retrieve the actual values.

Answer 2

(Already said in comments, but pulling out into an actual answer since it seems to be the issue here:)
If it happens that the url being passed into websError is the same string (meaning, the same piece of memory) as wp->url, then that call to bfreeSafe is freeing it before fmtValloc tries to use it, in which case it's no surprise if fmtValloc sees something garbled.