Testing shared memory ,strange thing happen
- by barfatchen
I have 2 program compiled in 4.1.2 running in RedHat 5.5 ,
It is a simple job to test shared memory , shmem1.c like following :
#define STATE_FILE "/program.shared"
#define NAMESIZE 1024
#define MAXNAMES 100
typedef struct
{
char name[MAXNAMES][NAMESIZE];
int heartbeat ;
int iFlag ;
} SHARED_VAR;
int main (void)
{
int first = 0;
int shm_fd;
static SHARED_VAR *conf;
if((shm_fd = shm_open(STATE_FILE, (O_CREAT | O_EXCL | O_RDWR),
(S_IREAD | S_IWRITE))) > 0 ) {
first = 1; /* We are the first instance */
}
else if((shm_fd = shm_open(STATE_FILE, (O_CREAT | O_RDWR),
(S_IREAD | S_IWRITE))) < 0) {
printf("Could not create shm object. %s\n", strerror(errno));
return errno;
}
if((conf = mmap(0, sizeof(SHARED_VAR), (PROT_READ | PROT_WRITE),
MAP_SHARED, shm_fd, 0)) == MAP_FAILED) {
return errno;
}
if(first) {
for(idx=0;idx< 1000000000;idx++)
{
conf->heartbeat = conf->heartbeat + 1 ;
}
}
printf("conf->heartbeat=(%d)\n",conf->heartbeat) ;
close(shm_fd);
shm_unlink(STATE_FILE);
exit(0);
}//main
And shmem2.c like following :
#define STATE_FILE "/program.shared"
#define NAMESIZE 1024
#define MAXNAMES 100
typedef struct
{
char name[MAXNAMES][NAMESIZE];
int heartbeat ;
int iFlag ;
} SHARED_VAR;
int main (void)
{
int first = 0;
int shm_fd;
static SHARED_VAR *conf;
if((shm_fd = shm_open(STATE_FILE, (O_RDWR),
(S_IREAD | S_IWRITE))) < 0) {
printf("Could not create shm object. %s\n", strerror(errno));
return errno;
}
ftruncate(shm_fd, sizeof(SHARED_VAR));
if((conf = mmap(0, sizeof(SHARED_VAR), (PROT_READ | PROT_WRITE),
MAP_SHARED, shm_fd, 0)) == MAP_FAILED) {
return errno;
}
int idx ;
for(idx=0;idx< 1000000000;idx++)
{
conf->heartbeat = conf->heartbeat + 1 ;
}
printf("conf->heartbeat=(%d)\n",conf->heartbeat) ;
close(shm_fd);
exit(0);
}
After compiled :
gcc shmem1.c -lpthread -lrt -o shmem1.exe
gcc shmem2.c -lpthread -lrt -o shmem2.exe
And Run both program almost at the same time with 2 terminal :
[test]$ ./shmem1.exe
First creation of the shm. Setting up default values
conf->heartbeat=(840825951)
[test]$ ./shmem2.exe
conf->heartbeat=(1215083817)
I feel confused !! since shmem1.c is a loop 1,000,000,000 times , how can it be
possible to have a answer like 840,825,951 ?
I run shmem1.exe and shmem2.exe this way,most of the results are conf-heartbeat
will larger than 1,000,000,000 , but seldom and randomly ,
I will see result conf-heartbeat will lesser than 1,000,000,000 ,
either in shmem1.exe or shmem2.exe !!
if run shmem1.exe only , it is always print 1,000,000,000 , my question is ,
what is the reason cause conf-heartbeat=(840825951) in shmem1.exe ?
Update: Although not sure , but I think I figure it out what is going on , If shmem1.exe run 10 times for example , then conf-heartbeat = 10 , in this time shmem1.exe take a rest and then back , shmem1.exe read from shared memory and conf-heartbeat = 8 , so shmem1.exe will continue from 8 , why conf-heartbeat = 8 ? I think it is because shmem2.exe update the shared memory data to 8 , shmem1.exe did not write 10 back to shared memory before it took a rest ....that is just my theory... i don't know how to prove it !!
