https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89899
Bug ID: 89899
Summary: g++ compiler error report
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: hos.lee at samsung dot com
Target Milestone: ---
Dear,
I have a question about g++ compiler when running the below example.
[ used compiler ]
- Welcome to Ubuntu 16.04.5 LTS (GNU/Linux 4.4.0-17134-Microsoft x86_64)
- g++ test.cpp -lpthread
There is a "test program" I checked in the below.
With next statement in test program, error is occurred.
- que[head++] = data;
However, with next statement in test program, error is not occurred.
- que[head] = data;
head++;
"que[head++] = data;" and "que[head] = data; head++;" is exactly same.
I confused why "que[head++] = data;" statement happened error only.
I also checked FAQ below and I do not find why this error is happened.
Please check this bug report and feedback to me.
Thank you very much.
[ Test Program ]
---------------------------------------------------------------------
#include <stdio.h>
#include <pthread.h>
volatile int DONE = 0;
volatile int que[65536];
volatile unsigned short head = 0;
volatile unsigned short tail = 0;
void in(int data)
{
while ((tail - head == 1) || (head - tail == 65535));
que[head++] = data;
// que[head] = data;
// head++;
}
int out(void)
{
while (head == tail);
return que[tail++];
}
void* test0(void* param)
{
for (int c = 0; c < 10000000; c++) in(c);
putchar('0'); DONE++;
}
void* test1(void* param)
{
for (int c = 0; c < 10000000; c++) if (out() != c) putchar('E');
putchar('1'); DONE++;
}
int main(void)
{
pthread_t thread1;
pthread_t thread2;
pthread_create(&thread1, NULL, test0, (void*)0);
pthread_create(&thread2, NULL, test1, (void*)0);
while (DONE < 2);
}
------------------------------------------------------------------------
[ Frequently Reported Bugs ]
Increment/decrement operator (++/--) not working as expected - a problem with
many variations.
The following expressions have unpredictable results:
x[i]=++i
foo(i,++i)
i*(++i) /* special case with foo=="operator*" */
std::cout << i << ++i /* foo(foo(std::cout,i),++i) */
since the i without increment can be evaluated before or after ++i.
The C and C++ standards have the notion of "sequence points". Everything that
happens between two sequence points happens in an unspecified order, but it has
to happen after the first and before the second sequence point. The end of a
statement and a function call are examples for sequence points, whereas
assignments and the comma between function arguments are not.
Modifying a value twice between two sequence points as shown in the following
examples is even worse:
i=++i
foo(++i,++i)
(++i)*(++i) /* special case with foo=="operator*" */
std::cout << ++i << ++i /* foo(foo(std::cout,++i),++i) */
This leads to undefined behavior (i.e. the compiler can do anything).
