(DIR) <- Back
       
       A minimal example of C's syscall mmap() to share memory between processes
       =========================================================================
       
       Recently I decided to add a feature to bsleep:
       My idea was to show the last button pressed (other than 'b', which would
       interrupt the sleep).
       
       The problem to overcome was the following:
       When fork() is used, parent and child no longer can access each others
       variables.
       Since it's the parent-process reading the button pressed, while the child
       is printing the update. Therefore the child needs to read the character
       from the parent process.
       Using mmap() I added a pointer to 1 byte of memory which both processes
       can access. It stores the character of the last button pressed.
       
       As a result bsleep has become a minimal example of:
       - how to fork a process in C
       - how to exchange data between parent and child process using mmap()
       
             git clone git://kroovy.de/bsleep
       
       Build dependencies
       - C compiler
       - libc
       
       
             #define _POSIX_SOURCE
             #include <signal.h>
             #include <stdio.h>
             #include <stdlib.h>
             #include <sys/mman.h>
             #include <unistd.h>
             
             int
             main(void)
             {
                 int i;
                 char *shmem = mmap(NULL, sizeof(char), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
                 pid_t pid = fork();
                     
                 if (pid == 0) {
                     /* CHILD */
                     for (i=1;;i++) {
                         printf("\r[ press 'b' to interrupt: %ds ] [ '%c' was pressed ] ", i, *shmem);
                         fflush(stdout);
                         sleep(1);
                     }
                 } else {
                     /* PARENT */
                     system("/bin/stty raw -echo");
                     while ((*shmem = getchar()) != 'b');
                     kill(pid, SIGKILL);
                     system("/bin/stty cooked echo");
                     printf("\n");
                 }
             }
       
       
       In the near future I will consider adding error-handling and will have a
       look at pagesize. It might be a good idea to let mmap claim a page-aligned
       amount of bytes. Not sure at this point.