svshm/svshm_xfr_writer.c

+/*  svshm_xfr_writer.c
+
+   Read buffers of data data from standard input into a System V shared memory
+   segment from which it is copied by svshm_xfr_reader.c
+
+   We use a pair of binary semaphores to ensure that the writer and reader have
+   exclusive, alternating access to the shared memory. (I.e., the writer writes
+   a block of text, then the reader reads, then the writer writes etc). This
+   ensures that each block of data is processed in turn by the writer and
+   reader.
+
+   This program needs to be started before the reader process as it creates the
+   shared memory and semaphores used by both processes.
+
+   Together, these two programs can be used to transfer a stream of data through
+   shared memory as follows:
+
+        $ svshm_xfr_writer < infile &
+        $ svshm_xfr_reader > out_file
+*/
 #include "semun.h"              /* Definition of semun union */
 #include "svshm_xfr.h"
 
 int
 main(int argc, char *argv[])
 {
     int semid, shmid, bytes, xfrs;
     struct shmseg *shmp;
     union semun dummy;
 
+    /* Create set containing two semaphores; initialize so that
+       writer has first access to shared memory. */
+
     semid = semget(SEM_KEY, 2, IPC_CREAT | OBJ_PERMS);
     if (semid == -1)
         errExit("semget");
 
     if (initSemAvailable(semid, WRITE_SEM) == -1)
         errExit("initSemAvailable");
     if (initSemInUse(semid, READ_SEM) == -1)
         errExit("initSemInUse");
 
+    /* Create shared memory; attach at address chosen by system */
+
     shmid = shmget(SHM_KEY, sizeof(struct shmseg), IPC_CREAT | OBJ_PERMS);
     if (shmid == -1)
         errExit("shmget");
 
     shmp = shmat(shmid, NULL, 0);
     if (shmp == (void *) -1)
         errExit("shmat");
 
     /* Transfer blocks of data from stdin to shared memory */
 
     for (xfrs = 0, bytes = 0; ; xfrs++, bytes += shmp->cnt) {
         if (reserveSem(semid, WRITE_SEM) == -1)         /* Wait for our turn */
             errExit("reserveSem");
 
         shmp->cnt = read(STDIN_FILENO, shmp->buf, BUF_SIZE);
         if (shmp->cnt == -1)
             errExit("read");
 
         if (releaseSem(semid, READ_SEM) == -1)          /* Give reader a turn */
             errExit("releaseSem");
 
         /* Have we reached EOF? We test this after giving the reader
            a turn so that it can see the 0 value in shmp->cnt. */
 
         if (shmp->cnt == 0)
             break;
     }
 
     /* Wait until reader has let us have one more turn. We then know
        reader has finished, and so we can delete the IPC objects. */
 
     if (reserveSem(semid, WRITE_SEM) == -1)
         errExit("reserveSem");
 
     if (semctl(semid, 0, IPC_RMID, dummy) == -1)
         errExit("semctl");
     if (shmdt(shmp) == -1)
         errExit("shmdt");
     if (shmctl(shmid, IPC_RMID, NULL) == -1)
         errExit("shmctl");
 
     fprintf(stderr, "Sent %d bytes (%d xfrs)\n", bytes, xfrs);
     exit(EXIT_SUCCESS);
 }