shm.c

中科院徐志伟老师一书《操作系统 原理·技术与编程》的源代码和习题接

#include "shm.h"

int shmid = -1;             /* Shared Memory IPC ID */
int semid = -1;             /* Table locking semaphore */
char *shmp = 0;             /* Pointer to shared memory */
int I = 0;                  /* 0=starter/ 1=second */
int you = 0;                /* 1=second / 0=starter */

struct S_MEM * table;

int main(int argc,char **argv){
    union semun semarg;         /* for semctl() */
    unsigned short seminit[] = { 1, 0 };/* Initial sem values */
    pid_t p1, p2;               /* PID for worker 1 & 2 */
    char wbuf[256];         /* For fgets() */
    //char rbuf[256];             

if(argc == 1){
    atexit(release);
    shmid = shmget(IPC_PRIVATE,sizeof(struct S_MEM),0666); /* Create Shared Memory */
    if ( shmid == -1 ) {
            perror("shmget()");
            exit(1);
    }
    shmp = shmat(shmid,0,0);     /* Attach the shared memory */
    if ( shmp == (char *)(-1) ) {
        perror("shmat()");
        exit(1);
    }
    table = (struct S_MEM *)shmp;

    semid = semget(IPC_PRIVATE,2,0666); /* Create a binary semaphore set*/
    if ( semid == -1 ) {
        perror("semget()");
        exit(1);
    }
    
    semarg.array = seminit;        /* Initialize semaphores  */
    if ( semctl(semid,0,SETALL,semarg) == -1 ) {
        perror("semctl(SETALL)");
        exit(1);
    }
    
    I = 0;
    you = 1;
/* Initialize shared memory table : */
    LOCK;                       /* Wait on semaphore */
    table->semid = semid;       /* Make IPC ID public */
    table->worker[0] = getpid();
    table->worker[1] = 0;   /* No second yet */
/* write into shared memory */
    fputs("Write a character string into shared memory: ",stdout);
    fgets(wbuf,sizeof(wbuf),stdin);
    strncpy(table->buf,wbuf,strlen(wbuf)+1);
    UNLOCK;
    printf("\n*** Shared memory IPC ID is # %d ***\n",shmid);
    puts("Waiting for second...");
    WAIT;
    puts("\nTHE communication BEGINS!\n");
    WAIT;
}else { /* Second is joining a game : */
    I = 1;                 /* We're worker[1] */
    you = 0;               /* They're worker[0] */

    shmid = atoi(argv[1]);  /* Simple int conversion */

    shmp = shmat(shmid,0,0);     /* Attach the shared memory */
    if ( shmp == (char *)(-1) ) {
          perror("shmat()");
          exit(1);
    }
    table = (struct S_MEM *)shmp;

    semid = table->semid;   /* Locking semaphore ID */

    LOCK;                   /* Wait on semaphore */
    p1 = table->worker[0];
    p2 = table->worker[1];
    if ( p2 == 0 )          /* No opponent yet? */
        p2 = table->worker[1] = getpid();
    UNLOCK;
/* Notify semaphore */
    NOTIFY;
    LOCK;
/* read from shared memory */
    printf("Read the character string from shared memory: %s\n", table->buf);
    UNLOCK;                 
    if (shmdt(shmp) == -1 )
    {
       perror("shmdt()");
       exit(1);
    }
    puts("Operation is over!");
    NOTIFY;
    return 0;
 }
}