c - 将结构数组初始化为共享内存

Question

我正在制作 4 个程序来创建一个 POSIX 共享内存对象，一个结构数组，它将被其他 3 个进程共享。基本上这个项目模拟文件。

程序 #1 创建对象。程序 #2 将文件名和字符串作为参数，然后将文件名和字符串(文件内容)作为结构保存到共享内存中，并将其放入数组的可用元素中。程序#3 将列出文件名。程序 #4 将搜索给定文件并显示其内容。

我遇到的问题是将一个结构数组初始化到共享内存中。我不断收到以下错误，这告诉我我使用了不正确的方法来初始化指针:

myformat.c:36: 警告:从不兼容的指针类型初始化

我搜索了这个主题并发现了一些类似的问题，但与我的问题无关。

那么，如何正确地将结构数组初始化到共享内存中？

根据我的研究，我编写了以下代码。谢谢!

程序#1(myformat.c):

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

struct MyFiles
{    
    char *fileName;    
    char *fileContents;    
};

int main()    
{    
    /* the size of shared memory object */    
    int size = sizeof(struct MyFiles)* 20;

    /* name of the shared memory object */    
    const char *name = "/PROJ4_SHARED_MEM";     

    /* shared memory file descriptor */    
    int shm_fd;

    /* pointer to shared memory obect */    
    void *ptr;

    /* create the shared memory object */    
    shm_fd = shm_open(name, O_CREAT | O_RDRW, 0666);    

    /* configure the size of the shared memory object */    
    ftruncate(shm_fd, size);    

    /* memory map the shared memory object */    
    ptr = mmap(0, size, PROT_WRITE, MAP_SHARED, shm_fd, 0);    
    struct MyFiles* file = (struct MyStruct*)ptr;           

    /* save struct array to the shared memory object. Initialize first element. */    
    file[0]->fileName = "\0";    
    file[0]->fileContents = "\0";       

    return 0;

}

程序#2(mycreate.c):

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

struct MyFiles
{
    char *fileName;
    char *fileContents;
};

int main()
{
    char *file_name = argv(0);
    char *file_contents = argv(1);

    /* the size of shared memory object */
    int size = sizeof(struct MyFiles)* 20;

    /* name  of  the  shared  memory  object */
    const  char  *name = "/PROJ4_SHARED_MEM";

    /* shared  memory  file descriptor */
    int shm_fd;

    /* pointer to  shared  memory  object  */
    void  *ptr;

    /* open the  shared  memory  object */
    shm_fd = shm_open(name, O_RDRW, 0666);

    /* memory map the shared memory object */
    ptr = mmap(0, size, PROT_WRITE, MAP_SHARED, shm_fd, 0);
    struct MyFiles* file = (struct MyStruct*)ptr;

    /*write to first available array slot in shared  memory  object.  Initialize next. */
    for (int i = 0; i < 20; i++)
    {
        if (file[i].fileName == "\0")
        {
            sprintf(file[i]->fileName,"%s",file_name);
            sprintf(file[i]->fileContents,"%s",file_contents);
            file[i + 1]->fileName = "\0";
            file[i + 1]->fileContents = "\0";
            break;
        }
        else if (i == 19)
        {
            prinf("ERROR: The Shared Memory Object is full.\n\n");
            shm unlink(name);
            exit(1);
        }
    }

    /* remove the  shared  memory  object */
    shm unlink(name);

    return 0;
}

程序#3(myls.c):

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

struct MyFiles
{
    char *fileName;
    char *fileContents;
};

int main()
{
    char *file_name = argv(0);
    char *file_contents = argv(1);

    int counter = 0;

    /* the size of shared memory object */
    int size = sizeof(struct MyFiles)* 20;

    /* name  of  the  shared  memory  object */
    const  char  *name = "/PROJ4_SHARED_MEM";

    /* shared  memory  file descriptor */
    int shm_fd;

    /* pointer to  shared  memory  object  */
    void  *ptr;

    /* open the  shared  memory  object */
    shm_fd = shm_open(name, O_RDONLY, 0666);

    /* memory map the shared memory object */
    ptr = mmap(0, size, PROT_READ, MAP_SHARED, shm_fd, 0);
    struct MyFiles* file = (struct MyStruct*)ptr;

    if (file[0].fileName == "\0")
    {
            prinf("ERROR: There are no saved files in the shared memory object.\n\n");
            exit(1);
    }

    /*List all filenames */
    while (file[counter].fileName != "\0";)
    {
        prinf("%s \n", file[counter]->fileName);
        counter++;
    }

    /* remove the  shared  memory  object */
    shm unlink(name);

    return 0;
}

Answer 1

    struct MyFiles* file = (struct MyStruct*)ptr;

这显然是一个拼写错误，因为文件中的其他任何地方都没有 MyStruct。正如 rici 评论的那样，C 不要求您为赋值强制转换 void*，所以

    struct MyFiles *file = ptr;

足够了。

    file[0]->fileName = "\0";    
    file[0]->fileContents = "\0";

下标 [0] 已经表示一个间接； file[0] 的类型是 struct MyFiles，所以

    file[0].fileName = "\0";    
    file[0].fileContents = "\0";       

是正确的。但是，rici 的评论 你不能假设共享内存在共享它的每个进程中都具有相同的地址 也是正确的，除非你在每个 中指定相同的地址(不是 NULL，依赖于系统) mmap()(并检查结果是否等于该地址)。即便如此，正如 Chris Dodd 所写，您永远不会为字符串分配空间。您将它们设置为指向非共享……字符串…… - 对于您的项目，如果您在 struct MyFiles 中分配一定数量的空间，这将是最简单的方法:

struct MyFiles
{
    char fileName[12];
    char fileContents[500];
};
…
    /* Initialize first element. */
    // We can well omit this, since newly allocated bytes of a
    // shared memory object are automatically initialized to 0.
    file[0].fileName[0] = '\0';
    file[0].fileContents[0] = '\0';
…
    /* write to first available array slot in shared memory object */
    for (int i = 0; i < 20; i++)
    {
        if (file[i].fileName[0] == '\0')
        {
            sprintf(file[i].fileName, "%11s", file_name);
            sprintf(file[i].fileContents, "%499s", file_contents);
…
    /* List all filenames. */
    while (file[counter].fileName[0] != '\0')
    {
        puts(file[counter]->fileName);
        counter++;
…