c - Semget 和 fork() : no such file or directory

Question

我在子进程中访问信号量时遇到问题。在子进程中时，我无法通过 semget() 获得已创建的信号量。

这是 make_semaphores() 函数；

int make_semaphores(key_t key)
{ 
int semid;
semid = semget(key, 3, IPC_CREAT|/*IPC_EXCL|*/0666);
if(semid == -1)
{
    perror("Creating an array of semaphores");
    exit(1);
}
if(semctl(semid,0,SETVAL, (int)MAX)==-1)
{
    perror("Initializing 'empty' semaphore");
    exit(1);
}
if(semctl(semid,1,SETVAL, (int)0)==-1)
{
    perror("Initializing 'full' semaphore");
    exit(1);
}
if(semctl(semid,2,SETVAL, (int)1)==-1)
{
    perror("Initializing mutex");
    exit(1);
}
return semid;
}

这是 allocate_memory 函数:

int* allocate_memory(int *buf, key_t key)
{
int shmid;
shmid=shmget(key,(MAX+1)*sizeof(int),IPC_CREAT|/*IPC_EXCL|*/0666);
if(shmid==-1)
{
    perror("Creating shared memory segment");
    exit(1);
}
buf=(int*)shmat(shmid,NULL,0);
if(buf==NULL)
{
    perror("Including shared memory segment");
    exit(1);
}
buf[0]=0;
return buf;
}

我进行了搜索，但没有找到满意的答案，如果我不必要地发送垃圾邮件，那么很抱歉。预先感谢您提供的任何帮助。

这是完整的主要内容:

 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/types.h>
 #include <sys/ipc.h>
 #include <sys/sem.h>
 #include <sys/shm.h>
 #include <sys/wait.h>

#define S1KEY 467221
#define S2KEY 379231
#define S3KEY 217411
#define MAX 10



int main(int argc, char* argv[])
{
srand(time(NULL));
int fork_id;
int *buf1, *buf2, *buf3;
int semid1, semid2, semid3, i, j, r, smd1;
int prod_info[3]={0,0,0}, cons_info[3]={0,0,0};

printf("-------------------\n");
buf1=allocate_memory(buf1,S1KEY);
buf2=allocate_memory(buf2,S2KEY);
buf3=allocate_memory(buf3,S3KEY);
printf("-------------------\n");
semid1=make_semaphores(S1KEY);
semid2=make_semaphores(S2KEY);
semid3=make_semaphores(S3KEY);

smd1=semget(S1KEY,3,0666);//here is okay
if(semid1!=smd1)
{
    printf("semid1: %d              smd1: %d\n",semid1,smd1);
    perror("S1KEY does not exist");
}
printf("----------------\n");
for(i=0;i<4;++i)
{
    fork_id=fork();
    if(fork_id<0)
    {
        perror("fork()");
        exit(1);
    }
    else if(fork_id==0)// one of four children
    {
        smd1=semget(S1KEY,3,0666);//here is NOT okay, smd returns -1
            if(smd1==-1)
            {
                printf("[%d]semid1: %d              smd1: %d\n",i,semid1,smd1);
                perror("S1KEY does not exist");
            }
        if(i==3) //producer for 3 buffers
        {
            for(j=0;j<100;j++)
            {
                r=(rand()%3)+1;
                printf("-------------------------\n");
                if(r==1)
                {
                    printf("Prod:1||%d||%d\n",prod_info[0],getval_semaphores(semid1,0));
                    printf("        %d\n", buf1[0]);
                    produce(buf1,semid1,i);
                    prod_info[0]++;
                }
                else if(r==2)
                {
                    printf("Prod:2||%d||%d\n",prod_info[1],getval_semaphores(semid2,0));
                    printf("        %d\n", buf2[0]);
                    produce(buf2,semid2,i);
                    prod_info[1]++;
                }
                else
                {
                    printf("Prod:3||%d||%d\n",prod_info[2],getval_semaphores(semid3,0));
                    printf("        %d\n", buf3[0]);
                    produce(buf3,semid3,i);
                    prod_info[2]++;
                }
            }
            printf("Produced: 1:%d 2:%d 3:%d\n", prod_info[0],prod_info[1],prod_info[2]);
            exit(1);
        }
        else if(i==2) //3 consumers for 3 buffers (1,2,3)
        {
                for(j=0;j<100;j++)
                {
                    //usleep(1);
                    printf("Cons:3\n");
                    printf("%d\n", buf1[0]);
                    printf("%d\n", buf2[0]);
                    printf("%d\n", buf3[0]);
                    cons_info[0]=consume(buf1,semid1);
                    cons_info[1]=consume(buf2,semid2);
                    cons_info[2]=consume(buf3,semid3);
                    printf("Buf1: %d\n",cons_info[0]);
                    printf("Buf2: %d\n",cons_info[1]);
                    printf("Buf3: %d\n",cons_info[2]);
                }
                exit(1);
        }
        else if(i==1) //2 consumers for 2 buffers (2,3)
        {
                for(j=0;j<100;j++)
                {
                    //usleep(1);
                    printf("Cons:2\n");
                    printf("%d\n", buf2[0]);
                    printf("%d\n", buf3[0]);
                    cons_info[1]=consume(buf2,semid2);
                    cons_info[2]=consume(buf3,semid3);
                    printf("Buf2: %d\n",cons_info[1]);
                    printf("Buf3: %d\n",cons_info[2]);
                }
                exit(1);
        }
        else if(i==0) //1 consumer for 1 buffer (3)
        {
                for(j=0;j<100;j++)
                {
                    //usleep(1);
                    printf("Cons:1\n");
                    printf("%d\n", buf3[0]);
                    cons_info[2]=consume(buf3,semid3);
                    printf("Buf3: %d\n",cons_info[2]);
                }
                exit(1);
        }
    }
    else
    {
        if(i==4) //Parent, waits after making children
        {
            smd1=semget(S1KEY,3,0666);
            if(semid1!=smd1)
            {
                printf("[%d]semid1: %d              smd1: %d\n",i,semid1,smd1);
                perror("S1KEY does not exist");
            }
            printf("Parent\n");
            wait(NULL);
        }
    }
}
break_semaphores(semid1);
break_semaphores(semid2);
break_semaphores(semid3);
return 0;
}

Answer 1

恐怕您误解了进程创建的工作原理。

父/子流程模型

流程创建模型一般如下

pid_t p1 = fork(); 

if (p1 < 0) {
    perror("problem forking"); 
}
else if (p1 == 0) {
    // parent process
    printf("parent process\n"); 
    // do_parent_process_stuff()
}
else {
    // child process
    printf("child process: %d\n", p1); 
    // do_child_process_stuff()
}

更正您的代码

至于你的代码，在你的主函数中你需要替换

for(i=0;i<4;++i)//parent creates four children
{
    fork_id=fork();

    smd1=semget(S1KEY,3,0666);//here is NOT okay (when in child process), smd returns -1
    if(smd1==-1)
    {
        printf("[%d]semid1: %d              smd1: %d\n",i,semid1,smd1);
        perror("S1KEY does not exist");
    }
    //the rest

像这样的东西

/* Start children. */
for (i = 0; i < n; ++i) {
    if ((pids[i] = fork()) < 0) {
        perror("error forking");

    }
    // parent
    else if (pids[i] == 0) {
        exit(0);
    }
    // child process
    else {
        smd1=semget(S1KEY,3,0666);
        if(smd1==-1)
        {
            printf("[%d]semid1: %d              smd1: %d\n",i,semid1,smd1);
            perror("S1KEY does not exist");
        }
        else {
            // do stuff with semaphores
        }
    }
}