2018-08-06

【转】父子进程通信的两种方式--消息队列和共享内存

man mmap

EXAMPLE

The  following  program prints part of the file specified in its first command-line argument to standard output.  The range of bytes to be printed is specified via offset and
length values in the second and third command-line arguments.  The program creates a memory mapping of the required pages of the file and then uses  write(2)  to  output  the
desired bytes.

#include 
#include 
#include 
#include 
#include 
#include 

#define handle_error(msg) \
   do { perror(msg); exit(EXIT_FAILURE); } while (0)

int
main(int argc, char *argv[])
{
   char *addr;
   int fd;
   struct stat sb;
   off_t offset, pa_offset;
   size_t length;
   ssize_t s;

   if (argc < 3 || argc > 4) {
       fprintf(stderr, "%s file offset [length]\n", argv[0]);
       exit(EXIT_FAILURE);
   }

   fd = open(argv[1], O_RDONLY);
   if (fd == -1)
       handle_error("open");

   if (fstat(fd, &sb) == -1)           /* To obtain file size */
       handle_error("fstat");

   offset = atoi(argv[2]);
   pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);
       /* offset for mmap() must be page aligned */

   if (offset >= sb.st_size) {
       fprintf(stderr, "offset is past end of file\n");
       exit(EXIT_FAILURE);
   }

   if (argc == 4) {
       length = atoi(argv[3]);
       if (offset + length > sb.st_size)
           length = sb.st_size - offset;
               /* Can't display bytes past end of file */

   } else {    /* No length arg ==> display to end of file */
       length = sb.st_size - offset;
   }

   addr = mmap(NULL, length + offset - pa_offset, PROT_READ,
               MAP_PRIVATE, fd, pa_offset);
   if (addr == MAP_FAILED)
       handle_error("mmap");

   s = write(STDOUT_FILENO, addr + offset - pa_offset, length);
   if (s != length) {
       if (s == -1)
           handle_error("write");

       fprintf(stderr, "partial write");
       exit(EXIT_FAILURE);
   }

   exit(EXIT_SUCCESS);
}