函数shmdt()有什么问题？

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);
}

上面是在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

正如您所看到的，除了shmdt()，一切都进行得很好。为什么会失败？

进一步：

#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);
}

按照您的回答，我更改了上面的代码。但现在我有了新的错误！

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

我似乎认为strcpy会导致错误。但是为什么呢？

Answer 1

这就是问题所在：

shmadd="Hello";

这会将shmadd指针更改为指向内存中的字符串。我认为您打算将字符串复制到共享内存中。要做到这一点，你可以：

strcpy(shmadd,"Hello");

还请注意，您的错误检查是错误的，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)