一,创建共享内存
void *shmat(int shmid, void *shmaddr, int shmflg);该系统调用将shmid 对应的共享内存区映射到进程的虚拟地址空间中,shmaddr 为指定的映射起始地址,其值为NULL 时,映射空间由系统确定;shmflg 为标志字,其值一般指定为0。
/* * mkshm.c - Create and initialize shared memory segment */ #include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> #include <stdio.h> #include <stdlib.h> #define BUFSZ 4096 /* Size of the segment */ int main(void) { int shmid; if((shmid = shmget(IPC_PRIVATE, BUFSZ, IPC_CREAT | 0666)) < 0) { perror("shmget"); exit(EXIT_FAILURE); } printf("segment created: %d\n", shmid); system("ipcs -m"); exit(EXIT_SUCCESS); }
查看目前已有的共享内存命令:ipcs -m
二,向共享内存中写入数据
/*
* wrshm.c - Write data to a shared memory segment
*/
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <ctype.h>
#include <unistd.h>
#define BUFSZ 4096
int main(int argc, char *argv[])
{
int shmid; /* Segment ID */
char *shmbuf; /* Address in process */
key_t key;
char *msg;
int len;
key = ftok("/tmp", 0);
if((shmid = shmget(key, BUFSZ, IPC_CREAT|0666)) < 0) {
perror("shmget");
exit(EXIT_FAILURE);
}
printf("segment created: %d\n", shmid);
system("ipcs -m");
/* Attach the segment */
if((shmbuf = (char *)shmat(shmid, 0, 0)) < 0) {
perror("shmat");
exit(EXIT_FAILURE);
}
/* Write message to the segment */
msg = "this is the message written by wrshm program.";
len = strlen(msg);
strcpy(shmbuf, msg);
printf("%s\nTotal %d characters written to shared memory.\n", msg, len);
/* De-attach the segment */
if(shmdt(shmbuf) < 0) {
perror("shmdt");
exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}
三,从共享内存中读入数据
/*
* wrshm.c - Write data to a shared memory segment
*/
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <ctype.h>
#include <unistd.h>
#define BUFSZ 4096
int main(int argc, char *argv[])
{
int shmid; /* Segment ID */
char *shmbuf; /* Address in process */
key_t key;
int len;
key = ftok("/tmp", 0);
/* get the same share memory block */
if((shmid = shmget(key, 0, 0)) < 0) {
perror("shmget");
exit(EXIT_FAILURE);
}
/* Attach the segment */
if((shmbuf = (char *)shmat(shmid, 0, 0)) < 0) {
perror("shmat");
exit(EXIT_FAILURE);
}
/* read from share memory */
printf("Info read form shared memory is:\n%s\n", shmbuf);
/* De-attach the segment */
if(shmdt(shmbuf) < 0) {
perror("shmdt");
exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}