功能：用来创建或打开一个共享内存对象

原型

int shm_open(const char *name, int oflag, mode_t mode);

参数

name:共享内存对象的名字

oflag:与open函数类似，可以是O_RDONLY、O_RDWR，还可以按位或上O_CREAT、O_EXCL、O_TRUNC等。

mode:此参数总是需要设置，如果oflag没有指定了O_CREAT，可以指定为0

返回值：成功返回非负整数文件描述符；失败返回-1

功能：修改共享内存对象大小

原型

int ftruncate(int fd, off_t length);

参数

fd: 文件描述符

length:长度

返回值：成功返回0；失败返回-1

01shm_open.c

#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>        /* For mode constants */
#include <fcntl.h>           /* For O_* constants */




#include <stdlib.h>
#include <stdio.h>
#include <errno.h>


#define ERR_EXIT(m) \
        do \
        { \
                perror(m); \
                exit(EXIT_FAILURE); \
        } while(0)


typedef struct stu
{
char name[32];
int age;
} STU;


int main(void)
{
int shmid;
shmid = shm_open("/xyz", O_CREAT | O_RDWR, 0666);
if (shmid == -1)
ERR_EXIT("shm_open");


printf("shm_open succ\n");
if (ftruncate(shmid, sizeof(STU)) == -1)
ERR_EXIT("ftruncate");


struct stat buf;
if (fstat(shmid, &buf) == -1)
ERR_EXIT("fstat");


printf("size=%ld mode=%o\n", buf.st_size, buf.st_mode & 07777);
close(shmid);
return 0;
}

02shm_unlink.c

#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>        /* For mode constants */
#include <fcntl.h>           /* For O_* constants */




#include <stdlib.h>
#include <stdio.h>
#include <errno.h>


#define ERR_EXIT(m) \
        do \
        { \
                perror(m); \
                exit(EXIT_FAILURE); \
        } while(0)


typedef struct stu
{
char name[32];
int age;
} STU;


int main(void)
{
shm_unlink("/xyz");
return 0;
}

03shm_write.c

#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>        /* For mode constants */
#include <fcntl.h>           /* For O_* constants */




#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>


#define ERR_EXIT(m) \
        do \
        { \
                perror(m); \
                exit(EXIT_FAILURE); \
        } while(0)


typedef struct stu
{
char name[32];
int age;
} STU;


int main(void)
{
int shmid;
shmid = shm_open("/xyz", O_RDWR, 0);
if (shmid == -1)
ERR_EXIT("shm_open");


printf("shm_open succ\n");


struct stat buf;
if (fstat(shmid, &buf) == -1)
ERR_EXIT("fstat");


STU *p;
p = mmap(NULL, buf.st_size, PROT_WRITE, MAP_SHARED, shmid, 0);
if (p == MAP_FAILED)
ERR_EXIT("mmap");


strcpy(p->name, "test");
p->age = 20;
close(shmid);
return 0;
}

04shm_read.c

#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>        /* For mode constants */
#include <fcntl.h>           /* For O_* constants */




#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>


#define ERR_EXIT(m) \
        do \
        { \
                perror(m); \
                exit(EXIT_FAILURE); \
        } while(0)


typedef struct stu
{
char name[32];
int age;
} STU;


int main(void)
{
int shmid;
shmid = shm_open("/xyz", O_RDONLY, 0);
if (shmid == -1)
ERR_EXIT("shm_open");


printf("shm_open succ\n");


struct stat buf;
if (fstat(shmid, &buf) == -1)
ERR_EXIT("fstat");


STU *p;
p = mmap(NULL, buf.st_size, PROT_READ, MAP_SHARED, shmid, 0);
if (p == MAP_FAILED)
ERR_EXIT("mmap");


printf("name=%s age=%d\n", p->name, p->age);
close(shmid);
return 0;
}

makefile:

.PHONY:clean all
CC=gcc
CFLAGS=-Wall -g
BIN=01shm_open 02shm_unlink 03shm_write 04shm_read
all:$(BIN)
%.o:%.c
$(CC) $(CFLAGS) -c $< -o $@
01shm_open:01shm_open.o
$(CC) $(CFLAGS) $^ -o $@ -lrt
02shm_unlink:02shm_unlink.o
$(CC) $(CFLAGS) $^ -o $@ -lrt
03shm_write:03shm_write.o
$(CC) $(CFLAGS) $^ -o $@ -lrt
04shm_read:04shm_read.o
$(CC) $(CFLAGS) $^ -o $@ -lrt
clean:
rm -f *.o $(BIN)