#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
void die(const char *msg)
{
perror(msg);
exit(1);
}
int main() {
int a=5,b=7;
key_t keyshm; // or key_t keyshm=0x200;
keyshm=ftok("/tmp",32); //see up
int buffer[1];
int *point;
int shmid;
shmid=shmget(keyshm , sizeof(buffer),0666);
if(shmid < 0) die("Couldn't shmget");
point=(int *)shmat(shmid,NULL,0);
point[0]=a;
point[1]=b;
printf("FIRST = %d\nSECOND = %d",point[0],point[1]);
fflush(stdout);
sleep(300);
shmdt(point);
exit(0);
}
Code:
$ ./shm1
Couldn't shmget: No such file or directory
$ man shmget
SHMGET(2) Linux Programmer's Manual SHMGET(2)
...
ERRORS
On failure, errno is set to one of the following:
EACCES The user does not have permission to access the shared memory
segment, and does not have the CAP_IPC_OWNER capability.
EEXIST IPC_CREAT | IPC_EXCL was specified and the segment exists.
EINVAL A new segment was to be created and size < SHMMIN or size > SHM-
MAX, or no new segment was to be created, a segment with given
key existed, but size is greater than the size of that segment.
ENFILE The system limit on the total number of open files has been
reached.
ENOENT No segment exists for the given key, and IPC_CREAT was not spec-
ified.
ENOMEM No memory could be allocated for segment overhead.
ENOSPC All possible shared memory IDs have been taken (SHMMNI), or
allocating a segment of the requested size would cause the sys-
tem to exceed the system-wide limit on shared memory (SHMALL).
EPERM The SHM_HUGETLB flag was specified, but the caller was not priv-
ileged (did not have the CAP_IPC_LOCK capability).
...
So you want 0666 | IPC_CREAT
And you should check the return values of everything in case anything you weren't expecting fails.
Did you change the sizeof? You don't even use buffer in the first program, so you can get rid of that. Then when you shmget pass (sizeof(int) * 2) as the size argument.
Also, to the third argument, shmflg, you have to OR in IPC_CREAT. So the call should be:
Code:
key_t shmkey=0x200;
int id;
..,
id = shmget(shmkey, sizeof(int) * 2, 0666 | IPC_CREAT);
Without IPC_CREAT, it's likely that shmget is returning -1 which, since you're not doing error checking, is then passed to shmat which is returning NULL. When you attempt to reference through the NULL pointer you are seg faulting.
I suggest reading the man pages for shmget!
edit: I also suggest not using 0666 and using the proper mode flags; but that's your prerogative. For example:
Did you change the sizeof? You don't even use buffer in the first program, so you can get rid of that. Then when you shmget pass (sizeof(int) * 2) as the size argument.
Also, to the third argument, shmflg, you have to OR in IPC_CREAT. So the call should be:
Code:
key_t shmkey=0x200;
int id;
..,
id = shmget(shmkey, sizeof(int) * 2, 0666 | IPC_CREAT);
Without IPC_CREAT, it's likely that shmget is returning -1 which, since you're not doing error checking, is then passed to shmat which is returning NULL. When you attempt to reference through the NULL pointer you are seg faulting.
I suggest reading the man pages for shmget!
edit: I also suggest not using 0666 and using the proper mode flags; but that's your prerogative. For example:
i've another problem, now i want to add two numbers and print result, two number are written in the first and second elements of buffer of shared memory and after call an adder program and he make sum and put it into third element of buffer so the main program prints result.
well if i have 3+5 program give 114678723
why?
this is the code for main:
Code:
#include <sys/types.h>
#include <sys/sem.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#include <stdlib.h>
#define SEMPERM 0600
typedef union _semun {
int val;
struct semid_ds *buf;
unsigned short *array;
}semun;
int initsem (key_t semkey){
int status=0, semid;
semid=semget(semkey, 1 , SEMPERM | IPC_CREAT | IPC_EXCL);
if (semid==-1){
if (errno==EEXIST){semid=semget(semkey,1,0);}
} else {
semun arg;
arg.val=0;
status=semctl(semid,0,SETVAL,arg);
}
if ((semid==-1) || (status==-1)){
perror("initsem fallita");
return (-1);
}
return (semid);
}
int waitSem(int semid){
struct sembuf wait_buf;
wait_buf.sem_num=0;
wait_buf.sem_op=1;
wait_buf.sem_flg=SEM_UNDO;
if (semop(semid,&wait_buf, 1)==-1){
perror("waitSem Fallita");
exit(-1);
}
return 0;
}
int signalSem (int semid){
struct sembuf signal_buf;
signal_buf.sem_num=0;
signal_buf.sem_op=1;
signal_buf.sem_flg=SEM_UNDO;
if (semop(semid,&signal_buf,1)==-1){
perror("signalSem fallita");
exit(1);
}
return 0;
}
int main (){
key_t chiavemem=0x100,keysem=0x050;
int sem,a,id,*point;
pid_t figlio;
int buffer[2];
sem=initsem(keysem);
if(sem<0){
perror ("creazione semaforo fallita");
exit (-1);
}
id=shmget(chiavemem,sizeof(buffer[2]),0777 |IPC_CREAT);
if (id<0){
perror("id main errato");
exit(-1);
}
point=(int *)shmat(id,NULL,0);
if (point<0){
perror ("Errato attacco main");
exit (-1);
}
waitSem(sem);
point[0]=3;
point[1]=5;
signalSem(sem);
figlio=fork();
if (figlio<0){
perror("fork fallita");
exit (-1);
}
if (figlio==0){
execvp("/home/francesco/NetBeansProjects/Esercizio1/addizione",NULL);
a=point[2];
fflush(stdout);
exit(0);
}
printf("il risultato è %d",a);
shmdt(point);
shmctl(id,IPC_RMID,0);
exit(0);
}
instead adder is:
Code:
#include <sys/types.h>
#include <sys/sem.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#define SEMPERM 0600
typedef union _semun {
int val;
struct semid_ds *buf;
unsigned short *array;
}semun;
int initsem (key_t semkey){
int status=0, semid;
semid=semget(semkey, 1 , SEMPERM | IPC_CREAT | IPC_EXCL);
if (semid==-1){
if (errno==EEXIST){semid=semget(semkey,1,0);}
} else {
semun arg;
arg.val=0;
status=semctl(semid,0,SETVAL,arg);
}
if ((semid==-1) || (status==-1)){
perror("initsem fallita");
return (-1);
}
return (semid);
}
int waitSem(int semid){
struct sembuf wait_buf;
wait_buf.sem_num=0;
wait_buf.sem_op=1;
wait_buf.sem_flg=SEM_UNDO;
if (semop(semid,&wait_buf, 1)==-1){
perror("waitSem Fallita");
exit(-1);
}
return 0;
}
int signalSem (int semid){
struct sembuf signal_buf;
signal_buf.sem_num=0;
signal_buf.sem_op=1;
signal_buf.sem_flg=SEM_UNDO;
if (semop(semid,&signal_buf,1)==-1){
perror("signalSem fallita");
exit(1);
}
return 0;
}
int main(){
int id,*point,a,b,c;
key_t keysem=0x050,chiavemem=0x100;
int sem,buffer[2];
sem=initsem(keysem);
if (sem<0){
perror("errore creazione semaforo");
exit(-1);
}
id=shmget(chiavemem,sizeof(buffer[2]),0777|IPC_CREAT|IPC_EXCL);
if(id<0){
perror("errore creazione memoria condivisa");
exit(-1);
}
point=(int *)shmat(id,NULL,0);
if (point<0){
perror("Attacco additore errato");
exit(-1);
}
waitSem(sem);
point[0]=a;
point[1]=b;
c=a+b;
point[2]=c;
signalSem(sem);
shmdt(point);
exit(0);
}
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