mmap 函数实现共享内存
1. 匿名mmap
-
匿名存储映射：调用mmap函数时，指定
MAP_ANONYMOUS标识，并将文件描述符设置为-1。结果得到的是一个匿名共享存储区，是可被后代进程共享的存储区域。
接口：
void * Shm_Mmap_Anon(size_t len, int prot, int flags)
{
void *area = NULL;
if(MAP_FAILED == (area = mmap(0, len, prot, flags | MAP_ANONYMOUS, -1, 0)))
{
perror("mmap error:");
exit(-1);
}
return area;
}
2. 有名mmap
mmap函数：
#include <sys/mman.h>
void *mmap(void *addr, size_t length, int prot, int flags,
int fd, off_t offset);
参数：
addr：指定映射存储区的起始地址，一般指定为NULL，说明由系统指定。
len：映射存储区的长度。
prot：PROT_READ , PROT_WRITE, PROT_EXEC, PROT_NONE
flags：MAP_PRIVATE, MAP_SHARED, MAP_ANONYMOUS
fd：映射的文件描述符
offset：映射文件的其实位置，一般为0
munmap函数：
#include <sys/mman.h>
int munmap(void *addr, size_t length);
参数：
addr：起始地址
length：长度
shm系列函数实现共享内存
API接口：
shm.h
#ifndef _SHM_H
#define _SHM_H
void * Shm_Mmap_Anon(size_t len, int prot, int flags);
void * Shm_Mmap_File(void *addr, size_t len, int prot, int flags, int fd, off_t offset);
int Shm_Munmap(void *addr, size_t len);
int Shm_Get(key_t key, size_t size, int flag);
void *Shm_At(int shmid, const void *addr, int flag);
int Shm_Ctl(int shmid, int cmd, struct shmid_ds *buf);
int Shm_Remove(int shmid);
int Shm_Dt(const void *addr);
#endif
shm.c
#include <sys/mman.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include "shm.h"
void * Shm_Mmap_Anon(size_t len, int prot, int flags)
{
void *area = NULL;
if(MAP_FAILED == (area = mmap(0, len, prot, flags | MAP_ANONYMOUS, -1, 0)))
{
perror("mmap error:");
exit(-1);
}
return area;
}
void * Shm_Mmap_File(void *addr, size_t len, int prot, int flags, int fd, off_t offset)
{
void *area = NULL;
if(MAP_FAILED == (area = mmap(addr, len, port, flags, fd, offset)))
{
perror("mmap error:");
exit(-1);
}
return area;
}
int Shm_Munmap(void *addr, size_t len)
{
int val = 0;
val = munmap(addr, len);
if(val < 0)
{
perror("munmap error:");
}
return val;
}
int Shm_Get(key_t key, size_t size, int flag)
{
int shmid = 0;
shmid = shmget(key, size, flag);
if(shmid < 0)
{
perror("shmget error:");
}
return shmid;
}
void * Shm_At(int shmid, const void *addr, int flag)
{
void *area = NULL;
area = shmat(shmid, addr, flag);
if(area == NULL)
{
perror("shmat error:");
exit(-1);
}
return area;
}
int Shm_Ctl(int shmid, int cmd, struct shmid_ds *buf)
{
int val = shmctl(shmid, cmd, buf);
if(val < 0)
{
perror("shmctl error:");
}
return val;
}
int Shm_Remove(int shmid)
{
int val = shmctl(shmid, IPC_RMID, 0);
if(val < 0)
{
perror("shmctl error:");
}
return val;
}
//如果成功，shmdt将使相关shmid_ds 结构中的shm_nattch 计数器值减 1，类似于unlink的用法
int Shm_Dt(const void *addr)
{
int val = shmdt(addr);
if(val < 0)
{
perror("shmdt error:");
}
return val;
}
例程：
#include <stdio.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/types.h>
#include <semaphore.h>
#include "shm.h"
int main(void)
{
key_t key = 0;
pid_t pid = 0;
int shmid = 0;
key = ftok("/home", 0x01);
shmid = shmget(key, 20, IPC_CREAT | 0666);
void *mm0 = Shm_Mmap_Anon(20, PROT_READ | PROT_WRITE, MAP_SHARED);
void *mm2 = Shm_At(shmid, NULL, 0);
sem_unlink("/home");
char str[] = "Welcome to the world!\n";
pid = fork();
if(0 == pid)
{
key = ftok("/home", 0x01);
shmid = shmget(key, 20, IPC_CREAT | 0666);
//void *mm0 = Shm_Mmap_Anon(20, PROT_READ | PROT_WRITE, MAP_SHARED);
void *mm2 = Shm_At(shmid, NULL, 0);
char arr[] = "This is magic world!\n";
sem_t *sem = sem_open("/home", O_CREAT, 0666, 1);
while(1)
{
sem_wait(sem);
//printf("%s", mm0);
printf("%s", mm2);
//memcpy(mm0, arr, strlen(arr) + 1);
memcpy(mm2, arr, strlen(arr) + 1);
usleep(1000000);
//printf("%s===========\n", mm0);
printf("%s===========\n", mm2);
sem_post(sem);
usleep(1000000);
}
}
sem_t *sem = sem_open("/home", O_CREAT, 0666, 1);
while(1)
{
sem_wait(sem);
//printf("%s", mm0);
printf("%s", mm2);
//memcpy(mm0, str, strlen(str) + 1);
memcpy(mm2, str, strlen(str) + 1);
usleep(1000000);
//printf("%s###########\n", mm0);
printf("%s###########\n", mm2);
sem_post(sem);
usleep(1000000);
}
return 0;
}
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