shared memory hello world improved by signals

29 Dec 2009

/* file: shared-write.c
   compiling: gcc -o shared-write shared-write.c */
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <stdio.h>
#include <sys/shm.h>


/* Make this global, so visible inside handler */
static char * shared_memory;
static int segment_id;


 /* SIGNAL handler
 detach and deallocate  */
void
handler (int signumber)
{
  shmdt (shared_memory);
  shmctl (segment_id, IPC_RMID, 0);
  exit (0);
}

int
main (void)
{
  key_t key = 2525;
  int segment_size = 0x6400;
  /* Needed for signal handler */
  struct sigaction sa;
  /* install signal handler */
  sa.sa_handler = & handler;
  sigaction (SIGUSR1, &sa, NULL);
  
  /* get a shareable segment */
  segment_id = shmget (key, segment_size, IPC_CREAT | 0666);
  /* attach it with shared_memory */
  shared_memory = shmat (segment_id, NULL, 0);
  /* write into */
  sprintf (shared_memory, "0%d,Hello World", getpid());
  
  /* wait for signal */
  while (1)
 sleep (1);
  
  /* the program must exit through handler */
  return -1;
}

/* file: shared-read.c
   compiling: gcc -o shared-read shared-read.c */
#include <string.h>
#include <stdio.h>
#include <sys/shm.h>
#include <signal.h>
#include <sys/types.h>

int 
main (void)
{
  int segment_id;
  pid_t pid_of_brother;
  key_t key = 2525;
  char * shared_memory, string;
  int segment_size = 0x6400;
  char * ptr1, * ptr2, * ptr3;
  
  /* get the shared segment, using the same key */
  segment_id = shmget (key, segment_size, 0666);
  /* attaching */
  shared_memory = shmat (segment_id, NULL, 0);
  /* There is pid of the other process and a string
  split by a comma. Take the 2 values by walk
  through pointers */
  ptr1 = strchr (shared_memory, ','); 
  /* here I got the pid. If the pid is 4 numbers then
  one 0 is taken at begining of pid */
  ptr2 = ptr1 - 5;
  /* and here is string */
  ptr3 = ptr1 + 1;
  /* conver pid to pid type */
  pid_of_brother = (pid_t) atoi (ptr2);
  /* write string to stdout */
  printf ("%s\n", ptr3);
  /* send SIGUSR1 signal to other process
  saying HEY I FINISH 
  so it can detach and deallocate the 
  shared memory */
  kill (pid_of_brother, SIGUSR1);

  return 0;
}

running:
$ ./shared-write &
[1] 2026
$ ./shared-read
Hello World
[1]+ Done ./shared-write