[ Next Article |
Previous Article |
Book Contents |
Library Home |
Legal |
Search ]
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
- Open (or create) the file and save the file descriptor:
if( ( fildes = open( filename , 2 ) ) < 0 )
{
printf( "cannot open file\n" );
exit(1);
}
- 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>
- 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)
}
- Use the fsync subroutine to write changes to the copy of the file on disk to save the changes:
fsync( fildes );
- Close the file when the program is finished working with it:
close( fildes );
Related Information
Mapping Files with the shmat Subroutine.
Creating a Mapped Data File with the shmat Subroutine.
The shmat subroutine, shmctl subroutine, shmdt subroutine, shmget subroutine.
[ Next Article |
Previous Article |
Book Contents |
Library Home |
Legal |
Search ]