Linux \ Unix IPC process Communication instance analysis (1): Shared Memory communication-file ing MMAP Mode

Note: This example uses the form of shared file ing. The write end writes data and the read end reads data.

Write end:

/*-------------map_normalfile1.c-----------*/#include <sys/mman.h>#include <sys/types.h>#include <fcntl.h>#include <unistd.h>#include <stdio.h>#include <string.h>typedef struct{    char name[4];    int  age;}people;main(int argc, char** argv) // map a normal file as shared mem:{    int fd,i;    people *p_map;    char temp;        //open file    fd=open(argv[1],O_CREAT|O_RDWR|O_TRUNC,00777);    printf("file opened\n");/*  man openNAME     open, openat - open a fileSYNOPSIS     #include <sys/types.h>     #include <sys/stat.h>     #include <fcntl.h>     int open(const char *path, int oflag,  mode_t mode ...);DESCRIPTION     The open() function establishes  the  connection  between  a     file and a file descriptor. It creates an open file descrip-     tion that refers to a file and a file descriptor that refers     to  that  open file description. The file descriptor is used     by other I/O functions to refer  to  that  file.   The  path     argument points to a pathname naming the file.     The file offset used to mark the current position within the     file is set to the beginning of the file.     The file status flags and file access modes of the open file     description  are  set  according  to the value of oflag. The     mode argument is used only when O_CREAT  is  specified  (see     below.)     Values for oflag are constructed by  a  bitwise-inclusive-OR     of  flags  from  the  following  list, defined in <fcntl.h>.     Applications must specify exactly one  of  the  first  three     values (file access modes) below in the value of oflag:     O_RDONLY           Open for reading only.     O_WRONLY           Open for writing only.     O_RDWR           Open for reading and writing. The result is  undefined           if this flag is applied to a FIFO.     Any combination of the following may be used:     O_APPEND           If set, the file offset is set to the end of the  file           prior to each write.     O_CREAT           Create the file  if  it  does  not  exist.  This  flag           requires that the mode argument be specified.     .....*/        lseek(fd,sizeof(people)*5-1,SEEK_SET);    write(fd,"",1);/*NAME     lseek - move read/write file pointerSYNOPSIS     #include <sys/types.h>     #include <unistd.h>     off_t lseek(int fildes, off_t offset, int whence);DESCRIPTION     The lseek() function sets the file pointer  associated  with     the open file descriptor specified by fildes as follows:        o  If whence is SEEK_SET, the pointer is  set  to  offset           bytes.        o  If whence is SEEK_CUR,  the  pointer  is  set  to  its           current location plus offset.        o  If whence is SEEK_END, the pointer is set to the  size           of the file plus offset.     The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END  are     defined in the header <unistd.h>.*/        //create mmap    p_map = (people*) mmap(NULL, sizeof(people)*100, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);    close( fd );/*NAME     mmap - map pages of memorySYNOPSIS     #include <sys/mman.h>     void *mmap(void *addr, size_t len, int prot, int flags,  int     fildes, off_t off);DESCRIPTION     The  mmap()  function  establishes  a  mapping   between   a     process's  address space and a file or shared memory object.     The format of the call is as follows:          pa = mmap(addr, len, prot, flags, fildes, off);     The  mmap()  function  establishes  a  mapping  between  the     address  space of the process at an address pa for len bytes     to the memory object  represented  by  the  file  descriptor     fildes  at  offset  off  for len bytes. The value of pa is a     function of the  addr argument and values of flags,  further     described  below. A successful mmap() call returns pa as its     result. The address range starting at pa and continuing  for     len  bytes  will  be legitimate for the possible (not neces-     sarily current) address space of the process. The  range  of     bytes  starting  at off and continuing for len bytes will be     legitimate  for  the  possible  (not  necessarily   current)     offsets  in  the file or shared memory object represented by     fildes.*/        //write mmap    temp = 'a';    for(i=0; i<100; i++)    {        temp += 1;        memcpy(( *(p_map+i)).name, &temp, 2);        (*(p_map+i)).age = 20+i;    }    printf("initialize over\n");        //unmap the mmap    sleep(10);    munmap((char*)p_map, sizeof(people)*100);    printf("umap ok\n");    /*NAME     munmap - unmap pages of memorySYNOPSIS     #include <sys/mman.h>     int munmap(void *addr, size_t len);DESCRIPTION     The munmap() function removes the mappings for pages in  the     range  [addr,  addr  + len), rounding the len argument up to     the  next  multiple  of  the  page  size  as   returned   by     sysconf(3C).  If  addr is not the address of a mapping esta-     blished by a prior call to mmap(2), the  behavior  is  unde-     fined.  After  a  successful call to munmap() and before any     subsequent mapping of the unmapped pages, further references     to  these  pages  will result in the delivery of a SIGBUS or     SIGSEGV signal to the process.*/}

Read end:

 

/*-------------map_normalfile2.c-----------*/#include <sys/mman.h>#include <sys/types.h>#include <fcntl.h>#include <unistd.h>#include <stdio.h>#include <string.h>typedef struct{    char name[4];    int  age;}people;main(int argc, char** argv)  // map a normal file as shared mem:{    int fd,i;    people *p_map;        //open file    fd=open( argv[1],O_CREAT|O_RDWR,00777 );        //create mmap and read    p_map = (people*)mmap(NULL,sizeof(people)*100, PROT_READ|PROT_WRITE, MAP_SHARED,fd,0);    for(i = 0;i<100;i++)    {        printf( "name: %s age %d;\n",(*(p_map+i)).name, (*(p_map+i)).age );    }        //unmap    munmap((char*)p_map,sizeof(people)*100);}