我的项目有些问题。
我一直在尝试找到一个关于如何使用 fork() 的共享内存的示例文档，但是一直没有成功。
基本上，场景是这样的：当用户启动程序时，我需要将两个值存储在共享内存中： current_path 是一个char*类型的变量，而 file_name 也是char*类型的变量。
根据命令参数，使用 fork() 启动一个新进程，并且该进程需要读取和修改存储在共享内存中的 current_path 变量，而 file_name 变量是只读的。
你能否指导我找到一个好的共享内存教程（如果可能附带示例代码）？
有两种方法：shmget和mmap。我会谈论mmap，因为它更现代化、灵活，但如果您更喜欢使用旧的工具，请参考man shmget（或此教程）。
mmap()函数可用于分配内存缓冲区，并具有高度可定制的参数来控制访问和权限，并在必要时将它们与文件系统存储关联。
以下函数创建一个可以与其子进程共享的内存缓冲区：
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
void* create_shared_memory(size_t size) {
// Our memory buffer will be readable and writable:
int protection = PROT_READ | PROT_WRITE;
// The buffer will be shared (meaning other processes can access it), but
// anonymous (meaning third-party processes cannot obtain an address for it),
// so only this process and its children will be able to use it:
int visibility = MAP_SHARED | MAP_ANONYMOUS;
// The remaining parameters to `mmap()` are not important for this use case,
// but the manpage for `mmap` explains their purpose.
return mmap(NULL, size, protection, visibility, -1, 0);
}
#include <string.h>
#include <unistd.h>
int main() {
char parent_message[] = "hello"; // parent process will write this message
char child_message[] = "goodbye"; // child process will then write this one
void* shmem = create_shared_memory(128);
memcpy(shmem, parent_message, sizeof(parent_message));
int pid = fork();
if (pid == 0) {
printf("Child read: %s\n", shmem);
memcpy(shmem, child_message, sizeof(child_message));
printf("Child wrote: %s\n", shmem);
} else {
printf("Parent read: %s\n", shmem);
sleep(1);
printf("After 1s, parent read: %s\n", shmem);
}
}
shmget 是一种非常老式的、有些人认为已经被废弃的共享内存方法... 更好的选择是使用 mmap 和 shm_open、普通文件，或者直接使用 MAP_ANONYMOUS。 - R.. GitHub STOP HELPING ICE这里是一个共享内存的示例:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#define SHM_SIZE 1024 /* make it a 1K shared memory segment */
int main(int argc, char *argv[])
{
key_t key;
int shmid;
char *data;
int mode;
if (argc > 2) {
fprintf(stderr, "usage: shmdemo [data_to_write]\n");
exit(1);
}
/* make the key: */
if ((key = ftok("hello.txt", 'R')) == -1) /*Here the file must exist */
{
perror("ftok");
exit(1);
}
/* create the segment: */
if ((shmid = shmget(key, SHM_SIZE, 0644 | IPC_CREAT)) == -1) {
perror("shmget");
exit(1);
}
/* attach to the segment to get a pointer to it: */
if ((data = shmat(shmid, NULL, 0)) == (void *)-1) {
perror("shmat");
exit(1);
}
/* read or modify the segment, based on the command line: */
if (argc == 2) {
printf("writing to segment: \"%s\"\n", argv[1]);
strncpy(data, argv[1], SHM_SIZE);
} else
printf("segment contains: \"%s\"\n", data);
/* detach from the segment: */
if (shmdt(data) == -1) {
perror("shmdt");
exit(1);
}
return 0;
}
步骤：
使用ftok将路径名和项目标识符转换为System V IPC键
使用shmget分配一个共享内存段
使用shmat将由shmid标识的共享内存段附加到调用进程的地址空间中
在内存区域上执行操作
使用shmdt进行分离
这些是用于使用共享内存的包含文件
#include<sys/ipc.h>
#include<sys/shm.h>
int shmid;
int shmkey = 12222;//u can choose it as your choice
int main()
{
//now your main starting
shmid = shmget(shmkey,1024,IPC_CREAT);
// 1024 = your preferred size for share memory
// IPC_CREAT its a flag to create shared memory
//now attach a memory to this share memory
char *shmpointer = shmat(shmid,NULL);
//do your work with the shared memory
//read -write will be done with the *shmppointer
//after your work is done deattach the pointer
shmdt(&shmpointer, NULL);
尝试这段代码示例，我已经测试过了，来源：http://www.makelinux.net/alp/035
#include <stdio.h>
#include <sys/shm.h>
#include <sys/stat.h>
int main ()
{
int segment_id;
char* shared_memory;
struct shmid_ds shmbuffer;
int segment_size;
const int shared_segment_size = 0x6400;
/* Allocate a shared memory segment. */
segment_id = shmget (IPC_PRIVATE, shared_segment_size,
IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR);
/* Attach the shared memory segment. */
shared_memory = (char*) shmat (segment_id, 0, 0);
printf ("shared memory attached at address %p\n", shared_memory);
/* Determine the segment's size. */
shmctl (segment_id, IPC_STAT, &shmbuffer);
segment_size = shmbuffer.shm_segsz;
printf ("segment size: %d\n", segment_size);
/* Write a string to the shared memory segment. */
sprintf (shared_memory, "Hello, world.");
/* Detach the shared memory segment. */
shmdt (shared_memory);
/* Reattach the shared memory segment, at a different address. */
shared_memory = (char*) shmat (segment_id, (void*) 0x5000000, 0);
printf ("shared memory reattached at address %p\n", shared_memory);
/* Print out the string from shared memory. */
printf ("%s\n", shared_memory);
/* Detach the shared memory segment. */
shmdt (shared_memory);
/* Deallocate the shared memory segment. */
shmctl (segment_id, IPC_RMID, 0);
return 0;
}
shmget和shmat从不同的进程访问共享内存段，而这正是共享内存的重点... =(翻译：这是一份不错的代码，但我认为它没有展示如何在客户端中通过使用shmget和shmat来访问共享内存段，而这实际上是共享内存的核心所在... =( - étale-cohomology#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
/*
* pvtmMmapAlloc - creates a memory mapped file area.
* The return value is a page-aligned memory value, or NULL if there is a failure.
* Here's the list of arguments:
* @mmapFileName - the name of the memory mapped file
* @size - the size of the memory mapped file (should be a multiple of the system page for best performance)
* @create - determines whether or not the area should be created.
*/
void* pvtmMmapAlloc (char * mmapFileName, size_t size, char create)
{
void * retv = NULL;
if (create)
{
mode_t origMask = umask(0);
int mmapFd = open(mmapFileName, O_CREAT|O_RDWR, 00666);
umask(origMask);
if (mmapFd < 0)
{
perror("open mmapFd failed");
return NULL;
}
if ((ftruncate(mmapFd, size) == 0))
{
int result = lseek(mmapFd, size - 1, SEEK_SET);
if (result == -1)
{
perror("lseek mmapFd failed");
close(mmapFd);
return NULL;
}
/* Something needs to be written at the end of the file to
* have the file actually have the new size.
* Just writing an empty string at the current file position will do.
* Note:
* - The current position in the file is at the end of the stretched
* file due to the call to lseek().
* - The current position in the file is at the end of the stretched
* file due to the call to lseek().
* - An empty string is actually a single '\0' character, so a zero-byte
* will be written at the last byte of the file.
*/
result = write(mmapFd, "", 1);
if (result != 1)
{
perror("write mmapFd failed");
close(mmapFd);
return NULL;
}
retv = mmap(NULL, size,
PROT_READ | PROT_WRITE, MAP_SHARED, mmapFd, 0);
if (retv == MAP_FAILED || retv == NULL)
{
perror("mmap");
close(mmapFd);
return NULL;
}
}
}
else
{
int mmapFd = open(mmapFileName, O_RDWR, 00666);
if (mmapFd < 0)
{
return NULL;
}
int result = lseek(mmapFd, 0, SEEK_END);
if (result == -1)
{
perror("lseek mmapFd failed");
close(mmapFd);
return NULL;
}
if (result == 0)
{
perror("The file has 0 bytes");
close(mmapFd);
return NULL;
}
retv = mmap(NULL, size,
PROT_READ | PROT_WRITE, MAP_SHARED, mmapFd, 0);
if (retv == MAP_FAILED || retv == NULL)
{
perror("mmap");
close(mmapFd);
return NULL;
}
close(mmapFd);
}
return retv;
}
open 添加了文件 I/O 开销。请使用 shm_open 替代。 - osveinint。shared_malloc。#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <string.h>
#include <unistd.h>
void* shared_malloc(size_t size) {
int prot = PROT_READ | PROT_WRITE;
int flags = MAP_SHARED | MAP_ANONYMOUS;
return mmap(NULL, size, prot, flags, -1, 0);
}
int main() {
int* shared_x = (int *)shared_malloc(sizeof(int));
* shared_x = 3;
int pid = fork();
if (pid == 0) {
printf("Child read: %d\n", *shared_x);
* shared_x = 4;
printf("Child wrote: %d\n", *shared_x);
} else {
printf("Parent read: %d\n", *shared_x);
while(* shared_x == 3);
printf("After sync, parent read: %d\n", *shared_x);
}
}
shmget()等，以及使用MAP_ANON(又名MAP_ANONYMOUS)的纯mmap()方法——尽管MAP_ANON未被POSIX定义。此外，还有POSIXshm_open()和shm_close()用于管理共享内存对象。_ […继续…]_ - Jonathan Lefflershm_unlink()为止），而使用mmap()的机制需要一个文件和MAP_SHARED来持久化数据（而MAP_ANON排除了持久性）。在shm_open()规范的Rationale部分中有一个完整的示例。 - Jonathan Leffler