共享内存映射的进程空间shmat函数返回是什么

czq1024 2017-12-01 10:17:31

是共享内存的物理地址，还是进程的虚拟地址，如果是父子进程分别映射呢？返回地址一样吗？有点晕

看一下代码
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define BUFFER_SIZE 2048

int main()
{
pid_t pid;
int shmid;
char *shm_addr;
char flag[] = "WROTE";
char buffer[BUFFER_SIZE];

if((shmid = shmget(IPC_PRIVATE,BUFFER_SIZE,0666)) < 0)
{
perror("shmget");
exit(-1);
}
else
{
printf("Create shared-memory:%d\n",shmid);
}

system("ipcs -m");
pid = fork();
if(pid < 0)
{
perror("fork error");
exit(-1);
}
else if(pid == 0)
{
shm_addr = shmat(shmid,0,0);
if(shm_addr == (void *)-1)
{
perror("Child:shmat");
exit(-1);
}
else
{
printf("Child:Attach shared-memory:%p \n",shm_addr);
}

system("ipcs -m");

while(strncmp(shm_addr,flag,strlen(flag)))
{
printf("Child:wait for enable data...\n");
sleep(5);
}

strcpy(buffer,shm_addr + strlen(flag));
printf("Child:shared-memory:%s\n",buffer);

if((shmdt(shm_addr)) < 0)
{
perror("shmdt");
exit(-1);
}
else
{
printf("Child: Deattach shared-memory\n");
}
system("ipcs -m");

if(shmctl(shmid,IPC_RMID,NULL) == -1)
{
perror("Child:shmctl(IPC_RMID)");
exit(-1);
}
else
{
printf("Delete shmared-memory\n");
}

system("ipcs -m");
}
else
{
if((shm_addr = shmat(shmid,0,0)) == (void *)-1)
{
perror("parent:shmat");
exit(-1);
}
else
{
printf("Parent:Attach shmared-memory:%p\n",shm_addr);
}

sleep(1);
printf("\nInput some string:\n");
fgets(buffer,BUFFER_SIZE,stdin);
strncpy(shm_addr + strlen(flag),buffer,strlen(buffer));
strncpy(shm_addr,flag,strlen(flag));

if((shmdt(shm_addr)) < 0)
{
perror("Parent:shmdt");
exit(-1);
}
else
{
printf("Parent : Deattach shared-memory\n");
}

system("ipcs -m");
waitpid(pid,NULL,0);
printf("Finsihed\n");
}

return 0;
}

运行结果
Create shared-memory:196613

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 0 fs 700 76800 2 dest
0x00000000 32769 fs 700 1843200 2 dest
0x00000000 65538 fs 700 20196 2 dest
0x00000000 98307 fs 700 17028 2 dest
0x00000000 131076 fs 700 19800 2 dest
0x00000000 196613 fs 666 2048 0

Parent:Attach shmared-memory:0xb773d000
Child:Attach shared-memory:0xb773d000

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 0 fs 700 76800 2 dest
0x00000000 32769 fs 700 1843200 2 dest
0x00000000 65538 fs 700 20196 2 dest
0x00000000 98307 fs 700 17028 2 dest
0x00000000 131076 fs 700 19800 2 dest
0x00000000 196613 fs 666 2048 2

Child:wait for enable data...

Input some string:
xiaoqiang
Parent : Deattach shared-memory

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 0 fs 700 76800 2 dest
0x00000000 32769 fs 700 1843200 2 dest
0x00000000 65538 fs 700 20196 2 dest
0x00000000 98307 fs 700 17028 2 dest
0x00000000 131076 fs 700 19800 2 dest
0x00000000 196613 fs 666 2048 1

Child:shared-memory:xiaoqiang

Child: Deattach shared-memory

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 0 fs 700 76800 2 dest
0x00000000 32769 fs 700 1843200 2 dest
0x00000000 65538 fs 700 20196 2 dest
0x00000000 98307 fs 700 17028 2 dest
0x00000000 131076 fs 700 19800 2 dest
0x00000000 196613 fs 666 2048 0

Delete shmared-memory

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 0 fs 700 76800 2 dest
0x00000000 32769 fs 700 1843200 2 dest
0x00000000 65538 fs 700 20196 2 dest
0x00000000 98307 fs 700 17028 2 dest
0x00000000 131076 fs 700 19800 2 dest

Finsihed

...全文

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另外建议使用 POSIX 共享内存接口

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SHMOP(2)                                          Linux Programmer's Manual                                         SHMOP(2)

NAME
       shmat, shmdt - System V shared memory operations

SYNOPSIS
       #include <sys/types.h>
       #include <sys/shm.h>

       void *shmat(int shmid, const void *shmaddr, int shmflg);

       int shmdt(const void *shmaddr);

DESCRIPTION
       shmat()  attaches the System V shared memory segment identified by shmid to the address space of the calling process.
       The attaching address is specified by shmaddr with one of the following criteria:

       If shmaddr is NULL, the system chooses a suitable (unused) address at which to attach the segment.

       If shmaddr isn't NULL and SHM_RND is specified in shmflg, the attach occurs at the address equal to  shmaddr  rounded
       down to the nearest multiple of SHMLBA.  Otherwise shmaddr must be a page-aligned address at which the attach occurs.

       If  SHM_RDONLY  is specified in shmflg, the segment is attached for reading and the process must have read permission
       for the segment.  Otherwise the segment is attached for read and write and the process must have read and write  per‐
       mission for the segment.  There is no notion of a write-only shared memory segment.

       The  (Linux-specific)  SHM_REMAP  flag  may be specified in shmflg to indicate that the mapping of the segment should
       replace any existing mapping in the range starting at shmaddr and continuing for the size of the segment.   (Normally
       an  EINVAL  error would result if a mapping already exists in this address range.)  In this case, shmaddr must not be
       NULL.

       The brk(2) value of the calling process is not altered by the attach.  The segment will automatically be detached  at
       process  exit.   The  same  segment  may  be  attached  as a read and as a read-write one, and more than once, in the
       process's address space.

       A successful shmat() call updates the members of the shmid_ds structure (see shmctl(2)) associated  with  the  shared
       memory segment as follows:

              shm_atime is set to the current time.

              shm_lpid is set to the process-ID of the calling process.

              shm_nattch is incremented by one.

       shmdt()  detaches the shared memory segment located at the address specified by shmaddr from the address space of the
       calling process.  The to-be-detached segment must be currently attached with shmaddr equal to the value  returned  by
       the attaching shmat() call.

       On a successful shmdt() call the system updates the members of the shmid_ds structure associated with the shared mem‐
       ory segment as follows:

              shm_dtime is set to the current time.

              shm_lpid is set to the process-ID of the calling process.

              shm_nattch is decremented by one.  If it becomes 0 and the segment is marked  for  deletion,  the  segment  is
              deleted.

       After a fork(2) the child inherits the attached shared memory segments.

       After an execve(2) all attached shared memory segments are detached from the process.

       Upon _exit(2) all attached shared memory segments are detached from the process.

RETURN VALUE
       On  success  shmat() returns the address of the attached shared memory segment; on error (void *) -1 is returned, and
       errno is set to indicate the cause of the error.

       On success shmdt() returns 0; on error -1 is returned, and errno is set to indicate the cause of the error.

ERRORS
       When shmat() fails, errno is set to one of the following:

       EACCES The calling process does not have the required permissions for the requested attach type, and  does  not  have
              the CAP_IPC_OWNER capability.

       EIDRM  shmid points to a removed identifier.

       EINVAL Invalid  shmid  value,  unaligned  (i.e.,  not  page-aligned and SHM_RND was not specified) or invalid shmaddr
              value, or can't attach segment at shmaddr, or SHM_REMAP was specified and shmaddr was NULL.

       ENOMEM Could not allocate memory for the descriptor or for the page tables.

       When shmdt() fails, errno is set as follows:

       EINVAL There is no shared memory segment attached at shmaddr; or, shmaddr is not aligned on a page boundary.

CONFORMING TO
       SVr4, POSIX.1-2001.

       In SVID 3 (or perhaps earlier) the type of the shmaddr argument was changed from char * into const  void *,  and  the
       returned  type  of  shmat()  from  char * into void *.  (Linux libc4 and libc5 have the char * prototypes; glibc2 has
       void *.)

NOTES
       Using shmat() with shmaddr equal to NULL is the preferred, portable way of attaching a  shared  memory  segment.   Be
       aware  that  the  shared memory segment attached in this way may be attached at different addresses in different pro‐
       cesses.  Therefore, any pointers maintained within the shared memory must be made relative (typically to the starting
       address of the segment), rather than absolute.

       On  Linux,  it  is  possible  to attach a shared memory segment even if it is already marked to be deleted.  However,
       POSIX.1-2001 does not specify this behavior and many other implementations do not support it.

       The following system parameter affects shmat():

       SHMLBA Segment low boundary address multiple.  Must be page aligned.  For the current implementation the SHMLBA value
              is PAGE_SIZE.

       The implementation places no intrinsic limit on the per-process maximum number of shared memory segments (SHMSEG).

SEE ALSO
       brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_overview(7), svipc(7)

COLOPHON
       This  page  is  part  of  release 3.54 of the Linux man-pages project.  A description of the project, and information
       about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                                    2013-02-12                                                 SHMOP(2)
 Manual page shmat(2) line 90/122 (END) (press h for help or q to quit)

zhxianbin 2017-12-01