Question

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

#define BUFSZ 2048

int main()
{
    int shmid,i,fd,nwrite,nread;
    char *shmadd;
    char buf[5];
    buf[5] = '\0';

    if((shmid=shmget(IPC_PRIVATE,BUFSZ,0x666))<0)
    {
        perror("shmget");
        exit(1);
    }
    else
        printf("created shared-memory: %d\n",shmid);

    if((shmadd=shmat(shmid,0,0))<(char *)0)
    {
        perror("shmat");
        exit(1);
    }
    else
        printf("attached shared-memory\n");

    shmadd="Hello";

    if((fd = open("share",O_CREAT | O_RDWR,0666))<0)
    {
        perror("open");
        exit(1);
    }
    else
        printf("open success!\n");

    if((nwrite=write(fd,shmadd,5))<0)
    {
        perror("write");
        exit(1);
    }
    else
        printf("write success!\n");

    lseek( fd, 0, SEEK_SET );

    if((nread=read(fd,buf,5))<0)
    {
        perror("read");
        exit(1);
    }
    else
        printf("read %d form file:%s\n",nread,buf);

    if(close(fd) == -1)
        printf("close fd fails!\n");
    else
        printf("close fd succeeds!\n");

    if((shmdt(shmadd))<0)
    {
        perror("shmdt");
        exit(1);
    }
    else
        printf("deleted shared-memory\n");

    exit(0);
}

以上是在Linux中演示共享内存的代码。运行结果如下：

$ ./ex2
created shared-memory: 1572887
attached shared-memory
open success!
write success!
read 5 form file:Hello
close fd succeeds!
shmdt: Invalid argument

正如你所看到的，除了shmdt（）之外，一切都很顺利。为什么失败？

的此外： 的

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <string.h>

#define BUFSZ 2048

int main()
{
    int shmid,i,fd,nwrite,nread;
    char *shmadd;
    char buf[5];
    buf[5] = '\0';

    if((shmid=shmget(IPC_PRIVATE,BUFSZ,0x666)) < 0)
    {
        perror("shmget");
        exit(1);
    }
    else
        printf("created shared-memory: %d\n",shmid);

    if((shmadd=shmat(shmid,0,0)) < (char *)0)
    {
        perror("shmat");
        exit(1);
    }
    else
        printf("attached shared-memory\n");

    strcpy(shmadd, "Hello");

    if((fd = open("share",O_CREAT | O_RDWR,0666)) < 0)
    {
        perror("open");
        exit(1);
    }
    else
        printf("open success!\n");

    if((nwrite=write(fd,shmadd,5)) < 0)
    {
        perror("write");
        exit(1);
    }
    else
        printf("write success!\n");

    lseek( fd, 0, SEEK_SET );

    if((nread=read(fd,buf,5)) < 0)
    {
        perror("read");
        exit(1);
    }
    else
        printf("read %d form file:%s\n",nread,buf);

    if(close(fd) == -1)
        printf("close fd fails!\n");
    else
        printf("close fd succeeds!\n");

    if((shmdt(shmadd))<0)
    {
        perror("shmdt");
        exit(1);
    }
    else
        printf("deleted shared-memory\n");

    exit(0);
}

根据你的回答，我改变了上面的代码。但是现在我遇到了新的错误！

$ ./ex2
created shared-memory: 2129948
attached shared-memory
Segmentation fault (core dumped)

我似乎strcpy会导致错误。但为什么呢？

Answer 1

这是问题所在：

shmadd="Hello";

这会将shmadd指针更改为指向内存中的字符串。我想你打算将字符串复制到共享内存中。要做到这一点，你会这样做：

strcpy(shmadd,"Hello");

另请注意，检查错误是错误的，应该是：

if((shmadd=shmat(shmid,0,0)) == (void *)-1) { ... error ... }

你的权限应该是八进制，而不是十六进制：

if((shmid=shmget(IPC_PRIVATE,BUFSZ,0666)) < 0)

Answer 2

您在此行中使用shmadd检查错误是不正确的：

if((shmadd=shmat(shmid,0,0)) < (char *)0)

您需要根据手册页对-1进行显式比较，因为对指针的-1强制转换为无符号值（32位系统上为0xFFFFFFFF）。用这一行替换该行：

if((shmadd=shmat(shmid,0,0)) == (char *)-1)

并且您将获得权限被拒绝错误，因为您不是root。

函数shmdt（）有什么问题？

2 个答案: