我正试图共享这样的结构
例子:
typedef struct {
int* a;
int b;
int c;
} ex;
在进程之间,问题是当我用malloc初始化“a”时,它对执行此操作的进程堆是私有的(或者至少我认为这是发生的事情)。有没有办法用这个结构来创建一个共享内存(使用shmget、shmat)?
编辑:我在Linux上工作。
编辑:我有一个初始化缓冲区的过程,如下所示:
key_t key = ftok("gr", 'p');
int mid = shmget(key, sizeof(ex), IPC_CREAT | 0666);
ex* e = NULL;
status b_status = init(&e, 8); //init gives initial values to b c and allocate space for 'a' with a malloc
e = (ex*)shmat(mid, NULL, 0);
另一个进程将自己连接到共享内存中,如下所示:
key_t key = ftok("gr", 'p');
int shmid = shmget(key, sizeof(ex), 0);
ex* e;
e = (ex*)shmat(shmid, NULL, 0);
然后从a中得到一个元素,在这种情况下,在位置1中
int i = get_el(e, 1);
最佳答案
首先,要共享int *a
字段指向的内容,需要复制与之相关的整个内存。因此,您需要一个至少可以保存size_t shm_size = sizeof(struct ex) + get_the_length_of_your_ex();
的共享内存。
从现在开始,既然您提到了shmget和shmat,我假设您运行的是Linux系统。
第一步是创建共享内存段。如果可以确定int *a
内容大小的上限,这将是一件好事。这样就不必反复创建/删除共享内存段。但是如果这样做,就需要额外的开销来说明实际数据需要多长时间。我假设一个简单的size_t
就可以实现这个目的。
然后,在创建段之后,必须正确设置数据,使其保持所需的状态。注意,虽然内存段的物理地址总是相同的,但是当调用shmat
时,您将得到虚拟指针,这些指针只在调用shmat
的进程中可用。下面的示例代码应该为您提供一些技巧。
#include <sys/types.h>
#include <sys/ipc.h>
/* Assume a cannot point towards an area larger than 4096 bytes. */
#define A_MAX_SIZE (size_t)4096
struct ex {
int *a;
int b;
int c;
}
int shm_create(void)
{
/*
* If you need to share other structures,
* You'll need to pass the key_t as an argument
*/
key_t k = ftok("/a/path/of/yours");
int shm_id = 0;
if (0 > (shm_id = shmget(
k, sizeof(struct ex) + A_MAX_SIZE + sizeof(size_t), IPC_CREAT|IPC_EXCL|0666))) {
/* An error occurred, add desired error handling. */
}
return shm_id;
}
/*
* Fill the desired shared memory segment with the structure
*/
int shm_fill(int shmid, struct ex *p_ex)
{
void *p = shmat(shmid, NULL, 0);
void *tmp = p;
size_t data_len = get_my_ex_struct_data_len(p_ex);
if ((void*)(-1) == p) {
/* Add desired error handling */
return -1;
}
memcpy(tmp, p_ex, sizeof(struct ex));
tmp += sizeof(struct ex);
memcpy(tmp, &data_len, sizeof(size_t);
tmp += 4;
memcpy(tmp, p_ex->a, data_len);
shmdt(p);
/*
* If you want to keep the reference so that
* When modifying p_ex anywhere, you update the shm content at the same time :
* - Don't call shmdt()
* - Make p_ex->a point towards the good area :
* p_ex->a = p + sizeof(struct ex) + sizeof(size_t);
* Never ever modify a without detaching the shm ...
*/
return 0;
}
/* Get the ex structure from a shm segment */
int shm_get_ex(int shmid, struct ex *p_dst)
{
void *p = shmat(shmid, NULL, SHM_RDONLY);
void *tmp;
size_t data_len = 0;
if ((void*)(-1) == p) {
/* Error ... */
return -1;
}
data_len = *(size_t*)(p + sizeof(struct ex))
if (NULL == (tmp = malloc(data_len))) {
/* No memory ... */
shmdt(p);
return -1;
}
memcpy(p_dst, p, sizeof(struct ex));
memcpy(tmp, (p + sizeof(struct ex) + sizeof(size_t)), data_len);
p_dst->a = tmp;
/*
* If you want to modify "globally" the structure,
* - Change SHM_RDONLY to 0 in the shmat() call
* - Make p_dst->a point to the good offset :
* p_dst->a = p + sizeof(struct ex) + sizeof(size_t);
* - Remove from the code above all the things made with tmp (malloc ...)
*/
return 0;
}
/*
* Detach the given p_ex structure from a shm segment.
* This function is useful only if you use the shm segment
* in the way I described in comment in the other functions.
*/
void shm_detach_struct(struct ex *p_ex)
{
/*
* Here you could :
* - alloc a local pointer
* - copy the shm data into it
* - detach the segment using the current p_ex->a pointer
* - assign your local pointer to p_ex->a
* This would save locally the data stored in the shm at the call
* Or if you're lazy (like me), just detach the pointer and make p_ex->a = NULL;
*/
shmdt(p_ex->a - sizeof(struct ex) - sizeof(size_t));
p_ex->a = NULL;
}
请原谅我的懒惰,因为它在共享内存中是完全未使用过的,所以不复制struct ex的
int *a
指针的所有值是空间优化的,但是我为自己保留了额外的代码来处理这个问题(以及一些指针检查,如p eu ex参数完整性)。但是,完成后,您必须找到一种方法在进程之间共享SHM ID。这可以通过使用插座、管道…或者使用相同输入的
ftok
。