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* [Bug libc/29313] New: MIN/MAX macros can cause double evaluation of arguments
@ 2022-07-01 11:13 gkoumplias at gmail dot com
2022-07-01 12:21 ` [Bug libc/29313] " schwab@linux-m68k.org
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: gkoumplias at gmail dot com @ 2022-07-01 11:13 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29313
Bug ID: 29313
Summary: MIN/MAX macros can cause double evaluation of
arguments
Product: glibc
Version: 2.36
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: libc
Assignee: unassigned at sourceware dot org
Reporter: gkoumplias at gmail dot com
CC: drepper.fsp at gmail dot com
Target Milestone: ---
* MIN, MAX macros as defined in misc/sys/param.h
https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=misc/sys/param.h;hb=HEAD
can cause the double evaluation of one of the passed in arguments.
Currently MAX is defined as:
#define MAX(a,b) (((a)>(b))?(a):(b))
This will evaluate the maximum argument twice.
* A test case to reproduce this would go like this:
int a = 1000;
int b = 10;
MAX(a++, b++) ;
// This will fail as a will be 1002 due to double evaluation by the MAX macro
EXPECT_EQ(a, 1001)
* solution
#define max(a,b) \
({ __typeof__ (a) _a = (a); \
__typeof__ (b) _b = (b); \
_a > _b ? _a : _b; })
* A discussion (and a solution) for the matter can be found here
https://stackoverflow.com/questions/3437404/min-and-max-in-c
Thanks
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* [Bug libc/29313] MIN/MAX macros can cause double evaluation of arguments
2022-07-01 11:13 [Bug libc/29313] New: MIN/MAX macros can cause double evaluation of arguments gkoumplias at gmail dot com
@ 2022-07-01 12:21 ` schwab@linux-m68k.org
2022-07-01 12:32 ` adhemerval.zanella at linaro dot org
2022-07-01 17:00 ` goldstein.w.n at gmail dot com
2 siblings, 0 replies; 4+ messages in thread
From: schwab@linux-m68k.org @ 2022-07-01 12:21 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29313
Andreas Schwab <schwab@linux-m68k.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |WORKSFORME
--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
The non-standard header <sys/params.h> is copied directly from BSD. Don't use
it.
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* [Bug libc/29313] MIN/MAX macros can cause double evaluation of arguments
2022-07-01 11:13 [Bug libc/29313] New: MIN/MAX macros can cause double evaluation of arguments gkoumplias at gmail dot com
2022-07-01 12:21 ` [Bug libc/29313] " schwab@linux-m68k.org
@ 2022-07-01 12:32 ` adhemerval.zanella at linaro dot org
2022-07-01 17:00 ` goldstein.w.n at gmail dot com
2 siblings, 0 replies; 4+ messages in thread
From: adhemerval.zanella at linaro dot org @ 2022-07-01 12:32 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29313
Adhemerval Zanella <adhemerval.zanella at linaro dot org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |adhemerval.zanella at linaro dot o
| |rg
--- Comment #2 from Adhemerval Zanella <adhemerval.zanella at linaro dot org> ---
(In reply to Andreas Schwab from comment #1)
> The non-standard header <sys/params.h> is copied directly from BSD. Don't
> use it.
Can't we do better? We can use the __typeof__ to avoid double evaluation at
least for gcc/clang.
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* [Bug libc/29313] MIN/MAX macros can cause double evaluation of arguments
2022-07-01 11:13 [Bug libc/29313] New: MIN/MAX macros can cause double evaluation of arguments gkoumplias at gmail dot com
2022-07-01 12:21 ` [Bug libc/29313] " schwab@linux-m68k.org
2022-07-01 12:32 ` adhemerval.zanella at linaro dot org
@ 2022-07-01 17:00 ` goldstein.w.n at gmail dot com
2 siblings, 0 replies; 4+ messages in thread
From: goldstein.w.n at gmail dot com @ 2022-07-01 17:00 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=29313
Noah Goldstein <goldstein.w.n at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |goldstein.w.n at gmail dot com
--- Comment #3 from Noah Goldstein <goldstein.w.n at gmail dot com> ---
(In reply to Vasilis Gkoumplias from comment #0)
> * MIN, MAX macros as defined in misc/sys/param.h
> https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=misc/sys/param.h;
> hb=HEAD
> can cause the double evaluation of one of the passed in arguments.
> Currently MAX is defined as:
> #define MAX(a,b) (((a)>(b))?(a):(b))
> This will evaluate the maximum argument twice.
>
> * A test case to reproduce this would go like this:
> int a = 1000;
> int b = 10;
> MAX(a++, b++) ;
> // This will fail as a will be 1002 due to double evaluation by the MAX macro
> EXPECT_EQ(a, 1001)
>
> * solution
> #define max(a,b) \
> ({ __typeof__ (a) _a = (a); \
> __typeof__ (b) _b = (b); \
> _a > _b ? _a : _b; })
>
> * A discussion (and a solution) for the matter can be found here
> https://stackoverflow.com/questions/3437404/min-and-max-in-c
>
I don't think we can change the existing macros to that because
they will no longer be usable outside of functions.
#define MAX(x, y) ((x) > (y) ? (x) : (y))
#define max(a, b)
\
({
\
__typeof__(a) _a = (a);
\
__typeof__(b) _b = (b);
\
_a > _b ? _a : _b;
\
})
enum { A = 1, B = 2 };
// Okay
enum { C = MAX(A, B) };
// Compile error
enum { D = max(A, B) };
>
> Thanks
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2022-07-01 11:13 [Bug libc/29313] New: MIN/MAX macros can cause double evaluation of arguments gkoumplias at gmail dot com
2022-07-01 12:21 ` [Bug libc/29313] " schwab@linux-m68k.org
2022-07-01 12:32 ` adhemerval.zanella at linaro dot org
2022-07-01 17:00 ` goldstein.w.n at gmail dot com
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