General Programming Concepts:
Writing and Debugging Programs

Creating a Copy-On-Write Mapped Data File with the shmat Subroutine

Prerequisite Condition

The file to be mapped is a regular file.

Procedure

1. Open (or create) the file and save the file descriptor:

   if( ( fildes = open( filename , 2 ) ) < 0 )
   {
           printf( "cannot open file\n" );
           exit(1);
   }

2. Map the file to a segment as copy-on-write, with the shmat subroutine:

   file_ptr = shmat( fildes, 0, SHM_COPY );

   The SHM_COPY constant is defined in the /usr/include/sys/shm.h file. This constant indicates that the file is a copy-on-write mapped file. Include this header file and other shared memory header files in a program with the following directives:

   #include <sys/shm.h>

3. Use file_ptr as a pointer to the start of the data file, and access the data as if it were in memory.

   while ( file_ptr < eof)
   {
         .
         .
         .
         (references to file using file_ptr)
   }

4. Use the fsync subroutine to write changes to the copy of the file on disk to save the changes:

   fsync( fildes );

5. Close the file when the program is finished working with it:

   close( fildes );