POSIX 和 System V 内存共享用法

名词解释

POSIX: Portable Operating System Interface
A cross-platform specification supported by UNIX operating systems and those considered UNIX-like, such as Linux. The X in the name originally denoted that the interface was “based on UNIX.”

System V
The specification that defines the requirements for an operating system to be considered UNIX.

下表列出了System V IPC 和 POSIX IPC的区别：

SYSTEM V	POSIX
AT & T introduced (1983) three new forms of IPC facilities namely message queues, shared memory, and semaphores	Portable Operating System Interface standards specified by IEEE to define application programming interface (API). POSIX covers all the three forms of IPC
SYSTEM V IPC covers all the IPC mechanisms viz., pipes, named pipes, message queues, signals, semaphores, and shared memory. It also covers socket and Unix Domain sockets.	Almost all the basic concepts are the same as System V. It only differs with the interface
Shared Memory Interface Calls shmget(), shmat(), shmdt(), shmctl()	Shared Memory Interface Calls shm_open(), mmap(), shm_unlink()
Message Queue Interface Calls msgget(), msgsnd(), msgrcv(), msgctl()	Message Queue Interface Calls mq_open(), mq_send(), mq_receive(), mq_unlink()
Semaphore Interface Calls semget(), semop(), semctl()	Semaphore Interface Calls Named Semaphores sem_open(), sem_close(), sem_unlink(), sem_post(), sem_wait(), sem_trywait(), sem_timedwait(), sem_getvalue() Unnamed or Memory based semaphores sem_init(), sem_post(), sem_wait(), sem_getvalue(),sem_destroy()
Uses keys and identifiers to identify the IPC objects.	Uses names and file descriptors to identify IPC objects
NA	POSIX Message Queues can be monitored using select(), poll() and epoll APIs
Offers msgctl() call	Provides functions (mq_getattr() and mq_setattr()) either to access or set attributes 11. IPC - System V & POSIX
NA	Multi-thread safe. Covers thread synchronization functions such as mutex locks, conditional variables, read-write locks, etc.
NA	Offers few notification features for message queues (such as mq_notify())
Requires system calls such as shmctl(), commands (ipcs, ipcrm) to perform status/control operations.	Shared memory objects can be examined and manipulated using system calls such as fstat(), fchmod()
The size of a System V shared memory segment is fixed at the time of creation (via shmget())	We can use ftruncate() to adjust the size of the underlying object, and then re-create the mapping using munmap() and mmap() (or the Linux-specific mremap())

实例！

话不多说，来分别看两个实例吧:

POSIX IPC

//C program for Producer process illustrating POSIX shared-memory API.

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

#define SHM_FILE "/apc.shm.kashyap"

void lg(const char *oper, int result) {
  printf("%s %d\n", oper, result);
  if (result < 0) {
    perror(oper);
  }
}

void child(char *result) {
  for (int i = 0; i < 50; ++i) {
    strcpy(result, "child ::: hello parent\n");
    usleep(2);
    printf("child ::: %s", result);
  }
  usleep(5);
}

void parent(char *result) {
  usleep(1);
  for (int i = 0; i < 50; ++i) {
    strcpy(result, "parent ::: hello child\n");
    usleep(2);
    printf("parent ::: %s", result);
  }
  usleep(5);
}

int main() {
  int integerSize = 1024 * 1024 * 256; //256 mb
  
  int descriptor = -1; 
  int mmap_flags = MAP_SHARED;

#ifdef SHM
  // Open the shared memory.
  descriptor = shm_open(SHM_FILE, 
      O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);

  // Size up the shared memory.
  ftruncate(descriptor, integerSize);
#else
  //descriptor = -1;
  descriptor = creat("/dev/zero", S_IRUSR | S_IWUSR);
  mmap_flags |= MAP_ANONYMOUS;
#endif
  char *result = mmap(NULL, integerSize, 
      PROT_WRITE | PROT_READ, mmap_flags, 
      descriptor, 0 );

  perror("mmap");
  printf("%X\n", result);

  pid_t child_pid = fork();

  switch(child_pid) {
    case 0:
      child(result);
      break;
    case -1:
      lg("fork", -1);
      break;
    default:
      parent(result);
  }

  lg("msync", msync(result, integerSize, MS_SYNC));
  lg("munmap", munmap(result, integerSize));

  usleep(500000);

#ifdef SHM
  if (child_pid != 0) {
    lg("shm_unlink", shm_unlink(SHM_FILE));
  }
#endif

  return 0;
}

编译命令：

gcc ipc_posix.c -o ipc_posix -lrt

System V IPC

#include <stdio.h> 
#include <sys/shm.h> 
#include <sys/stat.h> 

int main () 
{
  int segment_id; 
  char* shared_memory; 
  struct shmid_ds shmbuffer; 
  int segment_size; 
  const int shared_segment_size = 0x6400; 

  /* Allocate a shared memory segment.  */ 
  segment_id = shmget (IPC_PRIVATE, shared_segment_size, 
                 IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR); 
  /* Attach the shared memory segment.  */ 
  shared_memory = (char*) shmat (segment_id, 0, 0); 
  printf ("shared memory attached at address %p\n", shared_memory); 
  /* Determine the segment's size. */ 
  shmctl (segment_id, IPC_STAT, &shmbuffer); 
  segment_size  =               shmbuffer.shm_segsz; 
  printf ("segment size: %d\n", segment_size); 
  /* Write a string to the shared memory segment.  */ 
  sprintf (shared_memory, "Hello, world."); 
  /* Detach the shared memory segment.  */ 
  shmdt (shared_memory); 

  /* Reattach the shared memory segment, at a different address.  */ 
  shared_memory = (char*) shmat (segment_id, (void*) 0x5000000, 0); 
  printf ("shared memory reattached at address %p\n", shared_memory); 
  /* Print out the string from shared memory.  */ 
  printf ("%s\n", shared_memory); 
  /* Detach the shared memory segment.  */ 
  shmdt (shared_memory); 

  /* Deallocate the shared memory segment.  */ 
  shmctl (segment_id, IPC_RMID, 0); 

  return 0; 
} 

编译命令：

gcc ipc_sysv.c -o ipc_sysv

Refence

UNIX System V
System V & Posix
Linux kernel interfaces
How to use shared memory with Linux in C