linux

다음은 mmap 예입니다.

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

/*
 * pvtmMmapAlloc - creates a memory mapped file area.
 * The return value is a page-aligned memory value, or NULL if there is a failure.
 * Here's the list of arguments:
 * @mmapFileName - the name of the memory mapped file
 * @size - the size of the memory mapped file (should be a multiple of the system page for best performance)
 * @create - determines whether or not the area should be created.
 */
void* pvtmMmapAlloc (char * mmapFileName, size_t size, char create)
{
  void * retv = NULL;
  if (create)
  {
    mode_t origMask = umask(0);
    int mmapFd = open(mmapFileName, O_CREAT|O_RDWR, 00666);
    umask(origMask);
    if (mmapFd < 0)
    {
      perror("open mmapFd failed");
      return NULL;
    }
    if ((ftruncate(mmapFd, size) == 0))
    {
      int result = lseek(mmapFd, size - 1, SEEK_SET);
      if (result == -1)
      {
        perror("lseek mmapFd failed");
        close(mmapFd);
        return NULL;
      }

      /* Something needs to be written at the end of the file to
       * have the file actually have the new size.
       * Just writing an empty string at the current file position will do.
       * Note:
       *  - The current position in the file is at the end of the stretched
       *    file due to the call to lseek().
              *  - The current position in the file is at the end of the stretched
       *    file due to the call to lseek().
       *  - An empty string is actually a single '\0' character, so a zero-byte
       *    will be written at the last byte of the file.
       */
      result = write(mmapFd, "", 1);
      if (result != 1)
      {
        perror("write mmapFd failed");
        close(mmapFd);
        return NULL;
      }
      retv  =  mmap(NULL, size,
                  PROT_READ | PROT_WRITE, MAP_SHARED, mmapFd, 0);

      if (retv == MAP_FAILED || retv == NULL)
      {
        perror("mmap");
        close(mmapFd);
        return NULL;
      }
    }
  }
  else
  {
    int mmapFd = open(mmapFileName, O_RDWR, 00666);
    if (mmapFd < 0)
    {
      return NULL;
    }
    int result = lseek(mmapFd, 0, SEEK_END);
    if (result == -1)
    {
      perror("lseek mmapFd failed");
      close(mmapFd);
      return NULL;
    }
    if (result == 0)
    {
      perror("The file has 0 bytes");
      close(mmapFd);
      return NULL;
    }
    retv  =  mmap(NULL, size,
                PROT_READ | PROT_WRITE, MAP_SHARED, mmapFd, 0);

    if (retv == MAP_FAILED || retv == NULL)
    {
      perror("mmap");
      close(mmapFd);
      return NULL;
    }

    close(mmapFd);

  }
  return retv;
}                                                                                                     

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