Many C code freeing pointers calls:
if (p)
free(p);
But why? I thought C standard say the free function doesn't do anything given a NULL pointer. So why another explicit check?
views:
163answers:
6
+7
A:
As I understand, the no-op on NULL was not always there.
In the bad old days of C (back around 1986, on a pre-ANSI standard cc compiler) free(NULL) would dump core. So most devs tested for NULL/0 before calling free.
The world has come a long way, and it appears that we don't need to do the test anymore. But old habits die hard;)
http://discuss.joelonsoftware.com/default.asp?design.4.194233.15
Tom
2009-12-16 04:42:26
+1 This is what I remember but I couldn't find the reference to it.
Matt Davis
2009-12-16 04:45:36
As I understand it, this flaw existed in some compilers even after standardization.
Michael Burr
2009-12-16 07:45:50
It was always there, even in the original UNIX implementation. This is mentioned in P.J. Plauger's article on the standardisation process - can't find exact reference at this moment though.
anon
2009-12-16 11:45:50
THe day that Joel is quoted as a technical authority is a sad one for the human race.
anon
2009-12-16 11:48:11
Neil: this is quote from the forum on joel website, actual quote not from him but someone named Guidii. so i think human race has been saved today
zaharpopov
2009-12-16 13:10:14
A:
http://www.owasp.org/index.php/Double_Free ?
as far as I understand it, calling free() more than once on the same pointer (without that pointer being set to NULL/0) can lead to interesting stuff.
just somebody
2009-12-16 04:47:02
+3
A:
I tend to write "if (p) free(p)" a lot, even if I know it's not needed.
I partially blame myself because I learned C the old days when free(NULL) would segfault and I still feel uncomfortable not doing it.
But I also blame the C standard for not being consistent. Would, for example, fclose(NULL) be well defined, I would not have problems in writing:
free(p);
fclose(f);
Which is something that happens very often when cleaning up things. Unfortunately, it seems strange to me to write
free(p);
if (f) fclose(f);
and I end up with
if (p) free(p);
if (f) fclose(f);
I know, it's not a rational reason but that's my case :)
Remo.D
2009-12-16 11:41:24
A:
If you rely on that free(0) is OKAY, and it's normal for your pointer to be null at this point, please say so in comment // may be NULL
This may be merely self-explanatory code, saying yes I know, I also use p as a flag.
Pavel Radzivilovsky
2009-12-16 11:46:42
+2
A:
The construct:
free(NULL);
has always been OK in C, back to the original UNIX compiler written by Dennis Ritchie. Pre-standardisation, some poor compilers might not have fielded it correctly, but these days any compiler that does not cannot legitimately call itself a compiler for the C language. Using it typically leads to clearer, more maintainable code.
anon
2009-12-16 12:01:59
A:
there can be a custom implementation of free() in mobile environment. In that case free(0) can cause a problem. (yeah, bad implementation)
sevity
2009-12-16 12:31:23