[英]Initializing an array of structs into shared memory
我正在制作4个程序,这些程序创建POSIX共享内存对象(结构数组),该对象将由其他3个进程共享。 基本上,该项目模拟文件。
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,该结构放入数组的可用元素中。 程序#3将列出文件名。 程序4将搜索给定的文件并显示其内容。
我遇到的麻烦是将结构数组初始化为共享内存。 我不断收到以下错误,这表明我使用的是不正确的方法初始化指针:
myformat.c:36: warning: initialization from incompatible pointer type
我搜索了这个主题,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为共享内存?
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;
}
程式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;
}
计划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;
}
struct MyFiles* file = (struct MyStruct*)ptr;
显然有错别字,因为文件中其他任何地方都没有MyStruct
。 正如rici所评论的那样, C并不需要您为指定赋值void*
struct MyFiles *file = ptr;
就足够了。
file[0]->fileName = "\\0"; file[0]->fileContents = "\\0";
下标[0]
已经表示间接; 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,这也是正确的,除非您在每个mmap()
指定相同的地址(非NULL,取决于系统mmap()
(并检查结果是否等于该地址)。 即使那样,正如克里斯·多德(Chris Dodd)所写, 您也永远不会为字符串分配空间。 您将它们设置为指向非共享的…字符串… -对于您的项目,如果在struct MyFiles
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++;
…
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