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POTesting shared memory ,strange thing happen
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copied!<p>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 :</p> <pre><code>#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 </code></pre> <p>And shmem2.c like following :</p> <pre><code>#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); } </code></pre> <p>After compiled :</p> <pre><code> gcc shmem1.c -lpthread -lrt -o shmem1.exe gcc shmem2.c -lpthread -lrt -o shmem2.exe </code></pre> <p>And Run both program almost at the same time with 2 terminal :</p> <pre><code> [test]$ ./shmem1.exe First creation of the shm. Setting up default values conf->heartbeat=(840825951) [test]$ ./shmem2.exe conf->heartbeat=(1215083817) </code></pre> <p>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 ? </p> <p>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 ,<br> either in shmem1.exe or shmem2.exe !!</p> <p>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 ?</p> <p>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 !!</p>

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