14, Linux shared memory shm_open opens shared memory file.

shm_openBrief introduction of function

shm_open：To create a memory file, the path requires something like / filename, starting with / and then the filename, with no /.
shm_openThe prototype and header files of the function are as follows:

 1 NAME
 2        shm_open, shm_unlink - create/open or unlink POSIX shared memory objects
 3 
 4 SYNOPSIS
 5        #include <sys/mman.h>
 6        #include <sys/stat.h>        /* For mode constants */
 7        #include <fcntl.h>           /* For O_* constants */
 8 
 9        int shm_open(const char *name, int oflag, mode_t mode);
10 
11        int shm_unlink(const char *name);
12        //Delete open file descriptor
13 
14        Link with -lrt.
15        //Note that when you compile, you should add the link library, for example:
16        gcc a.out -o a -lrt
17 RETURN VALUE
18        On success, shm_open() returns a nonnegative file descriptor.  On failure, shm_open() returns -1.
19        shm_unlink() returns 0 on success, or -1 on error.

【Example]

　　Writing files to shared memory:

　　Compile:

gcc shm_open_w.c -o shm_open_w -lrt

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include<fcntl.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<sys/mman.h>

/*Package print error function*/
void sys_err(const char *str,int num){
    perror(str);
    exit(num);
}

int main(int argc,char *argv[])
  {
    int fd = shm_open("/hello.txt",O_RDWR|O_CREAT|O_EXCL,0777);
    /*O_EXCL|O_CREAT,If the document already exists, it is reported that it is wrong.*/
    if(fd < 0){
        fd = shm_open("/hello.txt",O_RDWR,0777);
        /*Open file to read and write directly*/
    }else
        ftruncate(fd,4096);
        /*If you create files for yourself, allocate memory space for files.*/
    void *ptr = mmap(NULL,4096,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);

    puts("start writeing  data....");
    /*Here is the file for writing.*/
    strcpy((char*)ptr,"i am lixiaogang");
    puts("write over");
    getchar();
    shm_unlink("/hello.txt");
    close(fd);
    return 0;
  }

Read files to shared memory
Compile:

gcc shm_open_r.c -o shm_open_r -lrt

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include<fcntl.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<sys/mman.h>

void sys_err(const char *str,int num){
    perror(str);
    exit(num);
}

int main(int argc,char *argv[])
  {
    int fd = shm_open("/hello.txt",O_RDWR|O_CREAT|O_EXCL,0777);
    if(fd < 0){
        fd = shm_open("/hello.txt",O_RDWR,0777);
    }else
        ftruncate(fd,4096);
    void *ptr = mmap(NULL,4096,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);

    puts("start reading data....");
    puts((char*)ptr);
    puts("read over");
    shm_unlink("/hello.txt");
    close(fd);
    return 0;
  }

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