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Re: [libvirt] [PATCH] libxl: Check for regcomp failure

On 09/04/2013 02:03 PM, Jim Fehlig wrote:
>> Yeah, good question.  I found a few occurrences of regcomp() and friends
>> throughout the sources and most seem to do regfree() even when regcomp()
>> fails.  The man page is not very clear, but the notes on regfree()
>> suggest it is not necessary
>>   POSIX Pattern Buffer Freeing
>>     Supplying regfree() with a precompiled pattern buffer, preg will
>>     free the memory allocated to the pattern buffer by the compiling
>>     process, regcomp().
>> But does the pattern buffer contain any allocated memory when regcomp()
>> fails?  The notes on regcomp() are not clear about this.

Thankfully, we can read the source :)

In glibc, regcomp assigns into preg, but is careful to undo any
allocation on failure; it is also careful to make regfree() a no-op on
an already-freed buffer (whether by calling regfree() twice in a row, or
using it on preg after a failed regcomp).  Gnulib copies this behavior.
 But it is not universally standard:

> The System Interfaces volume of POSIX.1-2008 [1] says this about
> regcomp() return value
> Upon successful completion, the regcomp() function shall return 0.
> Otherwise, it shall return an integer value indicating an error as
> described in /<regex.h>/
> <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/regex.h.html>,
> and the content of preg is undefined. If a code is returned, the
> interpretation shall be as given in /<regex.h>/
> <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/regex.h.html>.
> I don't think we want to call regfree() on an undefined preg right?

Correct - regfree() is only needed on successful regcomp().  We can
probably get away with calling regfree() even on failure because we use
gnulib, but that's not a good reason, so it wouldn't hurt to audit the
code and guarantee a free only on success.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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