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[Bug c/65712] New: pthread_self prints wrong result when used with ucontext
- From: "albertnetymk at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Thu, 09 Apr 2015 13:21:59 +0000
- Subject: [Bug c/65712] New: pthread_self prints wrong result when used with ucontext
- Auto-submitted: auto-generated
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65712
Bug ID: 65712
Summary: pthread_self prints wrong result when used with
ucontext
Product: gcc
Version: 4.9.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: albertnetymk at gmail dot com
When using no opitimization, the output is expected:
gcc test.c -pthread && ./a.out
B is 139778852222720
A is 139778843830016
A 139778843830016
B 139778852222720
However, once I turn on the opitmization, the output becomes:
gcc -O1 test.c -pthread && ./a.out
B is 140067829094144
A is 140067820701440
A 140067820701440
B 140067820701440
Inspect the assembly emitted, it seems that the two calls to `pthread_self` is
reduced to be one.
OS:
Linux
Program:
#define _XOPEN_SOURCE 800
#include <ucontext.h>
#include <signal.h>
#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <assert.h>
static int flag = 0;
void swap(ucontext_t *old, ucontext_t *new)
{
int ret = swapcontext(old, new);
assert(ret == 0);
}
#define SSIZE MINSIGSTKSZ
static char stack[SSIZE];
static ucontext_t a_ctx[2];
static ucontext_t b_ctx[2];
static void f1 (void)
{
printf("A %lu\n", pthread_self());
swap(&a_ctx[1], &a_ctx[0]);
printf("B %lu\n", pthread_self());
}
void *thread_a(void *arg)
{
printf("A is %lu\n", pthread_self());
ucontext_t ctx = a_ctx[1];
getcontext(&ctx);
ctx.uc_stack.ss_sp = stack;
ctx.uc_stack.ss_size = sizeof stack;
makecontext(&ctx, f1, 0);
swap(&a_ctx[0], &ctx);
__atomic_store_n(&flag, 1, __ATOMIC_RELAXED);
sleep(1);
return NULL;
}
void *thread_b(void *arg)
{
printf("B is %lu\n", pthread_self());
while(__atomic_load_n(&flag, __ATOMIC_RELAXED) == 0) ;
swap(&b_ctx[0], &a_ctx[1]);
return NULL;
}
int main(int argc, char **argv)
{
pthread_t a, b;
pthread_create(&b, NULL, &thread_b, NULL);
pthread_create(&a, NULL, &thread_a, NULL);
pthread_exit(NULL);
}