linux 进程通信之共享内存

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 4096int 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 4096int 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);}