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richard.greco
.novellino.

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Esercizio Sleep-Sort con semafori

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/shm.h>
#include <semaphore.h>
#include <sys/wait.h>
#include <sys/stat.h>

int main(){
int ID = shmget(IPC_PRIVATE, sizeof(sem_t), S_IRUSR | S_IWUSR), i, j=0;
sem_t *mutex = (sem_t * )shmat(ID, NULL, 0);
sem_init(mutex, 0, 0);
pid_t pid;

printf("Inserire numero (terminare con 0): ");
scanf("%d", &i);
while(i!=0){
j++;
if((pid = fork())==0){
printf("\nSto aspettando pid=%d, i=%d\n", getpid(), i);
sem_wait(mutex);
sem_post(mutex);
printf("\nVado pid=%d, i=%d\n", getpid(), i);
sleep(i);
printf("%d ", i);
shmdt(mutex);
return 0;
}
printf("Inserire numero (terminare con 0): ");
scanf("%d", &i);
}

printf("\npadre?");
sem_post(mutex);

for(i=0; i<j; i++)
wait(NULL);
printf("\n");
sem_destroy(mutex);
shmdt(mutex);
shmctl(ID, IPC_RMID, NULL);
return 0;
}

Non capisco come mai non funziona, qualcuno saprebbe aiutarmi? in sostanza il processo padre non esce dal while e non raggiunge mai il "sem_post", che sbloccherebbe tutti i figli. Non raggiunge neanche la printf addirittura.

09-05-2015 18:08
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spikey
.amico.

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Ciao,

dunque l'istruzione incriminata è questa inizializzazione:

sem_init(mutex, 0, 0);

se leggi la man page di sem_init guarda cosa ti dice sul secondo parametro:

$ man sem_init

int sem_init(sem_t *sem, int pshared, unsigned int value);

The pshared argument indicates whether this semaphore is to be shared between the threads of a process, or between processes.

If pshared has the value 0, then the semaphore is shared between the threads of a process, and should be located at some address that is visible to all threads (e.g., a global variable, or a variable allocated dynamically on the heap).

If pshared is nonzero, then the semaphore is shared between processes, and should be located in a region of shared memory (see shm_open(3), mmap(2), and shmget(2)). (Since a child created by fork(2) inherits its parent's memory mappings, it can also access the semaphore.) Any process that can access the shared memory region can operate on the semaphore using sem_post(3), sem_wait(3), etc.

Per cui nel tuo caso, usando una shared memory tra processi, il secondo parametro pshared non va inizializzato a 0 bensì ad un valore != 0. Difatti trasformando la tua istruzione incriminata in:

sem_init(mutex, 1, 0);

tutto funziona alla perfezione! :)

Ti do alcuni consigli per la vita:

1) Prima di usare qualsiasi library call, system call, ecc... guarda sempre la relativa man page
2) Studiati questi tool: ltrace e strace

Ciao

Last edited by spikey on 12-05-2015 at 13:22

12-05-2015 13:03
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