svshm/svshm_xfr_reader.c

This is svshm/svshm_xfr_reader.c (Listing 48-3, page 1005), an example program file from the book The Linux Programming Interface.

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+/* svshm_xfr_reader.c
+
+   Read data from a System V shared memory using a binary semaphore lock-step
+   protocol; see svshm_xfr_writer.c
+*/
 #include "svshm_xfr.h"
 
 int
 main(int argc, char *argv[])
 {
     int semid, shmid, xfrs, bytes;
     struct shmseg *shmp;
 
     /* Get IDs for semaphore set and shared memory created by writer */
 
     semid = semget(SEM_KEY, 0, 0);
     if (semid == -1)
         errExit("semget");
 
     shmid  = shmget(SHM_KEY, 0, 0);
     if (shmid == -1)
         errExit("shmget");
 
+    /* Attach shared memory read-only, as we will only read */
+
     shmp = shmat(shmid, NULL, SHM_RDONLY);
     if (shmp == (void *) -1)
         errExit("shmat");
 
     /* Transfer blocks of data from shared memory to stdout */
 
     for (xfrs = 0, bytes = 0; ; xfrs++) {
         if (reserveSem(semid, READ_SEM) == -1)          /* Wait for our turn */
             errExit("reserveSem");
 
         if (shmp->cnt == 0)                     /* Writer encountered EOF */
             break;
         bytes += shmp->cnt;
 
         if (write(STDOUT_FILENO, shmp->buf, shmp->cnt) != shmp->cnt)
             fatal("partial/failed write");
 
         if (releaseSem(semid, WRITE_SEM) == -1)         /* Give writer a turn */
             errExit("releaseSem");
     }
 
     if (shmdt(shmp) == -1)
         errExit("shmdt");
 
     /* Give writer one more turn, so it can clean up */
 
     if (releaseSem(semid, WRITE_SEM) == -1)
         errExit("releaseSem");
 
     fprintf(stderr, "Received %d bytes (%d xfrs)\n", bytes, xfrs);
     exit(EXIT_SUCCESS);
 }