test2.c

这个是学习嵌入式开发的重要例子

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

int *ptr;

int main(int argc, char **argv)
{
  if (argc < 2)
  {
    fprintf(stdout, "Usage: %s <filename>\n", argv[0]);
    exit(1);
  }

  int fd;

  // int open(const char *path, int oflag, ...  );
  //if ((fd = open(argv[1], O_RDONLY | O_CREAT | O_EXCL | O_TRUNC, 0644)) < 0)
  if ((fd = open(argv[1], O_RDONLY | O_CREAT | O_EXCL, 0644)) < 0)
  {
    if (errno != EEXIST)
    {
      fprintf(stderr, "open() failed: %s\n", strerror(errno));
      exit(1);
    }
    else
    {
      //if ((fd = open(argv[1], O_RDWR|O_TRUNC)) < 0)
      if ((fd = open(argv[1], O_RDWR)) < 0)
      {
	fprintf(stderr, "open() failed: %s\n", strerror(errno));
	exit(1);
      }
    }
  }

  lseek(fd, sizeof(int), SEEK_SET);
  write(fd, "", 1);

  //void *mmap(void *addr, size_t len, int prot, int flags, int fildes, off_t off);
  if ((ptr = (int *) mmap(NULL, sizeof(int), PROT_READ, MAP_SHARED, fd, 0)) == MAP_FAILED)
  {
    fprintf(stderr, "mmap() failed: %s\n", strerror(errno));
    exit(1);
  }

  fprintf(stdout, "mmap() successed.\n");

  //*ptr = 9;
  fprintf(stdout, "value = %d\n", *ptr);

  return 0;
}